Electricity rates vary widely. I’ve found rates from 12¢ to 50¢ per kWh from the same provider. If you want results that are anywhere close to accurate, you’ll need to check your own electric bill and find your actual kWh rate. Your bill might have multiple kWh rates (e.g., one for “delivery” and one for “fuel”), and in that case you should to add up them all up to get the total kWh rate. Most rates are tiered, meaning the higher your use, the higher the rate. You should generally enter your highest tier into the calculator, because any energy you save will save you money at that highest tiered rate.
Electrical use varies from model to model. I thought this would be obvious, but in hundreds of blogs and forums people say things like, “Mr. Electricity says a computer uses 150 watts,” which is a mis-characterization. The calculator and table figures are just examples. (See how to misquote this website.)
Some devices use varying amounts of electricity. An easy example is an oven, whose energy use depends on how high you crank it. Perhaps a less obvious example is a washing machine, which effectively uses phenomenally more energy if you wash in hot rather than cold. Then there’s the refrigerator, which alternates between periods of full energy use while the compressor is running, and then next to nothing when the compressor shuts off. (To solve this problem for fridges, the calculator lists the average wattage over time, not the higher amount used when the compressor runs.)
Most devices don’t run 24/7. Therefore the “Hours per Day” and “Days per Month” fields in the calculator are crucial. Even so, you might not have a good idea about how various appliances run. So below we’ll discuss which items use the most energy in a typical home. You can also measure a device’s usage yourself, which gives you the best information for your own situation.
Some devices use a little energy even when they’re not on. This is called standby power, or vampire power. In most cases it’s not especially significant, but in some cases it can be. My standby power page has more info.