SnapEnergyLab
Electricity

Watts vs kilowatt-hours

A watt is a rate. A kilowatt-hour is a quantity. Your bill only charges you for the second one.

Published 23 August 2026 · Published by SnapEnergyLab. Methodology and default assumptions are documented separately.

An electric kettle boiling on a bright kitchen counter in morning daylight, steam rising through the window light

Quick answer

A watt measures how fast a device draws electricity right now. A kilowatt-hour measures how much electricity it has used over a period of time. Power is a rate; energy is a quantity.

Your supplier bills the quantity, not the rate. Multiply power in kilowatts by hours of running to get kilowatt-hours: a 1,500 W heater running three hours uses 4.5 kWh, whatever its label says.

How to read the numbersYour inputCalculated resultSite default
Power
1,500 W
Your input
Time
3 h
Your input
Energy billed
4.5 kWh
Calculated result

A rate and a quantity are different questions

The confusion is easier to see with water. A tap has a flow rate — litres per minute. A bucket has a volume — litres. Turning the tap up increases the rate; leaving it running increases the volume. Nobody bills you for litres per minute, and no electricity supplier bills you for watts.

This is why two devices with the same wattage can have completely different places on your bill. A 2,000 W kettle and a 2,000 W immersion heater draw identical power. The kettle runs three minutes a day and the immersion heater runs two hours. Same rate, twenty-times different energy.

  1. energy (kWh) = power (W) ÷ 1000 × time (h)
  2. power (W) = energy (kWh) × 1000 ÷ time (h)
  3. cost = energy (kWh) × price (currency per kWh)

The 1000 is the only conversion involved: a kilowatt is a thousand watts, and a kilowatt-hour is a kilowatt sustained for one hour.

Energy accumulates; power does not

Hold the power constant and energy rises in a straight line. The slope of that line is the power. If you can read the chart below, you can read any consumption question: a steeper line means a higher-wattage device, and a longer line means a longer run.

Energy used by a 1,500 W heater against running time0 kWh5 kWh10 kWh0 h2 h4 h6 h8 hRunning time (hours)Energy used (kWh)
Constant 1,500 W draw, no cycling and no standby. Real heaters with a thermostat spend part of each hour switched off, which bends this line downwards — see the guide on nameplate watts versus actual use.
Show the plotted values
Running timeEnergy used
0 h0 kWh
1 h1.5 kWh
2 h3 kWh
3 h4.5 kWh
4 h6 kWh
5 h7.5 kWh
6 h9 kWh
7 h10.5 kWh
8 h12 kWh

Worked example: a 65 W television

A modern 55-inch television draws roughly 65 W while on. Watched five hours a day, its wattage sounds trivial — but the quantity is what matters.

Worked example

  1. Power while on65 WYour input
  2. Convert to kilowatts65 ÷ 1000 = 0.065 kWCalculated result
  3. Daily running time5 hYour input
  4. Daily energy0.065 × 5 = 0.325 kWhCalculated result
  5. Annual energy0.325 × 365.24 = 118.7 kWhCalculated result
  6. Electricity price0.18 per kWh (site default)Site default
Annual cost of the television≈ 21.37

Reading the three numbers on an appliance label

Watts (W)

Maximum or rated draw. Useful for checking whether a circuit, generator or inverter can supply the device at all, and almost never equal to average consumption.

Kilowatt-hours per year (kWh/yr)

Found on energy labels for fridges, washing machines and dishwashers. This is a measured test-cycle figure, not a nameplate rating, so it is far closer to what you will actually be billed for.

Amps (A)

Current draw. Multiply by supply voltage to get watts: 5 A at 230 V is 1,150 W. Comparing amps across different voltages tells you nothing on its own, the same trap that makes amp-hour battery comparisons misleading.

Turn watts and hours into a cost per month

Enter a wattage, the hours it runs and your own electricity price. The calculator shows cost per hour, day, month and year, with every step of the arithmetic exposed.

Open the electricity cost calculator

Assumptions and limitations

  • The $0.18/kWh used above is SnapEnergyLab's default rate — a rounded stand-in for the US EIA residential average, not your tariff. Your effective rate, including standing charges and taxes, is usually higher than the unit rate printed on a tariff sheet.
  • Constant-power examples ignore thermostats, compressors and standby modes. Devices that cycle use less than a constant-draw calculation suggests; devices with an always-on standby use more.
  • Nameplate wattage is a maximum, not an average. Where a measured annual figure exists on an energy label, prefer it.

Sources

  1. Units and calculators explained — electricity — U.S. Energy Information Administration

    Supports: Definition of the kilowatt-hour as the standard billing unit for electrical energy.

  2. The International System of Units (SI), 9th edition — BIPM

    Supports: The watt as the SI unit of power (one joule per second) and its relationship to energy.

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