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 can draw 5-8x their running current at start (NEMA code letters; see our locked-rotor amps calculator) while generator makers quote 2-3x "starting watts" for compressor loads, resistive heating elements reach full power almost instantly. Cited to NEMA MG 1. The standard text is not freely available to link; confirm against the current edition before acting. This makes heaters easier on inverters than their wattage suggests. Champion's generator wattage chart lists a space heater at 750-1,500W running watts with no starting watts, and notes: "These appliances do not require extra starting watts. Size based on running watts only."

That said, you still want an inverter rated above 1,500W continuous. An inverter's continuous rating falls as it gets warmer: Victron's datasheet prints each model's continuous output at 25°C and again, lower, at 40°C (the figures below).

This calculator adds a 25% margin on the continuous load: 1,500W x 1.25 = 1,875W. That is the top of the margin Renogy's inverter sizing guide gives: "add a margin of 20% to 25% power to the final answer." It is a margin for unplanned loads, not a surge allowance, so pick an inverter whose continuous rating is at least 1,875W. Victron's datasheet for its 1200 VA to 5000 VA inverters (230 VAC output) prints the continuous output at 25°C as 1,000W for the 1200 VA models, 1,300W for the 1600 VA models and 1,600W for the 2000 VA models, all below 1,875W. Its 3000 VA models print 2,400W, the first above 1,875W. At 40°C the same datasheet prints 900W, 1,200W, 1,450W and 2,200W for those four sizes. A larger inverter leaves room for other loads you run at the same time. Current is power over voltage: at 12V, 1,875W draws 1,875 / 12 = 156A from the battery bank, before the inverter's own losses. That is a large current that needs very thick cables. At 24V the same output draws 78A, and at 48V 39A. Our inverter cable sizing guide walks through the gauge selection step by step for each of these system voltages.

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 at the heater. The battery feeds the heater through the inverter, so divide by the inverter's efficiency: at 94%, 9,000 / 0.94 = 9,574Wh a day comes out of the battery.
2. Size the inverter. 1,500W x 1.25 = 1,875W continuous, a 25% margin on the load. Victron's 3000 VA inverter prints 2,400W continuous at 25°C, the first model in its 1200 VA to 5000 VA datasheet above 1,875W. A heater is a simple resistive load, so the waveform is not a requirement for it: AIMS says "Modified sine only for simple resistive loads", and Xantrex says a modified sine wave inverter can power heaters. AIMS gives pure sine for "sensitive electronics, motors, and mixed loads", so choose it if the same inverter will run those too.
3. Size the battery bank. For 1 day of autonomy at 80% DoD, with LiFePO4 at its 25.6V nominal: 9,000 / (25.6 x 0.80 x 0.94) = 468Ah. Five parallel strings of two 100Ah 12V LiFePO4 batteries in series (10 batteries, 24V 500Ah) cover it. A second day of autonomy doubles the energy the bank holds: the engine's bank at 2 days and the same inputs is 9,000 x 2 / (25.6 x 0.80 x 0.94) = 935Ah, or 20 batteries.
4. Size the solar array. Each 400W panel makes 400 x 4 sun hours x (1 - 0.14) x 0.98 (charge controller) x 0.94375 (panel heat) = 1,273Wh a day. 9,574 / 1,273 = 7.52, so eight 400W panels (3,200W).
5. Size the charge controller. 3,200W / 24V = 133A on the battery side. This divides by the system's 24V nominal voltage, not the bank's 25.6V nominal, because the lower voltage gives the larger current (3,200W / 25.6V is 125A). So the controller, or two controllers on separate sub-arrays, must handle at least 133A between them.

Here is the honest math for running a 1500W heater 8 hours per night in a northern US location in December. This example takes 3 peak sun hours a day. NASA POWER prints a December average of 1.90 kWh per square metre per day for Burlington, Vermont, on a panel tilted at the site's latitude, which is 1.90 peak sun hours, so a site that far north gets less than this example assumes.

Daily energy: 1,500 x 8 = 12,000Wh, or 12,000 / 0.94 = 12,766Wh from the battery through a 94% efficient inverter. Each 400W panel makes 400 x 3 x (1 - 0.14) x 0.98 x 0.94375 = 954Wh a day, so the array needs 12,766 / 954 = 13.4, or fourteen 400W panels (5,600W). One day of battery on a 48V LiFePO4 bank (51.2V nominal) at 80% DoD: 12,000 / (51.2 x 0.80 x 0.94) = 312Ah, and each further cloudy day you want covered adds another 312Ah. Price that bank and array from your own quotes before you commit.

Propane or wood heat takes its energy from the fuel, not from the battery, so it needs no bank or array of this size. Solar-powered electric heat fits short run times, or a supplement to a primary heat source: the Mild Climate preset (3 hours a day, 5 peak sun hours, 24V LiFePO4) needs a 234Ah bank and four 400W panels.

Consider a smaller 750W heater, a 12V diesel heater, or a heat pump before committing to powering a 1500W resistive heater with solar. A diesel heater burns fuel for its heat and draws only a small electrical load for its fan and pump, which its own datasheet prints. A heat pump delivers more heat than the electricity it draws, by the coefficient of performance its datasheet prints.