Watts, Amps and Volts, Kilowatt Hours (kWh) and Electrical Appliances—Basic Electricity Explained
A Guide to Understanding Watts, Amps and Volts
 Voltage is a measure of pressure in an electrical circuit.
 Amps is a measure of the current flowing.
 Watts is a measurement of power or the rate at which energy is used.
 Ohms is a measure of the restriction or resistance to current flow.
In this article, you'll learn all about volts, watts, amps, ohms, current, power, resistance and kilowatt hours (kWh). The equations are really quite simple, and you'll find some examples on how to apply them to home appliances.
Want to test yourself? See how you perform in Quiz A, B and C at the end of each section.
Calculating Power Consumption of Household Appliances
If you just want to know the power rating of, and how much it costs to run electrical appliances in the home, you can take a detour to this related hub which gives a comprehensive list: What is the Cost of Running Electrical Appliances?
Electric Current Is a Flow of Electrons in a Conductor
All matter is made from basic building blocks called atoms. A simplistic model of an atom, known as the Rutherford–Bohr model or Bohr model or Bohr diagram has a central nucleus made up of particles called protons and neutrons. The nucleus is surrounded by orbitals containing electrons. In some materials such as metals, electrons are bound loosely to the nucleus so they can detach and move when a voltage is applied. These materials are known as conductors and can conduct electricity. The flow of electrons is called a current.
What Are Volts, Amps and Watts?
Like any discipline, electrical engineering has jargon or specialized terminology. Voltage and current are like water pressure and water flow rate respectively, and reference is often made to pumps and water pipes as an analogy to explain electrical circuitry.
Voltage
This is the pressure in a circuit and is measured in volts. Think of a pump in a water pipe. The greater the pressure and the force which the pump exerts, the greater will be the flow of water through the pipe. Similarly a voltage source in a circuit is like a pump and pushes electrons around the circuit. The higher the voltage applied to a circuit, the greater the current which will be forced through it.
Load
This is the device connected to a voltage source. It could be a motor, bulb, heater, LED, or an electronic resistor.
Current
An electric current is due to the movement of electrons through a conductor and load and is measured in amps. High current means lots of electrons flowing through the circuit. The water analogy is water flow rate in gallons per minute.
Resistance
The resistance of the load is measured in ohms. Every electrical device or load has resistance. Resistance is like a restriction to the flow of electrons and electricity is dissipated as heat energy in a resistance. For a given voltage, the higher the resistance, the lower the current. Going back to the water analogy, when you stand on a hose, you increase the resistance and restrict the flow. The only way to restore the flow is by getting the pump to pump harder, and force water through the restriction, i.e. the pump needs to have a higher pressure. Alternatively if you take your foot off the hose, you increase the diameter and lower the resistance and more water can be forced through. In an electrical circuit, if the voltage is increased, more current is forced through the resistance. If the resistance is lowered, more current will flow even if the voltage doesn't change.
Even connecting wires in a circuit have resistance so when higher currents need to be carried by a cable, thicker gage cable must be used to avoid overheating.
Power
This is the rate at which energy is consumed and is measured in watts. A kilowatt is 1000 watts, also abbreviated to kW.
kWh or Kilowatt Hours
This is a measure of energy consumption. KWh are sometimes called units and are what you pay for on your electricity bill. A 1 kilowatt (1000 watt) appliance uses a kilowatt hour of electricity in one hour. Similarly a 500 watt device uses a kilowatt hour of electricity in 2 hours.
Frequency
For an AC supply, this is the number of times per second that the current changes direction, measured in cycles per second or hertz. Electricity is distributed to homes at 50 or 60 hertz.
How Does Current Flow in a Circuit?
In the photo below, an AA cell powers a torch bulb. Current first flows out the top of the battery, through the wire and bulb and then returns via the bottom wire.
We can represent this circuit in a simple manner using a schematic or circuit diagram. Looking at the schematic below, a voltage source V will force a current I around the circuit through the load (the bulb in this case) whose resistance is R.
In a real life circuit, the voltage source could be the 120 or 240 volts coming out of a socket outlet, a 12 volt car battery, or an AA cell and the resistance would be an appliance or component in an electronic circuit. The lines joining the source to the resistance would be the connecting wires inside an appliance or power flex, or tracks on a printed circuit board.
Note: Conventionally we think of current flowing out the positive terminal of a source such as a battery. However current is a flow of subatomic particles called electrons which are negatively charged, so current actually flows the other way, from the negative terminal of the battery
What Are Some Commonly Used Voltages ?
Voltage Source
 Voltage


AA or AAA cell
 1.5 volts

Mains supply in the home
 Nominally 120 or 240 volts

Car battery
 12 volts

Truck battery
 24 volts

Voltage input to transformer supplying home
 16kV (kilo volts)

High voltage transmission lines
 Up to 1.2 MV (Mega volts)

