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.
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Volts, Watts, Amps, Ohms Explained
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.
Electrons Surrounding the Nucleus of an Atom
Electron Flow in a Conductor
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.
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.
This is the device connected to a voltage source. It could be a motor, bulb, heater, LED, or an electronic resistor.
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.
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.
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.
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 sub-atomic particles called electrons which are negatively charged, so current actually flows the other way, from the negative terminal of the battery
A Simple Circuit
Schematic of a Simple Circuit
What are Some Commonly Used Voltages ?
AA or AAA cell
Mains supply in the home
Nominally 120 or 240 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.
Supply Voltages of the World
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.
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?
A Fluke non-contact detector "VoltStick" is a standard tool in any electricians tool kit, but useful for homeowners also. I use one of these for identifying which conductor is live whenever I'm doing any home maintenance. Unlike a neon screwdriver (phase tester), you can use one of these in situations when live parts/wires are shrouded or covered with insulation and you can't make contact with wires. It also comes in useful for checking whether there's a break in a power flex and where the break occurs.
Note: It's always a good idea to use a neon tester to double check that power is definitely off when doing any electrical maintenance.
Information Labels/ Specifications on Electrical Appliances
Test Yourself! - Quiz Aview quiz statistics
What 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 milli-seconds.
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.
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 model for general purpose use in the home or for car maintenance. This is an excellent meter and can measure AC and DC volts, current, resistance, check continuity and diodes. The meter is auto-ranging, so ranges don't have to be set. It is also a true-RMS meter. Fluke 113
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 Bview quiz statistics
What 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
Incandescent light bulb, LED, Fluorescent lamp
Electric heater, Incandescent light bulb
Radio transmitter, Microwave oven, Radar
Motor spinning a shaft or driving a vehicle
Winch or lift raising a load, Electromagnet tensioning a spring
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.
I = V / R
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.
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 cross-sectional 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.
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)
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
Nichrome (used in heating elements)
1.00×1011 to 1.00×1015 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,
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) = V2/ R
P = VI =(IR)I = I2R
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 = V2/ R = (240)2 / 100 = 576 watts
Test Yourself! - Quiz Cview quiz statistics
What is 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.
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.
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 = I2R 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
Delta - Star Transformer
A Delta - star (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, non-electronic, 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.
The Effects of High Current
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
© 2012 Eugene Brennan