How Those Kilowatt-Hours Add Up

Your electric bill is based on a simple measurement – how much power did you use? It’s calculated in something called kilowatt hours (kWh), but how do you come up with that number? Your electric meter does the math, but if you’d like to try it for yourself, here’s the formula:

wattage times the hours used divided by 1,000 = kilowatt hours

Every electrical device requires a certain amount of power to operate, and that’s measured in watts. Sometimes, it’s easy to figure out the wattage; for example, a 60-watt light bulb uses (you guessed it!) 60 watts of power. So, let’s plug that number into our equation and see how much electricity it takes to power that 60-watt bulb for 6 hours a day:

60 times 6 hours divided by 1,000 = .36 kWh per day

Not much, right? Using that 60-watt light bulb amounts to only 10 kWh per month. Some electrical appliances use a lot more power, but for a shorter time. A typical microwave oven uses 1,000 watts of power, so the frozen dinner that takes 12 minutes to cook would use:

1,000 times 6 minutes (.2 hour) divided by 1,000 = .1 kWh

By now you’re saying, “Hey, what’s the big deal? A few light bulbs and a microwave dinner doesn’t add up to much electricity!” But what about the heavyweights of wattage, like that air conditioner in the window? It uses 1,500 watts of power to operate and might run for 18 hours a day:

1,500 times 18 hours divided by 1000 = 27 kWh per day

That’s over 800 kWh per month. Multiply that times a few window air conditioners in your home, and it adds up fast. And a central air conditioning unit can draw 3,500 watts of power and use about 2,600 kWh per month.

Not every major appliance has such a big energy appetite. A newer refrigerator can run 24 hours a day and use about 50 kWh per month. And your washing machine is one of the few appliances that gives you a choice to save energy. A hot-water wash takes nearly ten times the electricity of a cold water cycle. That’s because up to 95% of the cost of washing in hot water is for the energy to heat the water.

How do other favorite appliances add up on your electric bill? That 50-inch flat-screen TV and cable box might use 2 kWh per day, and the video game console takes about the same amount of power, meaning your TV viewing and gaming totals about 100 kWh per month.

By now, you can see how your electric bill grows when you use all of these appliances. But you can’t just sit in the dark, not cook dinner, never watch TV and sweat it out with no AC, right?

Maybe not, but you can find savings with simple, smart energy choices. A CFL light bulb uses less than half the electricity of a comparable 60-watt bulb to provide the same amount of light, and LED bulbs use even less electricity.

That old refrigerator in the basement uses three times the electricity of a newer model, so unplug it. Make sure you do a full load of laundry each time you run the washer, and use cold water for the wash and rinse cycle whenever possible. And let the dishes air dry in the dishwasher – using the drying cycle takes up to six times the power!

A ceiling fan can greatly improve air circulation in a room for just 15 kWh per month. And using a thermostat with a timer function will let you dial down the air conditioning and dial up the savings when you’re not home. Even simple steps like leaving the drapes closed will keep a room cooler and reduce your AC costs.

When you see your electric bill each month, understand that it’s one number based on lots of smaller numbers – a kilowatt-hour or two here, or 10 or 20 there. When you manage those smaller numbers on a daily basis, you can worry less about having a big number with dollar signs when the monthly bill arrives.