What Is the Voltage Supplied to Our Homes?
In general, the voltage supply to your home is nominally 230 or 120 volts. Voltage in the USA is 120 volts, but two "hots" are supplied to homes so that a 240 volt supply is also available. The higher voltage is used for high powered appliances such as washers, driers, kitchen ranges (cookers) and air conditioning. 120 volts is used for lower power and portable devices. It is also safer because in the event of an electrical shock, less current flows through the body so there a lesser risk of electrocution.
In countries where 230 volts is standard, generators or step down isolating transformers are used to provide a 110 volt supply for power tools. This is normally mandatory on construction sites. Again the idea of the lower voltage is to lessen the danger of electrocution, if for example a power flex is inadvertently cut, or a tool gets wet.
Watts, Amps and Volts Equation
How to Convert Between Volts, Amps and Watts
We will consider Ohm's law later, but first let's examine the quantities which are usually of interest when dealing with appliances, such as volts, amps and watts and how to convert between them. If you look at the casing of an appliance (see photo below) you can usually find a specification label or panel which indicates the voltage supply, frequency, wattage and possibly current. On some appliances e.g. TVs and washing machines, this panel may be mounted at the back of the device.
So here are three simple equations for converting between volts, watts and amps:
Watts = Volts x Amps
e.g. A 120 volt appliance takes 2 amps, what is the power?
Power in watts = 120 x 2 = 240 watts
Amps = Watts / Volts
e.g. A 240 volt appliance consumes 480 watts of power, How much current does it draw?
Current in amps = 480 / 240 = 2 amps
Volts = Watts / Amps
e.g. A 720 watt appliance draws 3 amps, What voltage is it running on?
Voltage in volts = 720 / 3 = 240 volts
So it's really that simple. Notice I have chosen values in the examples so that everything works out nicely. You only really need to remember the first equation and if you know basic algebra you can rearrange to give the other two equations. However as you can see, you always need to know two of the quantities before you can work out the third quantity. From looking at the Google Analytics statistics and the questions which land people on this webpage, I often see questions asked such as "how many watts are in 480 volts?", which obviously makes no sense!
For high powered appliances, power is often specified in kilowatts ( abbreviated to kw)
1 kilowatt = 1000 watts
What Is a kWh?—How to Calculate Energy Consumption of Appliances
Power is the rate at which a device uses energy. So for instance an air conditioning unit, shower or powerful floodlight uses electrical energy much faster than a light bulb
Energy used = Power x Time
So to figure out the energy usage of an appliance, you multiply its power rating by the time period for which it is running. The standard unit of energy is the joule or calorie, but generally energy used in the home is measured in kWh, also known as "units". To work out the number of kwh, you divide the power in watts by 1000 to convert to kilowatt (kW) and then multiply by time in hours to give kWh. So:
kWh = Watts / 1000 x time in hours
Kilowatt hours, kWh or units are what you pay for on your bill. Your electricity meter counts and displays the number of units used by all the appliances and lighting in your home.
e.g. A 2500 watt drier runs for 3 hours a day, how many kWh does it consume and if electricity costs 12c per unit, what is the cost of running it?
kWh = watts/1000 x time = 2500 / 1000 x 3 = 7.5 kWh or units
Cost = 7.5 x 12c = 90 cents
Some appliances don't run continuously. Examples are devices controlled by a thermostat such as refrigerators, freezers, ovens in cookers and air conditioning systems. The time for which the appliance is powered on and consuming power is called the duty cycle and it is often quoted as a percentage. So for instance a fridge which stays on half of the time has a duty cycle of 50%.
This article gives a comprehensive list of domestic appliances and their energy use: What is the Cost of Running Electrical Appliances?
How Many Amps Are Supplied to Our Homes?
Typically for a 230 volt supply to a home, the main fuse rating is 80 to 100 amps at the consumer unit. So this is the maximum current that will flow before the fuse blows. At 80 A and 230 volts, this allows a power draw of 230 x 80 = 18.4 kW.
Test Yourself!  Quiz A
view quiz statisticsWhat Are Fuses For?
As we will see later, electrical cables, appliances, wires inside appliances, components etc all have resistance. This resistance produces heat when current flows through it. Any electrical conductor can get excessively hot if too much current flows and in the case of wires, this can cause the plastic insulation covering the cable to melt or even catch fire. So fuses are used in series with a cable or appliance to limit current flow and make everything safe. Fuses are like a "weak link" in a chain and blow before damage can occur. They have a specified rating and this is not the current they blow at, but the current they will carry without blowing. Once current exceeds the rating of the fuse, the fuse will blow. The length of time it takes for the fuse to blow is proportional to the current. So minor overloads can result in a fuse blowing in minutes, but if there is a large current or short circuit scenario, the fuse will blow in seconds or milliseconds.
