Before we see how much electricity costs, we have to understand how it's measured. When you buy gas they charge you by the gallon. When you buy electricity they charge you by the kilowatt-hour (kWh). When you use 1000 watts for 1 hour, that's a kilowatt-hour.

For example:

Device wattage	Device	Hours used	kWh
medium window-unit AC	1000 watts	one hour	1 kWh
large window-unit AC	1500 watts	one hour	1.5 kWh
small window-unit AC	500 watts	one hour	0.5 kWh
42" ceiling fan on low speed	24 watts	ten hours	0.24 kWh
light bulb	100 watts	730 hours (i.e., all month)	73 kWh
CFL light bulb	25 watts	730 hours	18 kWh

When the number is low we sometimes use watt-hours (Wh) instead of kWh. For example, we might say 240 watt-hours instead of 0.24 kWh.

 

Watts is the measure of the rate of electrical use at any moment. For example, a laptop computer uses about 50 watts.

If your device lists amps instead of watts, then just multiply the amps times the voltage to get the watts. For example:

2.5 amps   x   120 volts   =   300 watts

(If you're outside North America, your country probably uses 220 to 240 volts instead of 120.)

Understand the difference between watts and watt-hours:

• Watts is the rate of use at this instant.
• Watt-hours is the total energy used over time.

Here's a question I frequently get, which makes no sense:

"You say that some device uses 100 watts. What period of time is that for?"

It's not for any period of time, because watts is a rate at that instant. One might as well ask:

"The speedometer in my car says I'm going 35 miles an hour. What period of time is that for?"

It's not for any period of time. You're going 35 miles an hour at that instant.

To measure use over a period of time, we use watt-hours, not watts. The way it works is, watts or kilowatts for the amount at a given instant, and watt-hours or kilowatt-hours for the amount over a period of time.