The electric company bills not for power but for energy, using units of kilowatt-hours.

1 kW-h = 3.6 x 10⁶ J

One kW-h typically costs about 10 cents, which is really quite cheap. It does add up, though. The following equation gives the total cost of operating something electrical:

Cost = (Power rating in kW) x (number of hours it's running) x (cost per kW-h)

Example

The average household in the U.S. has a television on for about 3 hours every day. About how much does this cost every day?

1. 0.2 cents
2. 2 cents
3. 20 cents
4. $2

A typical TV is rated at around 75 W. The daily cost is therefore:

Cost = 0.075 kW x 3 hours x $0.1/kW-h = $0.0225, or about 2 cents.

Try this at home - figure out the monthly cost of using a particular appliance you use every day. Possibilities include hair dryers, microwaves, TV's, etc. The power rating of an appliance is usually written on the back or the base, and if it doesn't give the power it should give the current. Anything you plug into a wall socket runs at 120 V, so if you know that and the current you can figure out how much power it uses.

Power from a wall socket is relatively cheap, particularly when compared to battery power. $100 per kW-h, a thousand times more than what it costs for AC power from the wall socket, is a typical value for a battery.