Breaking Capacity of Fuses
Fuses have a max current they can carry without the encapsulation of the fuse rupturing. So fuses on the secondary of domestic power supplies in TVs, battery chargers and other electronic appliances are often glass types because the supply will only source a relatively small amount of energy if there is a fault. Ceramic types are used to resist the heat and shock that occurs when the inrush current can be perhaps hundred or thousands of amps, potentially feeding a huge amount of instantaneous power. If a short circuit occurs in an appliance, it's quite possible that the utility transformer in your street can feed current of this magnitude into the short. So for example the BS1362 fuse in a UK style plug has a ceramic body. Blown fuses should always be replaced by the same type, ceramic if necessary, to avoid a fire occurring.
Fuse Types
In general, fuses are fast blow (F) and time lag (T). Time lag types are often used for power supplies in electronic equipment because the capacitors take a surge of current as they charge up, which would blow a fast acting fuse.
What Is a Digital Multimeter?
A multimeter is an instrument which can measure voltage, current, resistance and possibly additional parameters. You can also use it to check continuity of cables and check fuses. If you don't know how to use one, read my guide How to Use a Digital Multimeter (DMM) to Measure Voltage, Current, and Resistance. Multimeters normally have a continuity range also, and this comes in useful for checking breaks in cables, fuses and loose connections.
Digital Multimeters From Amazon
Fluke, who are a leading manufacturer of digital instrumentation, recommend the Fluke 113 model for general purpose use in the home or for car maintenance. This is an excellent meter and can measure AC and DC volts, resistance, check continuity and diodes. The meter is autoranging, so ranges don't have to be set. It is also a trueRMS meter. If you also need to measure AC and DC current, the Fluke 106 is a suitable choice.
How to Convert Horsepower to Watts
Horsepower is a measure of....you guessed it!..... power!
Just as an engine's mechanical output can be measured in horsepower, so can the mechanical output of an electric motor.
1 horsepower = 746 watts
E.g. A fractional horsepower motor in a washing machine is rated at 1/2 horsepower
So the power output of the motor = 746 watts x 0.5 = 373 watts
A motor is not 100% efficient, in other words not all the electrical power input is converted into mechanical power at the output shaft, some being wasted as heat in the windings.
Test Yourself!  Quiz B
view quiz statisticsWhat Is an Electricity Usage Monitor?
An electricity usage monitor or tracker tells you everything you want to know about your appliance behavior. The parameters are displayed on an LCD and include voltage, current, power consumption, kwh used, cost of running and run time of appliance. The latter is useful for troubleshooting fridges, freezers, air conditioners etc which are controlled by a thermostat and switch on and off. A failed thermostat or waterlogged insulation can cause an appliance to run constantly, so this problem can be identified.
You can read about these devices here: Tracking the Power Consumption of Your Appliances
What Does It Mean to Consume Electricity?
What happens when an appliance is powered from electricity? Scientists tell us that energy cannot be destroyed, it just changes from one form to another. This process happens all the time  on Earth and throughout the Universe. For instance a rock on the edge of a cliff has potential energy, because of its altitude above the ground. If it falls over the edge of the cliff, it starts to pick up velocity, i.e. gains kinetic energy (motion energy) while losing potential energy. When it hits the ground, this energy is dissipated as heat (think of the heat produced by an asteroid impact). Similarly when an appliance is plugged in, the electricity doesn't get wasted or "consumed", in the sense of being destroyed, it simply changes form. So in the case of a lamp, it ends up as light energy or as heat energy when a heater is used. Electrical energy can also be converted to sound in a loudspeaker or electromagnetic radiation (microwave oven or radio transmitter), all forms of energy. Electrical energy can also be converted to kinetic energy in an electric motor or to potential energy when an elevator is raised in a building.
Power is a measure of the rate at which energy is used. So for instance a 1000 watt heater or high powered hvac air conditioning system uses energy at a higher rate than a 60 watt light bulb.
Electricity Can Be Converted to Other Forms of Energy
Energy Type
 Example


