Watts, Amps, Volts, Kilowatt Hours (kWh) and Electrical Appliances - Basic Electricity Explained

Volts, Watts, Amps for Dummies!


What are Volts, Amps and Watts?

"Voltage is a measure of pressure in an electrical circuit, amps is a measure of the current flowing and watts is a measurement of power or the rate at which energy is used."

In this article you'll learn all about volts, watts, amps, ohms, current, power and resistance. The equations are really quite simple and you'll find some examples on how to apply them to home appliances.

How Much Power Do Domestic Appliances Use?

If you just want to know the power rating of, and how much it costs to run common 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?

What's the Difference Between Volts, Watts and Amps? - A Simple Circuit

Reference is often made to pumps and water pipes as an analogy to explain electrical circuitry.

Looking at the simple circuit diagram or schematic below, a voltage source V will force a current I around the circuit through the load 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 or a 12 volt car battery, 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.

Current in a circuit
Current in a circuit | Source

Definitions of Volts, Amps and Watts

Like any discipline, electrical engineering has jargon or specialized terminology.

Some Definitions:

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 This is the flow of electrons through the circuit 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 magnitude of the load, 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.

What are Some Commonly Used Voltages ?

Voltage Source
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)

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, 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!

What is a Digital Multimeter?

A multimeter is an instrument which can measure voltage, current, resistance and possibly additional parameters. If you don't know how to use one, read 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

These multimeters are useful for measuring voltage, current, fuses and continuity of wires and connections. If you want a quality meter, Fluke, a US instrument manufacturer, specifically recommend the 113 model for general purpose home or car maintenance.

Fluke recommend the 113 meter for general purpose home use
Fluke recommend the 113 meter for general purpose home use | Source

Information Labels/ Specifications on Electrical Appliances

Typical electrical appliance labels/panels
Typical electrical appliance labels/panels | Source

What is a kWh? - Calculating Power and Energy Use 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 work 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 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

This article gives a comprehensive list of domestic appliances and their energy use: What is the Cost of Running Electrical Appliances?

A kilowatt hour meter counts the number of units of energy you've used
A kilowatt hour meter counts the number of units of energy you've used | Source

How to Convert Horsepower to Watts

Horsepower is a measure 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.

Power consumption monitoring adapter
Power consumption monitoring adapter | Source

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 uses energy at a higher rate than a 60 watt light bulb.

Electricity Can be Converted to Other Forms of Energy

Energy Type
Incandescent light bulb, LED, Fluorescent lamp
Electric heater, Incandescent light bulb
Electromagnetic radiation
Radio transmitter, Microwave oven, Radar
Loudspeaker, Thunder
Motor spinning a shaft or driving a vehicle
Winch or lift raising a load, Electromagnet tensioning a spring
Air compressor

Ohms's Law and Electrical Resistance

In the circuit above, voltage pushes current around the circuit and through the load whose resistance is R measured in ohms.

Current = Voltage / Resistance


Resistance = Voltage / Current

This is Ohm's law and basically says that the current is proportional to the voltage and inversely proportional to the resistance (As the resistance increases, the 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.

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 or battery acid or an 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 of a conductor 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 gage 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).


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.

Superconducting cables
Superconducting cables | Source

Alternative Way of Working Out Power

Remember watts = volts x amps? Another way to work out power is from the resistance in ohms:

Current is normally represented by the variable "I"

I = V / R from Ohm's law


If P is the power, then 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 = (240)2 / 100 = 576 watts

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.

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 modeled 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.

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.
AC sources include generators in power stations, transformers, DC to AC inverters, 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 before driving the motor.

AC waveform is a sine wave
AC waveform is a sine wave | Source
Transformer in an electrical sub-station
Transformer in an electrical sub-station | Source

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

Magnetic field lines around a conductor
Magnetic field lines around a conductor | Source

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 electric field under a high voltage power line is sufficient to produce an electric discharge in a fluorescent tube
The electric field under a high voltage power line is sufficient to produce an electric discharge in a fluorescent tube | Source

Summary of Equations For an Electric Circuit

V =
I =
R =
P =
I =
V =
P =
P =

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Comments 18 comments

prince uchenna 4 years ago

It is too good keep it up.

adil 2 years ago

Thanks man it's awesome.

