kWh calculator: convert watts to kWh and electricity cost

How this kWh calculator works

Your electricity is billed in kilowatt-hours. One kWh is what a 1,000-watt device uses in an hour. The calculator converts a power rating and running time into kWh and money, or turns a kWh figure from your meter or bill into cost per day, week, month and year.

In power mode, enter the wattage (or kW, horsepower, or amps at 230 V or 120 V), the hours of use per day and the number of days per month. Under the advanced fields you can set a duty cycle for devices that a thermostat switches on and off, and the number of identical devices (handy for lighting, servers or office equipment). In energy mode, enter kWh per day, week, month or year. For the tariff, give a single price or a day/night time-of-use tariff plus the share of consumption in the cheap period. A grid emission factor (optional) drives the CO₂ figure.

The formula

Worked example: a 1.5 kW space heater

The default is a 1,500 W heater used 3 hours a day, 30 days a month, at 0.25 per kWh:

• Energy per hour: 1.5 kWh, costing 0.375
• Per month: 1.5 kW × 3 h × 30 days = 135 kWh → 33.75
• Per year: 1,620 kWh → 405
• At 250 g CO₂ per kWh that is about 405 kg of CO₂ a year

Say the thermostat keeps it on only 60% of the time. Set the duty cycle to 60% and the cost drops to about 20 per month. A heat pump with a COP of 3.5 would deliver the same heat on less than a third of the electricity.

How to read the results

• Cost per month and year are the numbers to compare with your bill.
• Cost per hour of use is what each extra hour costs, which helps when you’re deciding whether to switch something off.
• The price chart shows how the yearly cost moves if your tariff changes. It’s a straight line.
• With a time-of-use tariff you also get the blended price and the yearly saving against a flat rate.

Typical power ratings and costs (at 0.25 per kWh)

Tips to cut the cost per kWh and the kWh

1. Shift flexible loads (EV charging, water heating, dishwashers) to off-peak hours if you’re on a time-of-use tariff.
2. Measure, don’t guess. A plug-in meter costs less than a month of running a big appliance and reveals standby waste.
3. Replace the biggest consumers first: resistive heating (switch to a heat pump), old fridges and freezers, halogen and incandescent lamps.
4. Generate your own. Over its life, rooftop solar produces electricity at 0.05–0.10 per kWh in most of Europe. Try the solar panel calculator.
5. For a detailed breakdown by appliance use the appliance energy cost calculator.

Limitations

The calculation assumes constant power, while real devices vary with load, temperature and settings. Bills often include a fixed daily standing charge, network fees and taxes; use your all-in average price per kWh (total bill ÷ kWh) for realistic results. The monthly figure uses an average month of 30.4 days.

Method & assumptions

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