Light
 Incandescent light bulb, LED, Fluorescent lamp

Heat
 Electric heater, Incandescent light bulb

Electromagnetic radiation
 Radio transmitter, Microwave oven, Radar

Sound
 Loudspeaker, Thunder

Kinetic
 Motor spinning a shaft or driving a vehicle

Potential
 Winch or lift raising a load, Electromagnet tensioning a spring

Pressure
 Air compressor

Chemical
 Battery

Ohms's Law and Electrical Resistance
In the circuit above, a voltage V pushes a current I around the circuit and through the load. As you may remember, this could be a device such as a bulb, electrical heater, motor, LED or other electrical appliance. The load resists the flow of current and the magnitude of its resistance is R ohms.
So
I = V / R
or
R = V / I
This is known as Ohm's law and basically says that the current is proportional to the voltage and inversely proportional to the resistance (As resistance increases, current decreases and vice versa) Remember the resistance measured in ohms is just a measure of how the load or appliance in the circuit "resists" the flow of current. In electronic circuits and some electrical appliances, components called resistors have precise values of resistance so that they can be used to control the value of current flowing in a circuit.
An example:
The resistance in a circuit is 100 ohms, a voltage of 120 volts is applied, what is the current?
Current = 120 / 100 = 1.2 amps
Electrical Resistance and Conductors
A conductor is a physical medium which carries an electric current. This could be a power cable, prongs on a plug, a liquid such as water, battery acid or ionized gas in a discharge lamp (e.g. fluorescent or sodium lamp).
In the case of a solid conductor such as copper wire, the electrical resistance is proportional to the length of the conductor and inversely proportional to its crosssectional area. In effect this means that the longer a piece of wire, the higher its resistance. Similarly the greater the diameter of the wire, the lower its resistance. This has implications for conductors used in appliances and power transmission. For example, the gauge of wire used in an extension lead is important, if the wire is too thin, the resistance will be high and the cable can overheat. If a power cable is very long, its resistance may be too high if not properly rated, resulting in an unacceptable voltage drop at the end of the cable (because of the resistance).
For a conductor with cross sectional area A and length l, the resistance R can be calculated using the equation:
R = ρl / A
ρ (Greek letter "rho") is a constant known as the resistivity. The lower the resistivity of a material, the lower will be the resistance of the conductor.
Copper has the lowest resistivity of most common materials and this is why it is widely used in the manufacture of cables. Silver has a lower resistivity than copper, but it is much more expensive. Aluminium is generally used for overhead cables and although it has a higher resistivity than copper, it is lighter. Gold has a resistivity about 1.5 times that of copper, however it is unreactive and doesn't oxidize (tarnish). A tarnish coating on a conductor increases contact resistance, so this is why gold is often used as a coating on audio / video connectors. Gold is also used for the miniature connecting wires in integrated circuits.
Insulators
An electrical insulator is a material which has a very high resistance because there are no free electrons to carry current. For all practical purposes an insulator can be considered to have infinite resistance. Because resistance is infinite (infinity is represented by the symbol ∞), then current through an insulator is:
Current = Voltage / resistance = voltage / ∞ = 0
Insulators are used to prevent current flow between two electrical points with differing voltage e.g. insulation on the individual cores of a power cable or glass/ceramic insulators on power lines, and also to prevent high voltage from causing electric shock. Typical insulators used for electrical purposes are various types of polymers (plastic), ceramic, glass, glass epoxy (used for PCBs) and Bakelite (an older style thermosetting plastic)
Superconductors
When certain materials are subjected to very low temperatures, their resistance falls to zero.
Since V = IR, if R is zero, then V becomes 0 even if I is non zero
The consequences of this are that a current can flow even if the voltage source is removed. Because resistance is zero, and no heat is dissipated, huge currents can be carried by thin cables. Superconductors are used for example in MRI machines to carry the high currents required by powerful magnets.
Resistivities of Various Materials
Material
 Resistivity


