What Size Inverter for a 1500W Heater?

A 1500W resistive space heater draws a steady 1,500 watts with virtually no surge. Unlike motors (which spike to 3-5x running watts at startup), resistive heating elements reach full power almost instantly. This makes heaters easier on inverters than their wattage suggests.

That said, you still want an inverter rated above 1,500W continuous. Running an inverter at 100% of its rated capacity generates excess heat, reduces efficiency, and shortens its lifespan. An inverter at 80% load runs cooler and more efficiently than one pegged at 100%.

Minimum inverter size: 2,000W continuous. Better: 2,500-3,000W continuous, which gives headroom for the heater plus other simultaneous loads (lights, phone charger, fan). At 12V, a 2,000W inverter draws 167A from the battery bank — that is an enormous current requiring very thick cables (2/0 AWG or larger). At 24V, the same load draws 83A, which is much more manageable. At 48V, only 42A.

This is why 24V or 48V systems are strongly recommended for loads above 1,000W. The lower current at higher voltages means thinner wires, smaller fuses, and less energy lost as heat in the cables.

1. Calculate daily energy. 1,500W x 6 hours = 9,000Wh per day. Add 20% for inverter and system losses: 9,000 x 1.2 = 10,800Wh total demand.
2. Size the inverter. Minimum 2,000W pure sine wave. Modified sine wave inverters can cause resistive heater elements to buzz and reduce their lifespan. Pure sine wave is the safe choice.
3. Size the battery bank. For 1 day of autonomy at 80% DoD: 9,000 / 0.80 = 11,250Wh. At 24V: 469Ah. Five 100Ah 12V LiFePO4 batteries in parallel with a series pair (10 batteries total for 24V 500Ah) meets this. A second day of autonomy doubles the battery bank to 20 batteries.
4. Size the solar array. 10,800Wh / 4 sun hours = 2,700W of solar panels. Seven 400W panels (2,800W) provide a small buffer.
5. Size the charge controller. 2,800W at 24V battery side = 117A. A 120A or 150A MPPT controller, or two 60A controllers on separate sub-arrays.

Here is the honest math for running a 1500W heater 8 hours per night in a northern US location (3 peak sun hours in December).

Daily energy: 1,500 x 8 = 12,000Wh. With losses: 14,400Wh. Solar needed: 14,400 / 3 = 4,800W — twelve 400W panels. Battery bank for 2 cloudy days: 12,000 x 2 / 0.80 = 30,000Wh at 48V = 625Ah. That is 24 batteries (4 series x 6 parallel of 12V 100Ah LiFePO4) at a cost of roughly $6,000 for batteries alone. Panels add another $3,000-4,000.

This is why most experienced off-grid builders use propane or wood heat instead of electric. A $500 propane heater with a $200 tank covers the entire winter heating season at a fraction of the solar-electric cost. Solar-powered electric heat makes sense in mild climates where you only need 2-3 hours per day, or as a supplement to a primary heat source.

Consider a smaller 750W heater, a 12V diesel heater (200W, far more efficient), or a heat pump (which produces 3W of heat per 1W of electricity) before committing to powering a 1500W resistive heater with solar.