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An earlier column emphasized understanding the numbers on your electric bill. Understanding what the bill means, however, doesn’t help much in understanding why you’ve used that much energy, nor does it help you reduce your energy costs. For that, you need to understand how much each of your electric devices contributes to the meter reading.

Electric devices have wattage ratings. Multiply the wattage by the amount of time the device is used to determine how much energy that device is using. For instance, turning a 60 watt incandescent lamp on for ten hours a day uses 600 (60 times 10) watt-hours, or 0.6 kilowatt-hours (kWh).

Most electrical devices don’t operate continuously. A refrigerator may use about 1,200 watts to keep food cool, but it doesn’t use 1,200 watts 24 hours a day. It only uses energy when the compressor is on – that is during a third of the day. So, a 1,200 watt refrigerator would use about 1,200 times 8, or 9,600 watt-hours, or 9.6 kWh, per day. A hand-held circular saw might require 1,200 watts, but most woodworking hobbyists aren’t likely to have a power saw on for more than an hour a day.

The 1,200 watt saw would use 1,200 watt-hours (1.2 kWh) in a day. The 1,200 watt refrigerator, running intermittently but more often, uses 9.6 kWh. The 60 watt light, running continuously for a long time, uses 0.6 kWh. Because it operates so intermittently, the saw uses one-eighth the amount of energy as the refrigerator, even though they use the same amount of energy when they’re actually running. The saw only uses twice the energy of the light over the whole day, even though it uses 20 times the energy during the period of time it is actually on.

Replacing the 60 watt light with a 9 watt LED light reduces the energy the light uses from 600 watt-hours/day to 90 watt-hours/day. Over a 30-day month, this saves 510 watt-hours a day, or 15,300 watt-hours (15.3 kWh). At $0.12/kWh, the new light bulb will take $1.84/month off your electric bill ($0.12 times 15.3).

Replacing the 1,200 watt refrigerator with a modern 600 watt refrigerator will reduce the energy the fridge uses from 9.6 kWh/day to 4.8 kWh/day. Total 30-day energy savings is 144 kWh; total 30-day dollar savings will be $17.28.

If you don’t want to estimate run times or look for wattage labels, get a personal meter like a Kill-a-watt (that is its real name) and plug each of your devices into it for a while. Personal meters will show you how much energy each device is using.

Heating, ventilation and air conditioning (HVAC) account for half to two-thirds of a typical electric bill. New technologies help you save money and energy while maintaining comfort. Using an energy-efficient heat pump reduces the energy used by the system. Using a programmable thermostat – actually programming it, not just installing it – enables you to let the house run to less comfortable temperatures during the hours you aren’t home. Your furniture doesn’t care if it’s 60 degrees inside from 7:30 am to 3:30 pm and you can program the system to return to 70 degrees by the time you get home.