JCH 24 months ago

Great article - keep it up!

saumya ranjan jena 17 months ago

thanks man, it is interesting keep it up_ 3 months ago

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 2-110V 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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eugbug 3 months ago from Ireland Author

Wow Ed, what a lot of questions, and by the way there are no dumb questions, just questions!

Q: Is 110V single phase electric a sine wave that oscillates between +55V, -55V; or +110V, -0 volts?

A: No, the peak voltage for a sinusoidal waveform is √2 times the RMS voltage. The RMS voltage in your example is 110 so the peak is √2 x 110 = 156 approx. So voltage ranges between -156 to +156

Q: I read that at a 90 degree phase angle difference between current and voltage the two would cancel each other out?

A: For a purely resistive load, voltage and current are in phase. For a purely capacitive or inductive load, voltage and current will be 90 degrees out of phase. Power dissipated is VICos(ɸ) where ɸ is the phase angle between current and voltage and power is zero. If two voltages are 180 degrees out of phase (as is the case with the two hot legs of a US supply), the voltages don't cancel each other out, in fact the voltage is doubled (which is where the 220/240 volt supply is derived from).

Q: What would happen if your circuit theoretically had 0 ohms resistance at 110V

A: In theory current flowing would be infinite (but infinity isn't actually a number!). In practice current flow in a real circuit would be limited by the resistance of the circuit cables but could potentially be thousands or tens of thousands of amps for a split second. This is why fuses should never be replaced by glass types which don't have a high rupturing capacity. Ceramic types must be used.

Q: Does a common household 110V circuit have a nominal resistance of 5.5 Ohms?

A: Well it depends on the length of the circuit cables and their gauge.

Q: Is 220V really 2-110V legs out of phase with each other, pulsing the power to twice as many end points.

A: One hot is 110 volts wrt neutral. The other hot is also 110 volts wrt neutral but 180 degrees out of phase (a diagram would be nice but think of the sine wave flipped on its head). So the 220 volt supply is derived from the difference between the two voltages. It's basically like putting 2 cells in series. Think of the point where they join as neutral and the total battery voltage is double the individual cell voltages. The supply transformer in the street is centre tapped and the centre tap is the neutral. The frequency never changes.

Q: 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?

A: No ground is needed for the 220 volt supply. You just use the voltage between the two hot legs.

Q: Why is 220 in the U.S. called single phase?

A: Not sure if it's called single phase, but there is only a single phase supply between the two hot legs. The two legs can be thought of as split phase. This differentiates it from a 3 phase supply which has 3 wires 120 degrees out of phase with each other. No neutral is used for distribution of power between transformers (delta system), but a neutral connection is created at the secondary of the supply transformer for supplying homes (star). This is the way it is here (Ireland). I'm not totally au fait with the setup in the US and how a transformer secondary can supply a three phase supply and also two hot legs. Maybe one of the phases is centre tapped?

Q: 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?

A: When voltage decreases, current decreases and wattage decreases (and visa versa). The meter measures both current and voltage and the product is what determines the speed of the disk in the older style meters. So if the supply voltage to your home is low, the meter runs slower and you're not being cheated!

Q: 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?

A: Universal motors, (the noisy ones with the brushes used in vacuum cleaners, power drills etc) are voltage dependant and will slow down and use less power when voltage drops. AC induction motors (the silent ones in fridges, freezers, washing machines) are less sensitive to variations in voltage. The speed of these motors is controlled by varying the frequency of the supply.

Kyle High 2 months ago

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.

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eugbug 2 months ago from Ireland Author

Brushless motors are like an AC induction motor and use an electronic commutation system to spin the magnetic field of the stator.


Your other battery is the same voltage but lower capacity (10 AH), so it would provide the same power to the motor but run out quicker. Because it has a lower capacity, it may also be less capable of supplying the current and power requirements of the motor and could possibly overheat.

Unlike controlling a DC motor, you can't simply reduce input voltage to reduce speed/torque/power because of the intervening electronic control of the motor. Maybe if you contact the supplier they may advise you on whether there is a setting in the motor which limits power. However it seems you may have to change the motor.

Kyle High 2 months ago

Thank you so much, eugbug. You've done a great job to help many people to understand more about electricity.

Amit 2 months ago

I have speaker of 100V/70W/50-60Hz.

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.