Silver
 1.59×10−8 Ωm

Copper
 1.68×10−8 Ωm

Gold
 2.44×10−8 Ωm

Aluminium
 2.82×10−8

Iron
 9.71×10−8 Ωm

Platinum
 1.06×10−7 Ωm

Nichrome (used in heating elements)
 1.10×10−6 Ωm

Glass
 1.00×1011 to 1.00×1015 Ωm

Hard Rubber
 1.00×1013 Ωm

Alternative Way of Working Out Power
Remember watts = volts x amps? Another way to work out power is from the resistance in ohms:
So if I is the current in amps, V is the voltage, R is the resistance in ohms and P is the power in watts,
Then:
I = V / R from Ohm's law
But also P =VI
So substituting the expression I =V/R into P = VI gives:
P = VI = V(V/R) = V^{2}/ R
similarly
P = VI =(IR)I = I^{2}R
It's unlikely when dealing with appliances in the home to need to use the last two equations. However here is an example.
A 240 volt supply is connected to a load of 100 ohms. What is the power consumption of the load?
Power = V^{2}/ R = (240)^{2} / 100 = 576 watts
Test Yourself!  Quiz C
view quiz statisticsWhat Are AC and DC?
The current produced by a power source can take one of two forms, AC or DC. The power source could be a battery, electrical generator, power transmitted along service cables to your home or the output of a signal generator, a device used in laboratories or by test personnel when testing or designing electronic systems.
DC
This stands for direct current so the current provided by the source only flows one way. A DC source will have a nominal value voltage level and this voltage will fall as the source is loaded and outputs more current. This drop is due to inherent internal resistance within the source. The resistance is not due to an actual resistor, but can be modelled as such, and is composed of actual resistance of conductors, electronic components, chemicals etc.
Examples of DC sources are batteries, DC generators known as dynamos, solar cells and thermocouples.
AC
This stands for "alternating current" and means that the current "alternates" or changes direction. So current flows one way, reaches a peak, falls to zero, changes direction, reaches a peak and then falls back to zero again before the whole cycle is repeated. The number of times this cycle happens per second is called the frequency. In the U.S. the frequency is 60 Hertz (Hz) or cycles per second. In other countries it is 50 Hz. The electricity supply in your home is AC.
The advantage of AC is the ease by which it can be transformed from one voltage level to another by a device known as a transformer.
AC sources include the electrical supply to your home, generators in power stations, transformers, DC to AC inverters (allowing you to power appliances from the cigarette lighter in your car), signal generators and variable frequency drives for controlling the speed of motors. The alternator in a vehicle generates electricity as AC before it is rectified and converted to DC. New generation brushless, cordless drills convert the DC voltage of the battery to AC for driving the motor.
Reducing Costs of Transmitting Electricity Over the Grid
Because AC can so easily be transformed from one voltage to another, it is more advantageous for power transmission over the electricity grid. Generators in power stations output a relatively low voltage, typically 10,000 volts. Transformers can then step this up to a higher voltage, 200,000, 400,000 volts or higher for transmission through the country. A step up transformer, converts the input power to a higher voltage, lower current output. Now this decrease in current is the desired effect for two reasons. Firstly, voltage drop is reduced in the transmission lines because of the lower current flowing through the resistance of cables (since V = IR). Secondly, reducing current reduces power loss as current flows through the resistance of the distribution cables (remember power = I^{2}R in the equations above?). Power is wasted as heat in transmission cables, which obviously isn't wanted. If current is halved, power loss becomes a quarter of what it was previously (because of the squared term in the equation for power), If current is made 10 times smaller, power loss is 1% of what it was, and so on.
3 Phase Voltage
Very long distance transmission lines may use DC to reduce losses, however power is normally distributed nationwide using a 3 phase system. Each phase is a sinusoidal AC voltage and each of the phases is separated by 120 degrees. So in the graph below, phase 1 is a sine wave, phase 2 lags by 120 degrees and phase 3 lags by 240 degrees (or leads by 120 degrees). Only 3 wires are needed to transmit power because it turns out that no current flows in the neutral (for a balanced load). The transformer supplying your home, has 3 phase lines as input and the output is a star source so it provides 3 phase lines plus neutral. In countries such as the UK, homes are fed by one of the phases plus a neutral. In the US, one of the phases is split to provide the two 'hot' legs of the supply.
Why Is 3 Phase Used?
 More power can be transmitted using just 1.5 times the number of wires
 Motors powered by 3 phase are smaller than a similar single phase motor of the same power
 Evening of output torque smooths operation and results in less vibration of motors powered by 3 phase
 Neutral conductor can be reduced in size because of lower current flow
 Neutral is unnecessary for transmitting power between substations and transformers
DeltaStar Transformer
A Deltastar (also known as delta  wye or delta Y) transformer is often used for producing a 3 phase, or single phase and neutral supply to homes and industry. The incoming supply is typically 11kv and output phase voltage is 230 volts (in countries which use this voltage)
How to Measure Voltage, Current and Resistance
As explained above, a multimeter is an instrument for measuring voltage in volts, current in amps and resistance in ohms. Each function usually has several ranges to allow large and small values to be measured. A multimeter has two probe leads which are connected to the circuit being tested, the measurement is then displayed on an LCD display.
Check out this hub: How to use a multimeter
What Are Other Effects When a Current Flows?
As mentioned above, when current flows through the resistance of a load, it gets hot. This is sometimes the desired effect, e.g. an electrical heater. However it is an unwanted effect in lamps, because the desired function of the device is to convert electricity to light, and not produce heat as a byproduct. Excessive current in power cables during an overload can potentially cause a fire if protective devices such as fuses or MCBs (Miniature Circuit Breakers) aren't included in line with the cable.
So what else happens when current flows through a conductor? One effect is that a magnetic field is produced. This phenomenon is used in a device called a solenoid or electromagnet which is basically like a spool or coil of wire through which a current flows. Electromagnets are used in the old style, nonelectronic, door and phone bells, water inlet valves on washing machines, relays (a switch operated by an electromagnet), starter motors on vehicles and in salvage for lifting iron and steel.
Current flowing through a conductor also produces an electric field. An extreme example of this is the high intensity field produced under a high voltage power line which is sufficient to illuminate a fluorescent tube held in the hand.
How Do Switches Work and What Are Sparks?
As you've discovered, if resistance is increased in a circuit, current decreases. If you just break the conductor in a circuit and create an air gap, the magnitude of the resistance for all practical purposes is infinite because air is a good insulator and no current will flow. I.e.
Current = Voltage / Resistance = Voltage / ∞ = 0
So this is how a switch works. Two contacts, usually made of brass in a domestic switch, are pressed together when the switch is on and closed. When the switch is turned off, the contacts rapidly separate and interrupt current.
What are Sparks?
Imagine two electrodes or points in a circuit separated by an air gap (e.g. the gap in an automotive spark plug). If voltage is high enough, the air between the two points becomes so stressed by the electric field that it becomes ionized, i.e. atoms have their electrons ripped off. These electrons are then able to traverse the gap, attracted by the positive electrode and in doing so, collide with other gas molecules and release more electrons. Eventually an avalanche of electrons occurs (all of this happening in a split second) and the result is called a spark or spark discharge A spark produces a flash of visible light, heat, UV radiation and sound and it's temperature can be about 5000 deg C, hotter than the surface of the sun. The voltage required to produce a spark is about 3000 volts per mm between rounded electrodes in air. Sparks can be small, e.g. automotive spark plug or gas lighter, or much larger.
An example of a large spark is lightning. When clouds get charged up, voltage becomes so high that a spark jumps from cloud to cloud or cloud to ground. The sound we call thunder is caused by the explosive heating and expansion of air by the electrical discharge.
Sparks occur in an air gap when voltage exceeds the breakdown voltage of the gap. When two electrodes are separated, current tends to continue to flow and heating of the metal electrodes causes material to vaporise and also ionise the air. This results is a continuous spark discharge called an arc which is similar to a spark. If the electrodes are separated sufficiently, the arc won't be sustained and will stop abruptly. Arc welding makes use of an arc between two electrodes to melt metal. Switches must also be designed so that their contacts separate sufficiently apart and quickly enough so that arcs are rapidly quenched and reduce damage to the contacts. In substations, large air gaps or oil filled circuit breakers are necessary to quench the high current arcs which occur when high voltage is switched.
Arc Between Switch Contacts at a Substation
Summary of Equations for an Electric Circuit
V =
 IR

