I have been paying PG&E bills in San Francisco for decades, and for most of those years I treated the bill like weather — something that happens to you. One month I finally sat down at the kitchen table and actually read the thing, and the unit at the center of it turned out to be wonderfully concrete.
A kilowatt-hour is exactly what it sounds like: run a device that draws 1,000 watts for one hour, and you have used one kilowatt-hour. Power multiplied by time. It is also a respectable amount of energy in physics terms — exactly 3.6 megajoules, enough to lift a small car more than 200 meters into the air — which makes it a little startling how affordable each one is.
A kilowatt-hour is a rate times a time
The recipe for any appliance is the same: watts times hours, divided by 1,000. My kettle draws about 2,000 watts; six minutes of it is 0.2 kWh. A 10-watt LED bulb burning for ten hours is 0.1 kWh. An electric oven at 2,400 watts for an hour is 2.4 kWh.
Run that arithmetic around the house and a pattern jumps out: anything that makes heat dwarfs everything else. Ovens, kettles, dryers, water heaters, space heaters. A space heater running all evening — a familiar ritual in our unheated-feeling SF houses — can use more energy than the Wi-Fi router does in months.
The quiet consumers that never clock out
The sneakier costs come from devices that draw little but never stop. My refrigerator averages only around 100 watts, but it works 24 hours a day, every day, which makes it one of the larger single items on the bill. Same logic for standby electronics and old chargers: small watts, enormous hours.
That is why energy labels quote estimated kWh per year rather than wattage. Duration matters just as much as draw.
Estimating any device’s monthly cost
Here is the whole method, the one I wish I had used decades sooner:
- Multiply the device’s watts by its daily hours of use
- Multiply by 30, then divide by 1,000 to get monthly kWh
- Multiply by the rate per kWh printed on your bill
A 100-watt device used 5 hours a day comes to 15 kWh a month; at, say, 20 cents per kWh, about three dollars. Do this once for your biggest appliances and you will likely find, as I did, that two or three of them dominate the entire bill — and the gadgets I had been guiltily unplugging barely registered.
Next time your bill arrives, find the kWh figure and the price per unit — those two numbers, plus the watts-times-hours recipe, are all you need to know exactly where the money goes.
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