Duty
05/11/2004 07:18 a.m.

The question of how much energy it takes to operate certain household items came up in the forum. It turns out you can’t just look at the UL label next to the cord running out of the back to get your wattage. Those are generally for peak conditions only, and you have to take “duty cycle” into account as well.

What is duty cycle? It’s the fraction of the time that an appliance is actually drawing power. This may not be the same as “operating time”. T

hink about a window air conditioner, for example. You might have a 6,000 BTU unit that is rated at 700 Watts. Tthat’s means it pulls 700 Watts to power its compressor and its fan and its on-board electronics. What happens when you come into your 80 degree bedroom in the evening after work and set the thermostat to 70 degrees? First thing that happens is that the compressor starts up, followed quickly by the fan. The heat exchanger starts to cool down on the room side and the fan blows room air across it and back into your room. At first the unit keeps the compressor on full time, but as the air in the room cools down, heat transfer isn’t as good, and the unit drops the compressor speed to 50% so that the coils don’t ice over. The fan is still going full blast, but now you’re only drawing 400 watts. And 15 minutes later, when the room hits 70 (You did remember to close your bedroom door, didn’t you?) the unit turns off. But slowly the room heats up, both from internal heat sources like the TV, the lights, and your metabolism; and from outside the room, such as through the glass window, through the crack under the door, and through the poorly insulated ceiling from the oven you call your attic. The unit clicks back on when the temp hits 72, and off again when it manages to get it down to 68. For the two hours you are in your room the fan is going 50% of the time and the compressor is at full for 30 minutes and half power for half an hour. So overall, for the two hours you were running your “700 Watt AC unit” you only averaged 225 W. That’s duty cycle. (Note that the less insulation you have, the more the unit will run. The longer it’s been since you cleaned the filter, the more the compressor is going to have to run. The more lights you have on in the room, etc.)

So! How much do appliances really draw on average?

How longer per day do you use these items?

lights	12 hours
computer	8 hours
oven	2 hours
vacuum	1 hour (riiiiight)
AC	??

And there's the problem... you turn on your AC and leave it on, you leave on your lights, you leave on your computer, etc. Go ahead, use your AC, but close the vents in rooms where you aren't, stuff a towel under the door and draw your curtains. And insulate that attic!