What does leaving a space heater on all day actually cost?
The real dollar cost of running a 1500W space heater for 8, 12, or 24 hours, at typical US electricity rates.
A typical 1500-watt space heater run for 8 hours a day costs about $0.48/day at the US average residential rate (16¢/kWh) — roughly $14.40/month. Run it 24 hours a day and that triples to about $1.44/day, or $43/month, for one heater.
The actual math
Running cost is watts × hours ÷ 1000 (to get kWh) × price per kWh. A 1500W heater running for one hour uses 1.5 kWh. At 16¢/kWh, that's 24¢/hour.
- 8 hours/day: 12 kWh/day → $1.92/day → ~$58/month
- 12 hours/day: 18 kWh/day → $2.88/day → ~$86/month
- 24 hours/day: 36 kWh/day → $5.76/day → ~$173/month
(These numbers correct the earlier estimate above for a heater actually left on, rather than cycling on a thermostat — a heater with a thermostat draws its rated wattage only while actively heating, so real-world cost is usually lower than the "always on" figure.)
Why electricity rate matters more than heater wattage
Rates vary roughly 3x across the US — from around 11¢/kWh in parts of the Pacific Northwest to 30¢+/kWh in parts of California and the Northeast. That spread changes the monthly cost of the exact same heater far more than swapping a 1500W unit for a 1200W one would. Check a real electricity bill for the actual per-kWh rate rather than assuming the national average applies.
Run the numbers for your own rate and heater
The appliance running cost calculator takes wattage, hours per day, and a local electricity rate and returns daily/monthly/yearly cost directly — useful for comparing a space heater against other high-draw appliances, or checking whether it's cheaper than raising the thermostat on central heat.