home, environment

energy conservation and single appliance electric meters

overview

Here in Northern California, we have some very high electric rates, which naturally gives rise to the question of what in the house is driving up the electric bill. If you go to most energy conservation web sites, they'll give you general pointers like better insulation, changing to flourescent lights, and so on. Those tips will help, but they don't solve the problem of finding the power hog in your house or apartment. Is it the fridge? The computer? The dryer? To answer that question, you need some way to measure the power that different suspects in the house are consuming until you can find the culprit.

Fortunately, such devices exist. They're known by various names (which can make them hard to track down), so I'm just going to call them Single Appliance Electric Meters. The idea is simple: you plug the meter into the wall, plug the appliance into the meter, and it tells you how many watts the appliance is using right now and, over time, how many kilowatt hours it's used.

The more enlightened power co-ops lend these meters to their users for free. Unfortunately, our local power company does not have them available. Fortunately, Radio Shack does. Radio Shack catalog number 63-1152 is a device called the Kill A Watttm, manufactured by a company called P3 International. As of May 2003, they sell for $29.99. They can be hard to find (they don't seem to be in the online catalog, at least not any more) but your local Shack should be able to track one down for you.

measurements

If you can't get your hands on one of these gizmos, there's always second best: this web page collects measurements that folks have made. You may be able to use them as a rough guide to finding the power hog in your house or apartment. The power numbers are in watts. The kilowatt hours number is how many kilowatt hours that appliance would add to your monthly electric bill if you left it on, 24 hours per day, for the whole month. You can figure it out by multiplying the number of watts by 0.72.[1]

item
 watts
 kilowatt hours
 measured by
Wirespeed DSL modem, Netscreen 5, Netgear FS108, Hewlett Packard Deskjet 812C (in standby mode)
 26
 19
 Jeff
1.8GHz Pentium IV computer with 2 disk drives, 400W PC Power and Cooling power supply
 69
 50
 Jeff
AcerView F51 15" LCD monitor, displaying image
 29
 21 Jeff
21 inch Nokia 445XiPlus monitor, displaying image
 98 71
 Arlen
21 inch Nokia 445XiPlus monitor, command sleep
 2
 1
 Arlen
21 inch Nokia 445XiPlus monitor, timeout sleep
 86
 62
 Arlen
Mac G4 933MHZ desktop computer with two hard drives, running
 94
 68
 Arlen
Mac G4 933MHZ desktop computer with two hard drives, command sleep
 7
 5
 Arlen
Mac G4 933MHZ desktop computer with two hard drives, timeout sleep
 80
 58
 Arlen

Sometimes it makes more sense to look at the total amount of energy to do something, like boiling a cup of water. Here are some measurements of total energy use.

item
 kilowatt hours
 measured by
Philips 10 Cup Water Kettle (hot pot), boiling 1 cup water
 0.03
 Arlen
G.E. Microwave Oven, boiling 1 cup water
 0.05
 Arlen
Philips Water Kettle, boiling 1 qt water
 0.10
 Arlen
G.E. Microwave Oven, boiling 1 qt water
 0.23
 Arlen

links

footnotes

[1] One watt for one month is (1 watt) * (30 days) * (86,400 seconds/day) = 2,592,000 watt-seconds. One kilowatt-hour is (1000 watts/kilowatt) * (3600 seconds/hour) = 3,600,000 watt-seconds. So one watt for one month is (2,592,000 watt-seconds) * (1 kilowatt / 3,600,000 watt-seconds) = 0.72 kilowatt-hours.