I =
 V/R

R =
 V/I

P =
 IV

I =
 P/V

V =
 P/I

P =
 V²/R

P =
 I²R

Questions & Answers
Is it volts or amps that can kill?
Volts is a measure of pressure and it's volts that drives a current, measured in amps or milliamps, through your body when you make contact with electrical conductors. Current equals voltage divided by resistance, but the total resistance is influenced by several factors. These include the resistance of the bulk of your body through which current flows, whether your skin is moist or dry, the texture of your skin (i.e smooth or calloused) and the shape of the conductors in contact with skin. Pointed electrodes can penetrate into skin and make better contact and also greater surface area of contact will reduce resistance. If current flows through your chest, it can cause fibrillation of the heart or cardiac arrest. Fibrillation is when the muscle fibres of the heart are all contracting randomly rather than in sync, so they don't pump blood properly.
Is it volts or amps that kills? Volts drives the current, but it's current that kills. A current as low as 30mA can cause ventricular fibrillation, so RCDs (residual current devices) that detect earth leakage faults are designed to trip and shut off power when a current of 30 mA flows (e.g. if you touch a power cable).
I have a 100w bulb, what supply voltage is required for it?
Either of the two common voltages can be used i.e. 230 volts or 120 volts. The wattage doesn't determine the voltage required.
Why is the mains electricity AC?
Electricity is generated at voltages of several thousand volts. For it to be transported via the electrical grid, the voltage is increased to hundreds of thousands of volts. This allows thinner conductors to be used because a current is lower when the voltage is increased. Also, lower current results in lower power losses in cables because a loss is proportional to a current squared. The advantage of AC is that voltage can be easily increased and decreased by the use of transformers. Also, induction motors that use AC can run much more silent than those that use DC (e.g., fridge, freezer, and washing machine motors).
How to convert horsepower to watts?
There are several different types of horsepower but usually, the conversion for mechanical horsepower or imperial horsepower (hp) is 1 hp = 745.7 watts and for metric horsepower, 1 hp = 735 watts.
See this Wikipedia article:
© 2012 Eugene Brennan
Comments
If the supply voltage of a load is doubled (Say from 60 Volts to 120 Volts), what effect does it have on power dissipation?
i need to ask the question in a different way then. if i want to charge a 100kwh supercap battery in 30secs, how do you calculate wire thickness
what wire thickness would you need to dump 100kw in 30secs
Hello sir,
I need help regarding running my 3hp electric motor. In my place the current supply is very low which cannot run my 3hp electric motor properly. I need to buy a stepup transformer to run it very well. I just want to know how much watts of transformer i have to use. Others say 3000 watts transformer is sufficient to run the electric motor for long time. Please help me,
Great really great definitions
Hi sir can you pls help me to solve this problem. Thanks
1_Assignment_PowerFactor @Ric Arnold Serrano Robantes
64. In order to correct the power factor, a threephase, 480 volt, 92 kVAR capacitor bank located 6 feet from the main service of a 3200 square foot office building has a minimum required ampere rating for the conductors in the capacitor bank of which of the following:
a. 240 amps
b. 180 amps
c. 180 amps
d. 110 amps
Hi Sir ,Re. To my previous Question,Yes all 3 Engines are synced.with 440v AC OUTPUT. ALL ENGINE has the same load ,for example around 600kw of all Engine 1,2&3. And 1,&2 showing the same Amps.But for the No.3 is higher than those two Engines.
Thank you for your response. Sir.
Hi Sir, I would like to ask a question of some problem in our AUX. Engine Power Generator,, The Problem is The No.3 Aux. Engine is showing higher Amperes than the other two Aux. Engine. when the load is high..
When i checked The No.3 Engine,there is no combustion happening in one of the Cylinder Liner due to clogged high pressure fuel inlet pipe, but after i fixed the problem the amps is normal, do you think it was the reason why it is giving higher amps than the other two Engines? hoping for your kind response .
Great article but it didn't help me solve one riddle regarding power pricing and metering.... if two countries charge for electricity by the kWh, say 20c per kWh but one country is 110v and the other 220v, are you getting twice the energy for your 20c in the 220v country?
hi sir eugene, how would i know how long my 12v 50A battery can support my 15kWH consumption? Thank you sir!
how to calculate capcitor value from the circuit
Hi sir/Madam, thanks for your presentation.
Hi Eugene,
Good to find this page. Perhaps you could assist with my below questions:
We are having a 65kVA 3phase generator and only single phase appliances are connected to the generator. The generator display (F.G. Wilson Power Wizard 1.1) gives me the following output:
V 412 417 419
A 48 25 19
Now I want to know how much kW the output of the generator approximately is (assuming a power factor of 0.9) to estimate the kWh production.
Would this be
A) (48+25+19) * 0.9 * 415V = 34kW
or
B) (48+25+19) * 0.9 * 230V = 19kW
or
C) another calculation
Any feedback would be much appreciated!
Pictures of the generator display can be found here:
https://drive.google.com/drive/folders/0ByJy69fcPB...
Best, Gijs
Thanks. It is a new appliance that comes from Japan. All their products like many Japanese manufacturers come with polarized 2 pin plugs. My problem is that nothing worth buying is available in UK as they are focused on Europe and Japanese. Korean, Chinese stuff is not main stream. The only Japanese version I can buy in UK is a restaurant version which is total overkill. Far too big and runs on its own 30 Amp circuit. I have tried to find a double isolated transformation but again they all seem aimed at commercial budgets. Does this mean my Windows computers are illegal in the UK? My Mac has a two pin connector on power supply with an extensions lead which has a grounded plug, is that illegal too? And my iphone, ipad, my other cell and my electric razor? I knew the EU would be involved. I thought US equipment grounded through neutral back to the sub station? I guess UK works on different standards as everything is wired with hot at 230 v not two taps of 120 v which are only used on the high current stuff. Long time since I did any electrical stuff, I remember we used 440 volt triple phase which had three taps. Maybe I wait for Britain to leave the EU and hope this is one of the bits that is dropped. Though I am not supplying anything as I am using it myself. Thanks for info, I think I will investigate double isolated transformers more.
