Electricity Usage Calculator

How to use the electricity usage calculator

Enter the device’s power in watts, how many hours a day it runs, and over how many days, and you get the energy it uses in kilowatt-hours — over the whole period, per day, and per year. This tool answers one question and one only: how much energy. It deliberately stops short of dollars, because the rate is a separate, personal number that varies by state, plan, and time of day. When you want the cost, take the kWh here straight into the electricity-cost-calculator, which multiplies it by your rate.

Watts can be the nameplate figure or the real draw. The number on the label or sticker is the maximum the device can pull; many run below it in normal use, and motor-driven ones — refrigerators, air conditioners — cycle on and off, so their average sits well under the nameplate. For a quick estimate the nameplate is fine. For accuracy, a $15 plug-in meter (a Kill A Watt and the like) reads the actual draw at the outlet, which matters most for anything thermostat-controlled.

Be honest about hours and days. A space heater that’s “on all evening” runs five or six hours, not twenty-four; a fridge sits in the kitchen around the clock but its compressor only works about a third of the time. Set the hours to real runtime rather than how long the thing is plugged in, and set the days to the window you’re measuring — 30 for a month, 365 for a year. The energy figure is only as good as those two numbers.

A kilowatt-hour is the unit your meter counts and your bill charges for, so the kWh this tool produces is directly comparable to the line on your statement. If you want to know what one appliance contributes to that line, run it here and weigh it against your whole-house total. The US residential average is around 900 kWh a month, but your bill is the real number — read it off the statement rather than trusting any average, because households vary enormously.

To measure usage straight from the meter, skip the watts entirely and subtract two readings. Your meter counts cumulative kilowatt-hours and only ever climbs; note the number today, note it again in a week or a month, and the difference is exactly the energy you used in between. That reading is the ground truth this calculator estimates toward — handy for sanity-checking a device tally against what the whole house actually drew over the same stretch.

The formula

Energy is power times time. Watts are divided by 1,000 to become kilowatts so the units line up with the kilowatt-hours your meter counts — a kilowatt sustained for an hour is one kilowatt-hour:

kWh = (watts ÷ 1000) × hours per day × days

Worked example with the defaults — a 1,500 W device, 6 hours a day for 30 days: 1,500 W ÷ 1000 × 6 h × 30 days = 270 kWh (that is 9 kWh a day, about 3,285 kWh a year). Change either the hours or the days and the energy moves in step, because the formula is just a straight multiplication.

A kilowatt-hour is one kilowatt sustained for an hour — ten 100 W bulbs running for an hour, or one 1,000 W microwave for an hour — and it is exactly the unit your meter counts and your bill charges for. That is why the kWh you get here is the same kWh on your statement: the calculator is just doing in advance what the meter does as it spins.

Frequently asked questions