Question about energy usage of electronics

[GreyianStorm]

I just remembered what the device in question was: a kettle.

I saw a kettle in a local shop, and on the box they had 3000W in big writing (does that really sell kettles to anyone?) and was curious about what exactly it meant. I assumed it meant that, if the kettle was boiling water for 1 hour straight (assuming it was working at full capacity the entire time, which wouldn't actually happen in the real world), it would use 3000Wh, or 3KWh, so 60c to run for 1 hour (in the 20c example given above).

I'm aware that my 100% load example isn't realistic, I just thought it work make it easier to get an answer (although, I suppose so long as I had chosen any simple enough percentage, 50%, 25% etc, it would have been pretty simple still).

It's nice to know for sure though, how this stuff works. I always assumed it measured the hourly usage (at 100% load), but it's nice to have it confirmed. A friend of mine is looking into building his own computer and has been asking what kind of power costs to expect, so it's nice to be able to give him an absolute maximum (PSU Wattage * 24) for daily usage costs, and then obviously explain that it won't be running at 100% load and (most likely) won't even be turned on 24 hours per day.

Thanks for your help!