A watt is a measure of rate of energy usage.

It is equal to one “joule” per second. A joule (J) is a quantity of energy whose definition escapes me at the moment.

Thus, my hair dryer, which says 1400 watts, uses electricity at a rate of 1400 joules per second.

Total *quantity* of energy is something different. This is where the joule comes in, and where we can start to answer such questions as, “How much energy does my toaster use to toast two slices of bread?”.

If the toaster used energy at a rate of 500 watts for two minutes, it would be 500 W x 120 s = 60 000 joules!

Most North Americans are not used to seeing their energy measured in joules; watt-hours are more common.

But a watt-hour is simply the *total amount* of energy you use when you use energy at a rate of 1 joule per second for 1 hour.

This equals 1 J/s x 1 h, or 1 J/s x 3600 s, or… 3600 joules!

So the toaster would have used 60 000 joules / 3600 joules/watt-hour = 16.67 watt-hours of electricity. Boris B, you’re right on the mark.

So, tomndebb, a 100-W bulb uses energy at a rate of 100 watts, or 100 joules per second. In one hour, it uses a total of 1 watt-hour, or 3600 joules, of energy.

With a single 100-W bulb, after ten hours, you would still be using energy at the same rate of 100 watts, but you would have used 10 watt-hours, or 36 000 joules, of energy.

It would take *ten* bulbs to use energy at the rate of 1000 watts, and after just one hour, they would have used 10 watt-hours of energy.

But you’re right–anything that uses electricity to heat (dryer, toaster, etc) will take a *lot* of power. Some friends of mine had to move out of an electrically-heated house; they literally could not afford the bills (~$600 per MONTH in the winter).

Those of us who are planning a solar-powered house have to be very aware of this kind of thing…