Volts, Watts, Amps, Kilowatt Hours, What Does it All mean ?

I wrote this hub in response to questions which often crop up on Webanswers.com about watts, amps, volts, kwh, ohms and other electrical quantities. It explains what they mean and how to convert between them. The hub also shows you how to work out the power consumption and cost of running appliances.

Reference is often made to hoses or water pipes as an analogy to explain electrical circuitry.

Looking at the circuit on the right, a voltage V will force a current I around the circuit through the resistance R

Voltage This is the pressure in a circuit and is measured in volts. Think of a pressure washer which has high water pressure. The higher the voltage applied to a circuit, the greater the current which will be forced around the circuit.

Resistance This is the restriction in the circuit and is measured in ohms. Every electrical device has resistance, bulbs, heaters, motors etc. 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.

Current This is the flow of electrons 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.

Power This is the rate at which energy is consumed and is measured in watts

kWh This is a measure of energy consumption. Kwh are what you pay for on your electricity bill

We will consider Ohms law later, but first let's examine the quantities of normal interest when dealing with appliances, such as volts, amps and watts and how to convert between them. Watts or amps are often written on the specification of an appliance so it is handy to convert between quantities.

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.

Now another equation for working out the power consumption of an appliance.

kWh = Watts / 1000 x time in hours

Kilowatt hours also known as units or kWh are what you pay for.

e.g. A 2500 watt drier runs for 3 hours a day, how many kWh does it consume and if electricity costs 18c per unit, what is the cost to run it?

kWh = watts/1000 x time = 2500 / 1000 x 3 = 7.5 kWh or units

Cost = 7.5 x 18c = $1.35

Now back to Ohms law.

In the circuit above the voltage pushes the current around the circuit and through the load whose resistance is R measured in ohms. If the voltage is V and the current is I then:

Current = Voltage / Resistance or

I = V / R

This is Ohms 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 visa 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

Now some more equations:

I = V / R

V = I x R

R = V / I

If P is the power then:

P = V²/ R

P = I² x 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 = (240)² / 100 = 576 watts