Watts to kWh Calculator
Energy in kilowatt-hours from power in watts and time in hours
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What this calculator computes
Watts to kilowatt-hours conversion turns a power rating into an energy figure by multiplying by the operating time and dividing by 1000. The formula is kWh = (W × h) / 1000, and the result represents the cumulative energy a constant-power load consumes over a stated duration. This is the conversion at the heart of every electricity bill: utilities meter cumulative energy in kWh, and the bill total is the kWh figure multiplied by the per-unit tariff rate (currently averaging about 16 ¢/kWh in the US, 27 p/kWh in the UK after the 2024 price-cap revisions, and varying widely by region in the EU). Knowing how to translate between watts and kWh lets you predict the electricity cost of running any specific appliance: a 1500 W space heater running 8 hours per night for 30 days consumes 1500 × 8 × 30 / 1000 = 360 kWh per month, which at 27 p/kWh is £97.20 in monthly running cost. This calculator accepts a wattage and a duration (hours, days, or months with appropriate scaling) and returns the cumulative energy in kWh, with helpful conversions to Wh and MJ for cross-checking against scientific data sources. The calculation assumes constant power draw for the entire duration, which is correct for resistive heaters, lighting at fixed dimming, and CPU at fixed load, but understates duty-cycled loads (refrigerators, HVAC, microwaves) where the load only runs a fraction of the time.
Calculator
The formula
Formula
kWh = (W × h) / 1000
Worked example
When to use this calculator
Use this calculator any time you need to predict the electricity cost of running a specific appliance, compare the operating cost of two alternatives (LED vs incandescent, electric vs gas heating, server-room A/C zoning), or budget the energy capacity of a battery bank or generator runtime. The most common scenarios are residential energy auditing (matching individual appliance consumption to the utility bill total), commercial peak-demand and time-of-use planning (where running a high-wattage process during off-peak hours can halve the per-kWh rate on time-of-use tariffs), solar-PV self-consumption analysis (matching daytime generation to known appliance loads), and EV-charging cost estimation (where the kWh delivered to the battery times the per-kWh tariff equals the per-charge cost). The calculator is most accurate for resistive constant-power loads; for cyclic loads (refrigerators, heat pumps, microwaves) measured kWh from a smart-plug meter or the utility bill itself is more reliable than a calculation from nameplate watts.
Common input mistakes
- Using nameplate wattage for cyclic loads. A refrigerator labelled 700 W only draws 700 W during the compressor-on portion of its duty cycle, typically 30–40% of the time, so the actual daily consumption is closer to 700 × 24 × 0.35 / 1000 = 5.9 kWh rather than the 16.8 kWh the constant-power calculation suggests. For duty-cycled loads, use the EnergyGuide label kWh/year figure or measure with a smart plug.
- Confusing watts and watt-hours when the appliance is rated by energy rather than power. Some battery banks, UPS units, and rechargeable devices give capacity in watt-hours (Wh) rather than watts. A 1000 Wh power station can deliver 1000 W for one hour or 100 W for ten hours; it is energy capacity, not a power rating, and should not be entered into the watts field of this calculator.
Frequently asked questions
How do I convert watts to kWh?
Multiply the wattage by the number of hours of operation and divide by 1000: kWh = (W × h) / 1000. The 1000 converts watts to kilowatts before multiplying by time, since kWh is by definition kilowatts × hours. A 1500 W heater running 4 hours consumes (1500 × 4) / 1000 = 6 kWh, which at 27 p/kWh costs £1.62.
How many kWh does a 100 W bulb use in a day?
A 100 W bulb running 24 hours consumes (100 × 24) / 1000 = 2.4 kWh per day, or 876 kWh per year if left on continuously. At the UK average tariff of 27 p/kWh, that is 65 p per day or £237 per year — far more than the bulb itself costs to buy. Replacing a 100 W incandescent with an equivalent 12 W LED reduces this to 0.29 kWh per day and £28 per year, a payback of weeks at typical lamp prices.
Is kWh the same as kW?
No — kW (kilowatts) is a unit of power, the rate at which energy is being used at any instant, while kWh (kilowatt-hours) is a unit of energy, the cumulative total over a period of time. A 2 kW heater running for 3 hours uses 2 × 3 = 6 kWh of energy. Utilities bill in kWh because they bill for cumulative energy delivered, not instantaneous draw.
How do I convert kWh back to watts?
If you know the energy consumed and the duration, watts = (kWh × 1000) / hours. A device that consumed 6 kWh over 4 hours had an average power draw of (6 × 1000) / 4 = 1500 W. Use the inverse calculator (kWh to watts) for this direction. For variable-load devices, the result is the time-averaged power, not the instantaneous peak.
Why does my electricity bill use kWh and not joules?
Joules is the SI unit of energy and is used throughout physics and engineering, but the joule is too small for practical billing — 1 kWh equals 3.6 million joules, so a typical UK household monthly consumption of 250 kWh would be 900 million joules, an unwieldy number to print on a bill. The kWh was adopted by utilities in the early 20th century as a practically-sized energy unit that maps directly to a 1 kW load running for 1 hour, an intuitive scale for both households and industry.