Space heater
kWh/month ≈ (watts ÷ 1,000) × hours/day × (duty ÷ 100) × 30. Cost = kWh × $/kWh.
Window air conditioner
Compressors cycle off when the room is cool. Use hours the AC is on, or set duty cycle to roughly 50–70% for a typical 8-hour evening run.
How the math works
Electricity is sold in kilowatt-hours (kWh). Power in watts tells you how fast a device draws energy. Divide watts by 1,000 to get kilowatts, multiply by hours running, and you get kWh for that period.
Space heaters turn nearly all their electricity into heat, so a 1,500 W setting draws 1.5 kWh for every full hour at high. Window ACs only draw full compressor power part of the time—that is why duty cycle matters for AC more than for a heater on “high” without a thermostat.
Your utility bill also includes fixed fees, taxes, and sometimes tiered rates. This tool estimates one appliance’s energy at the rate you enter (check the supply or “generation” line on your bill, or total kWh cost ÷ kWh used).
Worked example: 1,500 W heater, 6 hours/day
Effective power for a full 6 hours at 100% duty: 1.5 kW × 6 h = 9 kWh per day. Over 30 days: 270 kWh. At 18.4¢/kWh ($0.184): 270 × 0.184 ≈ $49.68 per month. If the thermostat cuts actual heating to half the time, use 3 hours or 50% duty—about $25 instead.
Quick FAQ
- 750 W or 1,500 W? High doubles the hourly cost. Low is enough for a small room; high heats faster in large or drafty spaces.
- Why is my bill higher than this? You have other loads (water heater, fridge, lights), rate tiers, and delivery charges not shown here.
- Window AC vs central air? One window unit usually uses less total power than whole-house cooling, but several window units can add up.
- Where does 18.4¢ come from? A recent U.S. average residential rate ballpark—replace with your bill’s actual $/kWh for accuracy.
About this tool
howmuchtorunaspaceheater.com is a browser-only electricity cost estimator for common room heaters and window AC units. Nothing you type is uploaded. Published by R7Global.
Questions: r7global@gmail.com