Electric power

The hair dryer says 2000 W, the charger says 20 W, and at the end of the year an electricity bill arrives in kWh. What do these numbers mean, and why is the hair dryer a hundred times hungrier than the charger? With a single formula you can work all of this out yourself and even unmask the real power guzzlers in your home.

Power: P = U · I

Electric power tells you how much energy a device converts per second. It comes from voltage times current: P = U · I. Voltage U is measured in volts (V), current I in amperes (A), power P in watts (W). A lamp on 230 V with 0.1 A flowing through it therefore has 23 W. You can also rearrange the formula: if you know power and voltage, then I = P divided by U.

Energy: power times time

For the electricity bill it's not just how powerful a device is that counts, but also how long it runs. Energy = power times time. The practical unit for this is the kilowatt hour (kWh): 1 kWh is what a 1000 W device uses when it runs for one hour. A kilowatt hour costs roughly 30 to 40 cents, and it achieves a surprising amount: one load of laundry, about 7 hours of television, or around 50 phone charges. The 2000 W hair dryer burns through it in just half an hour.

Power guzzlers and standby

The biggest power guzzlers in a household are devices that produce heat or run for a long time: the stove, hair dryer, kettle, fridge, tumble dryer. The quiet round the clock draw is often underestimated: an older TV easily pulls 10 W in standby mode, all day and all year. New devices in the EU may only draw 0.5 W in simple standby, but routers, set top boxes, and old appliances together still add up to 100 kWh per year. A power strip with an off switch solves the problem with one click.