What Is a Kilowatt-Hour? The Only Electricity Unit You Need to Understand
Updated 2026/06 · Basics
Your entire electric bill is built on one unit: the kilowatt-hour (kWh). Understand it, and every number on this site — and your bill — becomes obvious.
Watts measure speed, kilowatt-hours measure amount
A watt is a rate — how fast a device consumes energy right now, like the speedometer in a car. A kilowatt-hour is a quantity — how much energy was actually consumed, like the odometer. One kilowatt-hour is 1,000 watts sustained for one hour.
The two questions that matter for any device:
- How many watts does it draw while running?
- How many hours does it actually run?
Multiply them, divide by 1,000, and you have kWh. Multiply by your rate and you have dollars. That’s the whole formula — it’s what our calculator does with current state rates built in.
Getting an intuition for 1 kWh
One kilowatt-hour is:
- A space heater on high for 40 minutes
- A 55-inch TV for about 12 hours
- A refrigerator for about 7 hours
- An LED bulb for about 110 hours — four and a half days
- Charging a phone roughly 50 times
- About 3–4 miles of driving in a typical EV
Notice the pattern: anything that makes heat is expensive, everything else is cheap. A device that heats air or water (heater, dryer, oven, water heater) draws 1,000–5,000 watts. Everything else in your house — screens, lights, chargers, fans — mostly draws 5–200 watts. This single fact explains about 80% of every electric bill.
What a kilowatt-hour costs
The U.S. average residential rate is currently around 17¢/kWh, but the spread is huge — roughly 11¢ in the cheapest states to over 40¢ in Hawaii. Check your state’s current rate. Note that your effective rate (total bill ÷ kWh used) is usually higher than the advertised rate, because bills add fixed charges, delivery fees, and taxes.
The mental shortcut worth memorizing
Here’s the trick that makes you dangerous at parties:
At ~17¢/kWh, every continuous 100 watts costs about $12–15 per month.
A 100 W device running 24/7 uses 73 kWh/month — about $12.50 at the average rate. So:
- A home server idling at 100 W? ~$12/month.
- An old cable box at 20 W? ~$2.50/month.
- A hot tub averaging 200 W? ~$25/month.
- A space heater that averages 500 W across the day? ~$60/month.
Scale by your own state: in California, roughly double these; in North Dakota, roughly two-thirds.
Watts vs. volts vs. amps, in one paragraph
Volts × amps = watts. Your outlets supply ~120 volts; a device drawing 12.5 amps is pulling 1,500 watts — the practical maximum for a standard circuit, which is why space heaters, kettles, and hair dryers all cluster at 1,500 W. Big appliances (dryers, EV chargers, water heaters) use 240 V circuits precisely so they can draw more watts without enormous currents.
Where labels mislead
The wattage printed on a device is its maximum, not its average. A “1,200 W” microwave describes cooking power; a “500 W” PC power supply describes capacity, not consumption; a fridge’s label amperage implies a draw it only hits when the compressor starts. For realistic typical figures, that’s exactly what our appliance database provides — or measure the real number yourself with a $10 plug.