This page is a brief introduction to the important electrical units – volts, amps, watts, amp-hours and watt-hours – for those who aren’t quite sure what they mean. If it isn’t complete enough, How Stuff Works explains it all very well and at greater length.
Basic units
If we describe electrical current in terms of water in a hosepipe, the water pressure would be measured in Volts and the amount of water flowing through the hose would be measured in Amps.
- Multiplying Volts by Amps gives us a measure of power (how much work the flowing electricity can do) in Watts.
- Multiplying Watts by time gives us a measure of how much electricity has been used or stored. (This figure is really important to discussions of battery usage.)
- In equations (it’s okay, they are very simple)
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- Volts x Amps = Watts (power)
- Watts x hours = Watt-hours (battery capacity, or power used in a certain time)
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Volts, Amps and Watts in real life
- Most small devices (desk light, mobile phone) use voltages between 1.5 Volts (torch battery) and 12V (car battery) and currents much less than 1 Amp. Typical power consumption is less than 5 Watts.
- Mains power is 240V and large appliances (aircon, washing machine, stove-top) can use up to 15 Amps and consume 1000 – 3600W of power.
More units
The prefixes milli and kilo are used for electrical units just as they are for length (metres) and weight (grams), so we use:
- Volts V and, rarely, milliVolts mV (computer circuitry) or kilovolts kV (high voltage transmission lines)
- Amps A and often milliAmps mA (mobile phones)
- Watts W and often milliWatts mW (mobile phones) or kiloWatts kW (household electricity usage), rarely MegaWatts MW (whole cities).
The most useful measure of battery capacity is Watt-hours, Wh. If we return to our hosepipe and think of it filling or emptying a tub, Watt-hours are the litres of water in the tub.
Battery capacity is also often quoted in Amp-hours, Ah, and the capacity of very small batteries (e.g. AA cells) may be quoted in milliAmp hours, mAh. Amp-hour figures are fine if we are comparing batteries of equal voltage but can be tricky if we’re not. For instance, a 100Ah battery will always last twice as long as a 50Ah battery powering the same light, but a 48V 2Ah battery stores four times as much power as a 12V 2Ah battery.
To convert Ah to Wh, recall that W = V x A and simply multiply the Ah by the battery voltage.