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What Is a Kilowatt-Hour?

Watts say how fast energy is being used; kilowatt-hours say how much has been used. The electricity meter counts kilowatt-hours, so every question about cost comes down to watts multiplied by hours.

By the Akermin Editorial Team · 2 min read

A kilowatt-hour (kWh) is a unit of energy equal to one kilowatt (1,000 watts) of power used for one hour. It is what the electricity meter counts and what the bill charges for. Energy in kWh equals power in kilowatts multiplied by time in hours: a 100 W bulb for 10 hours is 1 kWh; a 3 kW kettle for 4 minutes is 0.2 kWh; a 1.5 kW heater for 8 hours is 12 kWh. Multiply by the tariff per kWh and you have the cost.

Power and energy

Power (watts, kilowatts) is a rate: how fast energy is flowing right now. Energy (watt-hours, kilowatt-hours) is a quantity accumulated over time. The speedometer versus the odometer. Appliance labels give power; bills give energy; the link between them is how long the appliance runs. See watts vs volts vs amps.

Calculating it

kWh = watts × hours ÷ 1,000

Appliance Power Daily use Energy per day
LED bulb 9 W 5 h 0.045 kWh
Laptop 45 W 8 h 0.36 kWh
Refrigerator (cycling) 150 W when running, about 30% duty 24 h About 1.1 kWh
Television 100 W 4 h 0.4 kWh
Electric kettle 3,000 W 10 min total 0.5 kWh
Space heater 1,500 W 6 h 9 kWh
Electric vehicle charging 7,000 W 4 h 28 kWh

The heater and the car dominate; the bulb is noise. The electricity cost calculator turns any wattage and hours into daily, monthly and yearly kWh and cost.

Cycling loads

Appliances with thermostats or compressors do not run continuously. A refrigerator rated 150 W may average 40 to 50 W over a day; a heater with a thermostat might run 50 percent of the time. Nameplate watts times 24 hours overstates these badly. A plug-in energy monitor that accumulates kWh is the only reliable way to know; see plug-in vs whole-home energy monitors.

Reading the meter

Electricity meters display cumulative kWh. The difference between two readings is the energy used in between. Read it at the same time on two days for daily use; smart meters and whole-home monitors do this continuously and show it per hour.

kWh in solar and storage

  • A solar system’s output is quoted in kWh per day or per year; a 5 kW array in a sunny location might produce 20 to 25 kWh on a summer day. See solar irradiance vs panel output.
  • Battery capacity is quoted in kWh (or amp-hours at a voltage). A 10 kWh home battery stores about a day’s use for a modest home.
  • Electric vehicle batteries are 40 to 100 kWh; efficiency is kWh per 100 km.

Cost

Bills list a price per kWh, sometimes with time-of-use variations and a separate standing charge. Multiply the kWh by the rate for the period it was used. For what a specific device costs, see how much electricity a 100-watt device uses.

P3 P4400 Kill A Watt Electricity Usage Monitor

P3 P4400 Kill A Watt Electricity Usage Monitor

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Emporia Vue 3 Home Energy Monitor - Smart Home Automation Module and Real Time Electricity Usage Monitor, Power Consumption Meter, Solar and Net Metering for UL Certified Safe Energy Monitoring

Emporia Vue 3 Home Energy Monitor - Smart Home Automation

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Products shown because their documented specifications fit this guide. Price and availability are on Amazon and can change.

Frequently Asked Questions

Is a kilowatt the same as a kilowatt-hour?

No. A kilowatt is power (a rate). A kilowatt-hour is energy (an amount): one kilowatt sustained for one hour. A 2 kW heater running 30 minutes uses 1 kWh.

How many kWh does a house use per day?

Widely variable. Around 8 to 12 kWh per day is typical for a European home without electric heating; US averages are around 25 to 30 kWh per day; homes with electric heating or air conditioning use far more.

How is kWh related to battery capacity?

Battery capacity in amp-hours times voltage gives watt-hours. A 100 Ah 12 V battery holds 1,200 Wh, or 1.2 kWh, before allowing for depth-of-discharge limits.

Affiliate disclosure: Akermin earns a commission from qualifying Amazon purchases made through links on this page. Our editorial picks are based on documented specifications and owner feedback, not commissions.