I am specifying a step down transformer so I can run a US cooking device on UK power. It has 13 amp glass fuse on 230 v side which is grounded and transformer has 3 pin socket but cooker is 2 pin polarized. 120 volt side has a reset button.
q Specifying transformer am I correct (assuming 110 v 1350 w and a recommendation to use 15 amp 110volt circuit) :110 volt 1350 watts is 12.3 amps the Volt/Amps is 1650vA or 1800 vA if you use 120 volt. So a Transformer Rated 3000 vA on a UK 13 amp 230 volt Circuit will work safely and give enough spare capacity for safety?
q Can I use a GPFI on wall socket or is that unsafe on an autotransformer with an ungrounded appliance?
Thanks
Sir
12v dc input energizer.
What its power (watts)
How can I calculate generator power need ? exp: I consume all equipment in watt 77300 watt and devise 1000 equate 773 Kw so do I need to devise by 3 phase? if the generator is 3 phase?
Thank you for your answer :)
hai i have a question . what is the rating of a motor in horsepower if the current is 20 Ampere and the voltage is 200 Voltage .. Pleasee .Answer this. Pls thankyou
Please calculate me electric watts.
8 energy savers
8 ceiling fans
1 computer
1 lcd tv
1 Water pump
2 pedestal fans
So, if a three phase delta plus N + E incoming line feeds a 3 phase supply and three single phase supplies, to a street distribution system (so that an industry therein connected can obtain 400v 3phase supply, whilst three houses can each obtain a single one phase supply and a fourth house obtains two single (different) phases  what kwh "poaer" is measured on either of it's two kwh meters, if a load is directly connected (NOT ACROSS EITHER PHASE AND N but) across two of the line phases, with out a n or e connection.
In other words obtaining a 400 v load source, using two single 230v line feeds  what power in kwh is measured and which single phase meter measures that power used, or is any power used measured at all?
The reason I ask is very simple.
Is it a possibility to obtain a 400 v supply / load power supply, from two different line voltages, in a two phase supplied building, when there IS no 400v kwh meter installed, just two single phase line/n meters connected.
As the load across the two lines IS NOT directly (or indirectly) connected to either meter's neutral wire.
Thus could either meter even see the 400 v load across the two incoming phases.?
Thank you, Eugene, for your response. I cherish answers, dearly.
The only way I can presume that a simulator can give diametrically opposite results from apparently two different reference frames might be on account of divergence cropping up during its calculations? Namely, two answers which  as far as the simulator can tell  are both valid at the same moment in time?
This assumption, if true, doesn't shock me since divergence is common in many of my simulated experiments of replicating an electrical surge to try and better understand this little understood phenomenon.
This sort of answer coming from a simulator also doesn't shock me from a realistic standpoint since complex/imaginary numbers can also produce multiple results during a lengthy course of calculations. The trick, I think, is to not interfere by throwing out one or the other result without considering the other option that: both results may in fact be true since two reference frames for a singular phenomenon may be created by that phenomenon creating two different references: one for energy and the other for time. Not to imply that time reversal is occurring, but that time and energy are not two different phenomena, but one and the same phenomenon looked at from two different perspectives. That's my take on this, at any rate.
Thanks, again.
Thank you very much.
HI,
CAN PLEASE HELP ME HOW TO CALCULATE SHORT CIRCUIT RATING OF SINGLE PHASE 1.2 HP MOTOR
Hi Eugene, I have a lot of electrical work to do on daily basis. But Im a non technical person. If you just tell me how watts, voltage, am pier and horse power related to each other, it will be a great help to me. I need just simple formulas and calculations. Hope u will reply
very nice
Thanks for all this information you have put here it was really easy to understand. My question is... Suppose i want to build a solar charger for my phone by using a prebuilt voltage regulator which outputs 5v and 2A... And takes 512V input... So since my charge gives me 5x2 =10 watts of power... When building the charger do I need to supply the required current as well? Or I could supply higher voltage and the deficient current will be compensated by the high voltage as the overall power is still the same.. In other words can we compensate for voltage with current or current with voltage? I don't know if that made sense but i hope you get it hahaha
Hey mate, just touching base, I used power meters to measure all my equipment, and from pumping it as loud as I could manage, I got three of the speakers to pull a peak of 800w .. so am guessing that 5A measurement is indeed for the 110V.. or that they are capable of earth shattering sound pressure movement if one were to put say a synthesiser on max volume and hold all the keys...
I have 7 kwt equipment so what amp used in one hour
Mr. Brennan you are an absolute legend and your efforts and knowledge is hugely appreciated.
Cheers!
Thank you! All the spec sheets I've been through haven't been very helpful, the speakers are indeed rated at 1500W output, so the 5A @ 230V does seem like a correct draw? But the 120V/230V 5A figure does confuse me..
edit: the speakers are JBL PRX 635 but I didn't want to cause you extra time and hassle
