Space Heater Size Calculator
Estimate the heat you need (BTU/h), convert it to required watts, and check whether your electrical circuit can handle it.
Updated for 2026 electrical load planning norms.
On this page: Quick guide · Calculator · Sizing tips · Equipment · Related calculators · FAQ
For full HVAC sizing, use the advanced BTU calculator. For whole-home heating, try the furnace size calculator or heat pump calculator.
Quick guide: what “1500W” means
Most plug-in heaters top out around 1,500W, which is roughly 5,100 BTU/h. If your room needs more heat than that, you may need multiple heaters on separate circuits, a 240V heater, or better insulation and air sealing.
Circuit capacity chart
Planning-grade usable watts at ~80% of breaker capacity (practical for continuous heating).
| Circuit | Max watts | Usable (~80%) | What it means |
|---|---|---|---|
| 120V / 15A | ~1,800W | ~1,440W | A 1,500W heater is near the practical limit |
| 120V / 20A | ~2,400W | ~1,920W | Better headroom for continuous use |
| 240V / 15A | ~3,600W | ~2,880W | Supports larger fixed/garage heaters |
| 240V / 20A | ~4,800W | ~3,840W | High-output heaters (often dedicated circuit) |
Watts to BTU/h conversion
Electric resistance heat converts at about 1 W ≈ 3.412 BTU/h.
| Watts | BTU/h | Typical use |
|---|---|---|
| 500W | ~1,700 | Desk / small spot heating |
| 750W | ~2,560 | Medium / eco setting |
| 1,000W | ~3,410 | Small rooms, mild cold |
| 1,500W | ~5,100 | Most common plug-in max |
| 2,000W | ~6,820 | Often 240V / fixed |
| 3,000W | ~10,240 | Garage / shop (240V) |
Shop shortcuts: 1,500W heaters · Oil-filled · Infrared · Garage
Enter your room details. The calculator estimates heat loss, converts to required watts, and checks whether your circuit can handle it.
Optional: estimate operating cost
If your result requires more than ~1,500W, consider a heat pump or a 240V fixed heater. For code-grade whole-home sizing, request an ACCA Manual J. Check local electricity rates for the cost estimate.
Space heater sizing tips
1,500W is the portable ceiling
Most plug-in heaters max out at 1,500W (~5,100 BTU/h). If your room needs more, look at 240V fixed heaters, or better yet, reduce heat loss with weatherstripping and spray foam.
Drafts matter more than heater size
Sealing leaks can reduce required watts dramatically and improves comfort. A thermal leak detector ($30–50) finds the worst spots.
Basements and garages need more heat
Cold surfaces and air leakage mean basements and garages often need 20–40% more watts than living areas. For garages, a dedicated garage heater on a 240V circuit is usually safer and more effective than multiple portable units.
Oil-filled radiators for steady comfort
For longer heating runs (bedrooms, offices), oil-filled radiators provide gentler, quieter, more stable heat than ceramic fan heaters. They’re the same wattage but distribute heat more evenly.
Ceiling fans help in tall rooms
For rooms with 9+ ft ceilings, gentle fan mixing pushes warm air down from the ceiling. Size one with the ceiling fan calculator.
Space heater shopping
Heaters by type
1,500W space heaters — most common plug-in size. Oil-filled radiators — steady, quiet comfort. Infrared heaters — spot heating. Small space heaters — compact rooms. Garage heaters — workshops and garages (often 240V). Browse all space heaters.
Reduce heat loss (and heater size)
Weatherstripping ($5–15), door sweeps ($8–20), window insulation kits ($10–30), and outlet gaskets ($5–10) reduce infiltration and can drop you to a smaller heater.
All equipment: heating & cooling equipment by BTU range.
Frequently asked questions
How many BTUs is a 1500W space heater?
About 5,100 BTU/h. Electric resistance heat converts at 1 watt ≈ 3.412 BTU/h, so 1,500 × 3.412 ≈ 5,118 BTU/h.
Can I run a 1500W heater on a standard outlet?
Often yes, but it’s close to the practical limit for a 120V/15A circuit (~1,440W at 80% continuous rating). For frequent or long use, a dedicated circuit or lower wattage setting is safer. See the circuit chart above.
What if my room needs more than 1500W?
Consider multiple heaters on separate circuits, a 240V fixed heater, or reduce heat loss with air sealing and insulation. For whole-room or whole-home heating, a heat pump or furnace is more practical and cost-effective.
Oil-filled vs ceramic vs infrared — which is best?
Oil-filled radiators provide steady, quiet heat and are good for longer runs. Ceramic fan heaters warm up quickly but can be noisier. Infrared heaters are best for spot heating (they warm objects, not air). All produce the same BTU per watt — the difference is heat delivery style.