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eugbug 2 months ago from Ireland Author

Hi Amit, is 300A the model of the battery? I'm not familiar with audio/speaker systems so I can't really advise, but if 50-60 Hz is quoted on the speaker, it sounds as if it is an active speaker requiring a mains supply. Your battery would then need an inverter to drive the speaker. However maybe the speaker has a 12 volt power input? The speaker has a 70W output so the power input requirement from the supply would be greater than this. You would be pushing it a bit with only 12 x 5 = 60 watt maximum available from the accessory o/p of the battery, if the speaker was driven to its maximum level.

What's the make and model of the speaker?

maynal 5 weeks ago

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


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eugbug 5 weeks ago from Ireland Author

Hi Maynal, I don't understand your question, can you rephrase it?

A constant 12kw load for a day is equivalent to 12 x 24 = 288 units.

In one month (eg 31 days), total number of units is 288 x 31 = 8921 units.

If you work the other way, 4000 units for a month is on average 4000/31 = 129 units per day or 129 /24 = 5.37 kw average load

Bebe 4 weeks ago

hello i have a question how much current does a 110v plug in cost a month for a ADT alaram system

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eugbug 4 weeks ago from Ireland Author

If the alarm is powered by a plug in power adaptor, you need to know the wattage/VA rating which should be printed on the adaptor. This would be the max rating of the adaptor though, and the alarm could be taking less power than this. Alternatively if you could find out the model number of the alarm, the specification would indicate the current drawn by the electronics.

I doubt whether the alarm consumes a lot of energy. So for instance if it uses 10 watts, then:

Units used in a day would be 10/1000 x 24 = 0.24 units

In a month units used = 0.24 x 31 = 7.4

If a unit costs 10 cent. Then cost = 7.4 x 10 = 74 cent

Arthur 4 weeks ago

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?

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eugbug 4 weeks ago from Ireland Author

Hi Arthur,

12 gage wire has a resistance of 0.001588 ohms/foot.

For a 290 foot run, total length of current path is 2 x 290 = 580 feet.

So resistance of the power line = 580 x 0.001588 = 0.919 ohms

A 1.5 H motor uses 1.5 x 746 = 1119 watts of power

So on full load it draws 1119/230 = 4.87 amps

If a motor was a linear device like a resistor, its resistance could be worked out from the wattage and voltage equation

So from equation P = V²/R then R = V²/P

Then you could just add the two resistances and total power would be less and = V² / (Rcable + Rmotor)

However the current drawn by an induction motor increases as voltage decreases for a fixed mechanical load. This is proportional so e.g. a 5% decrease in voltage causes a 5% increase in current. It would really be necessary to draw out a load line and voltage / current characteristic for the motor to calculate the voltage actually reaching the motor terminals and the current input. There are probably tables for working out on the Net, but couldn't find anything specific to motors. However the power drawn by your motor would stay the same.

The current in the supply cable is 4.87 amps, power dissipation is I²R = 4.87² x 0.919 ohms = 22 watts approx.

Your meter at 90 feet from the motor wouldn't register the power dissipated in the 200 feet upstream of the meter to the power source. So it would under-read by about 200/290 x 22 = 15 watts approx.

Voltage drop should be IR = 4.87 x 0.919 = 4.47 or 2% of supply

However because the motor draws more current because of the drop in voltage, current would end up being greater than this.

The only thing I would be concerned about is whether the voltage drop could cause difficulties on motor startup. Startup torque depends on voltage squared so for instance if voltage drops by 10 %, voltage is now 0.9 times what it was before and torque is 0.9 x 0.9 =0.81 times what it was. Also as I mentioned above, reduction in voltage causes an increase in current, which compounded with a heavy load could push the current drawn by the motor above its rated value. You could use a current clamp to measure this to make sure it's within limits.

This is an interesting discussion on the topic:

and also:

Try the EEWeb forum aswell:

It's over 30 years since I studied all this sort of stuff and I haven't actually worked specifically in the field of electrical engineering so I can't guarantee the calculations above are 100% correct. However the power un-measured by the meter (about 15 watts) is definitely much lower than the 1119 watts used by the motor.

Hope this helps!

Janet 2 weeks ago

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!

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    Eugene Brennan (eugbug)311 Followers
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    Eugene is a qualified control/instrumentation engineer Bsc(Eng) and has worked as a developer of electronics and software for SCADA systems

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