Why do published appliance running costs disagree so much?
Search for what a refrigerator costs to run and you will find answers between $3 and $20 a month. Most of them are defensible. Here is how to tell which one applies to you.
Reviewed
The spread is mostly real
It is tempting to assume that when sources disagree by a factor of five, most of them are wrong. For appliance running costs that is usually not the case.
A refrigerator built in 1990 genuinely uses around three times the electricity of a modern Energy Star model. And Hawaii charges about 4.0× what Nevada does for the identical kilowatt-hour. Multiply those two together and a spread of roughly 12× appears without anybody making an arithmetic error.
So the useful question is not which figure is correct. It is which set of assumptions the figure was built on, and whether they match your house.
Three questions that settle it
What wattage did they assume? This is the number most often stated without a source. A refrigerator can be 80 watts or 400. If a page gives you a single figure with no range and no explanation of where it came from, you cannot map it onto your appliance.
Did they apply a duty cycle? Anything with a compressor or a thermostat — refrigerators, freezers, air conditioners, dehumidifiers, electric water heaters — draws power for only part of the time it is switched on. A fridge is plugged in 24 hours a day and runs for roughly eight of them. Multiplying wattage by 24 gives a number about three times too high.
What price per kilowatt-hour? Some pages still quote 10 or 12 cents, which was a reasonable US average several years ago and is now well below it. A figure built on an old rate understates by a third or more even when everything else about it is right.
Doing it yourself in one line
The whole calculation is: watts, divided by 1,000, multiplied by the hours it actually draws power, multiplied by your price per kilowatt-hour.
For a 150 W refrigerator running about a third of a 24-hour day, at 18 cents: 0.15 × 8 × 0.18, which is about 22 cents a day, or roughly $6.50 a month.
If a published figure is several times that, check the duty cycle first. It is the most common reason two honest sources disagree.
When to stop estimating
Every figure on this site is an estimate, including ours. For a single appliance you actually care about, a plug-in energy meter costs about the price of two months of the thing you are measuring and removes the guesswork entirely.
For anything hard-wired — a water heater, a furnace, a well pump — the meter is not an option, and the next best evidence is your own bill. Read the meter at the same time on two consecutive days without changing anything, then switch the appliance off for a day and repeat.
The yellow EnergyGuide label on newer appliances states kilowatt-hours per year directly, which is better than any wattage estimate including this site's.