Hi, thank you very much for this explanation! Well paced and easier to follow than other examples I've tried to digest.
I have a query that someone may be able to help with
I'm trying to work out the max power draw for some powered speakers, the rear says "120V/230V 5A" they are on 230V mode however AFAIK that doesn't mean they will draw twice the power as 120V mode.. I'm assuming they just wrote "5A" because that's the max, and easier than writing "5A FOR 120V, 2.5A FOR 230V" does that sound right?
X watt device take X/1000 unit per hour
In 3 phase system at any instant sum of voltage is zero know and current also is zero so power will be zero know... But it isn't true.so what is the write answer
measuring P V I
P=50w , I=0.69A , V=222v
P'=VI=0.69 x 222=153w
why P'greater P explain
thank you i wait you
thanks for the education.
i am from Liberia West Africa, the cost of unit of electricity is 0.55c/kwh
i have notice over the time that my unit run faster with a low voltage so i have come to suspect some things which i want to be correct about.
my mini freezer voltage is 220240v and in recent time the voltage from the transformer to the house has drop to 160v Yet the freezer still come on. but i am noticing that it take a longer time to freezed and the unit run faster than when the transformer was not sending this low volt.
1. so than my question is: does the unit run out equally with a low volt the same way it run out with a normal or higher volt?
2. if at a 220v my frezer take 2 hrs to freezez water to 0 degree assuming that the consumption of the freezer is 1kwper hour.
will it take the same 2hours to freeze to 0 degree at 160v?
3. if it take 4 hour at 160 volt to freeze to 0 degree; will the cost be the same as when the voltage was 220?
i am trying to make a comparison with the water the author make mention of.
if the water pressure is high, it take a shorter time to fill a five gallon from the pump.
when the water pressure is low, it take a longer time to fill a five gallon but what is clear is that only five gallons of water the the meter will read and and if the cost is 10c per gallon the five gallon will cost 50c: in that case the time it take to fill the gallon has nothing with the cost.
i understand this water issue but need some clarity with the low voltage issue. thanks
16. The unit for measuring electric power is the
A. ampere.
B. watt.
C. volt.
D. ohm.
I'm purchasing an RV and trying to sort out all of the electrical basics. Your information answers my questions about electricity and is easy to understand and presented in a logical order. Thanks so much!
what i need to know is, how much more electicity am i using by having 290 feet of #12 copper wire on a 1.5 hp 230 volts motor? and am I getting a true reading from an installed meter at only 90 feet from the motor, of all of the electricity been total used?
hello i have a question how much current does a 110v plug in cost a month for a ADT alaram system
If I have just EB consumption reading suppose 4000 unit a month ,permission load 12 kW load in ampear 23 amps
Now how can find the EB availability in a day
...
I have speaker of 100V/70W/5060Hz.
I have portable battery 300A with DC Accessory Outlet of 12VDC,5A and USB Outlet of 5VDC,2A.
Thought to ask you if I can play speaker using this battery.
I've purchased a Front 36V 800W Electric Bicycle Hub Motor Brushless Conversion Kit 26" and 36v 20ah LiFePO4 Battery 5A Charger BMS E Bike Rechargeable Powerful USE 800W to converse my regular bike to an electric one. But now I learn that the 800W is not acceptable in the electric bike laws in my city. It's only 500W or less. I have a battery that is 36V 10Ah 350W at home. Can I use this battery for the bicycle hub motor mentioned above? Would like your answer please. Thank you very much.
SIMPLE questions. Is 110V single phase electric a sine wave that oscillates between +55V, 55V; or +110V, 0 volts? Also, I read that at a 90 degree phase angle difference between current and voltage the two would cancel each other out. I thought this happened at 180 degree intervals, where the VOLTAGE is a mirror of the current, thereby cancelling it and resulting in O power. What am I not seeing? You don't have to oversimplify the answer. I am just thinking in a unit circle that at 0 degrees sin=0, 90 degrees sin=1, 180 degrees sin=0, 270 degrees sin=1 and at 360 degrees sine =0 again. Two waves 180 degrees out of phase mirror each other and so cancel (or am I mistaken), just as the relationship of sin to its mirror happens every 180 degrees (0,180; 90,270; 180,360; etc.). I also learned that watts=voltsXamps. What would happen if your circuit theorhetically had 0 ohms resistance at 110V (besides tripping the breaker because of a dead short, if there was no limiter on it)? Does a common household 110V circuit have a nominal resistance of 5.5 Ohms? Is a 110V circuit +/ 55V, +110V,0V or another set of values? Is 220V really 2110V legs out of phase with each other, pulsing the power to twice as many end points, effectively increasing the frequency to a nominal 120Hz, each leg serving as the other's return or ground, and if so, what is the phase angle relationship (e.g. 180 (my thought), or 90))? If 220 represents 2 different phases, why is 220 in the U.S. called single phase? In Europe my understanding is that 240 IS single phase but in the U.S. the two 110 legs alternate being each other's grounds or returns so no ground/neutral connection is required. Finally, does either amperage or voltage have a greater effect on field or is a joint relationship where as voltage decreases amperage increases, to deliver the same wattage and that is what is measured by an electric meter? If current had a greater effect then the meter would spin faster at lower voltages to deliver the same wattage. Does a 220V motor driven appliance that will operate down to 197V slow down and deliver less power or does it need more amps and use the same amount of watts? I should remember these things from physics E/M but that was over 40 years ago when I was 16 or 17. Sorry to ask the dumb questions! Thank you!! I am a minor geek and electricity has always fascinated me. Ed S.
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