Power Consumption Calculator
Convert an appliance's wattage and daily usage into energy consumption in kWh — per day, month and year — and the running cost. The core calculation behind every energy-saving decision.
How to use this calculator
- Enter the appliance's power rating in watts.
- Enter hours per day and days per week of use.
- Add your rate per kWh for the running cost.
Formula used
Energy (kWh) is power (kW) times time (hours). This differs from the raw wattage, which is instantaneous power — a 2,000 W device used briefly can consume less energy than a 50 W device left on all day. Rate × kWh gives cost.
Example calculation
A 100 W device used 6 hours/day, 7 days: 0.1 kW × 6 = 0.6 kWh/day → 219 kWh/year.
At 0.30/kWh that's ~66/year — modest, but scale it to a dozen always-on devices and it adds up.
Power versus energy
The distinction this calculator makes concrete is power versus energy — the thing most people conflate. Power (watts) is the rate of use at an instant; energy (kilowatt-hours) is power accumulated over time, and it's energy that your bill charges for. A high-wattage appliance used briefly can cost less than a low-wattage one running constantly, which is why usage hours matter as much as the rating.
The practical upshot is where to focus energy-saving effort. Cutting the runtime of high-wattage heating and cooling appliances saves far more than switching off low-wattage electronics — though the always-on devices (routers, standby loads, fridges) earn their keep on the bill through sheer hours. Running the numbers reveals which is which for your own home.
Why use this calculator?
- Turn a wattage label into real annual energy and cost.
- Compare appliances fairly on energy, not just wattage.
- Identify whether runtime or rating is driving a device's cost.
Frequently asked questions
How do I convert watts to kWh?
Divide watts by 1,000 to get kilowatts, then multiply by hours of use. A 1,500 W appliance running 2 hours uses 1.5 × 2 = 3 kWh. Multiply by your rate for cost.
What's the difference between watts and kWh?
Watts measure power (the rate of energy use right now); kWh measure energy (power used over time). A 100 W bulb for 10 hours uses 1 kWh. Your electricity bill charges for kWh, not watts.
Does a higher wattage always cost more?
Only if used for the same time. A 2,000 W kettle for 3 minutes uses 0.1 kWh; a 40 W device left on all day uses ~1 kWh. Runtime matters as much as the rating — which is why always-on devices can quietly dominate a bill.