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Measuring Electricity Use

Posted May 04, 2009 04:06 PM by Alex Wilson
Related Categories: Energy Solutions
The Kill-A-Watt meter by P3 International Corporation.

I get a lot of questions about energy. Electricity consumption factors into many of them. Why are electric bills so high? How can I tell when it's time to replace a refrigerator? Most of us have electric meters on our houses, but these measure your total household electricity use. To figure out what accounts for that overall figure, you need to measure the consumption of individual appliances and pieces of equipment. A really useful gadget for figuring out these sorts of questions is an electricity monitor.

You plug one of these into a wall outlet, and then plug the device whose electrical consumption you're trying to measure into it. You can measure the instantaneous electrical consumption (wattage) and also the cumulative consumption over time in kilowatt-hours (kWh). It's a great way to figure out how much power your refrigerator is really using, whether your television set consumes electricity even when it's "turned off," and just how much electricity your pellet stove needs to power its fans (mine uses a maximum of about 320 watts, and during the winter consumes about 0.7 kWh per day).

For equipment that consumes electricity periodically, such as a refrigerator or pellet stove, the overall energy usage depends as much on the amount of time the compressor or motor or electronics is operating as on the wattage of that electrical load. For this reason, you really need to keep the device plugged into the monitor for a significant period of time. If you keep it plugged in for 24 hours, multiply the resultant kWh consumption by 365 to get an estimate of the annual usage. You'll get a more accurate estimate of yearly energy use, however, if you keep it plugged into that electricity monitor for a week and then multiply the number by 52.

To determine the "stand-by" or "phantom" electricity consumption of entertainment equipment (television, stereo, DVD player, VCR, etc.), you can plug these devices into the electricity monitor either singly or, if they're all in the same place, altogether using a power strip. My findings were quite surprising. Our television (I think it's a 32-inch Panasonic) didn't use a measurable amount of electricity when turned off, nor did our stereo receiver or DVD player. A VCR that we rarely use, however, consumed about 2.3 watts all the time--so we keep that unplugged. Results will, of course, vary from product to product and brand to brand.

With standby electricity use, a good rule-of-thumb is that each watt of electricity consumption that occurs 24/7 costs about a dollar per year, given our electricity prices (12¢/kWh).

Thus, if your cable modem and wireless router for Internet access are drawing 25 watts, and they're plugged in and turned on all the time, they're costing you about $25 per year. Those kinds of numbers can really add up if you factor in all of the appliances that draw these phantom loads. The transformers (AC adaptors) used to charge power tools, cell phones, and computers are other phantom loads that can be significant.

As for specific electricity monitors on the market, I have an affordable, cleverly named "Kill-A-Watt" meter, made by P3 International Corporation. This particular device, which works only for 110-volt equipment (not 220-volt), sells online for about $25. Brooks Library has one of these that you can check out, or if you're a Dummerston resident, there are two available for borrowing at the Town Office.

Efficiency Vermont has a different model available for lending. Note that these electricity monitors only work for electrical loads that have their own plugs; hard-wired kitchen light fixtures and water pumps can't be plugged into it.

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