Electricity Calculator
Estimate energy usage and monthly electricity cost from wattage and runtime.
Estimate the energy usage and monthly or annual electricity cost of running any device, based on its wattage, daily runtime, and your utility rate.
How Electricity Calculator Works
The calculator converts wattage and daily runtime into kilowatt-hours (kWh) per day — the unit utilities actually bill for — by multiplying watts by hours used and dividing by 1,000 (since 1 kilowatt equals 1,000 watts).
That daily energy figure is multiplied by the number of days per month you specify to get monthly kWh usage, which is then multiplied by your cost per kWh to get the monthly dollar cost. Annual cost is simply the monthly figure multiplied by 12.
See It In Action
Who Uses Electricity Calculator and Why
- Estimating how much a space heater, window AC unit, or other appliance adds to a monthly electric bill before buying or running it regularly.
- Comparing the running cost of two appliances with different wattages to decide which is cheaper to operate long-term.
- Budgeting for the added electricity cost of a new device like an EV charger or aquarium heater.
- Checking whether an appliance is worth replacing with a more energy-efficient model based on projected annual savings.
Mistakes to Avoid
- Using a device's maximum rated wattage instead of its typical running wattage — appliances like refrigerators and AC units cycle on and off rather than running continuously at full power, so actual usage is often lower than the nameplate figure alone suggests.
- Forgetting standby ("vampire") power draw isn't included — a device left plugged in when "off" often still draws a small amount of power that this calculator doesn't add unless you include that runtime separately.
- Entering a flat per-kWh rate when your utility uses tiered or time-of-use pricing, which can make the real bill impact higher or lower than this simple estimate.
Tips for Best Results
- Check a device's nameplate or use a plug-in power meter for the most accurate wattage figure, rather than guessing.
- Find your true cost per kWh from a recent bill rather than assuming a national average — rates vary significantly by utility and region.
Fixing Common Problems
My actual electric bill increase doesn't match the estimate. — Check whether your utility bills at a flat or tiered rate, and whether the appliance actually runs continuously at its rated wattage or cycles on and off (like a fridge or AC) — both can cause real-world cost to diverge from a simple wattage × hours estimate.
I don't know my device's wattage. — Check the nameplate/spec sheet on the device, calculate it as volts × amps if only amperage is listed, or use a plug-in power meter for the most accurate real-world reading.
Terms Explained
Kilowatt-hour (kWh): The standard unit utilities bill electricity by — the energy used by a 1,000-watt device running for one hour.
Standby power: Electricity a device draws while switched off or idle but still plugged in, sometimes called "vampire" or "phantom" power.