Inverter Surge Capacity Calculator

An inverter has two ratings: continuous power (what it can supply indefinitely) and surge power (what it can supply for a few seconds during motor startup).
Sizing only for the continuous load is the most common off-grid mistake.
A 2,000 W continuous inverter looks fine on paper for a 1,500 W load list — until the well pump kicks on and tries to pull 4,500 W for half a second, and the inverter trips.

The math:

continuous_required = sum of all loads running simultaneously surge_required = largest_motor × surge_multiplier + other_running_loads

Surge multipliers for common motor loads (multiply running watts by this for inrush current):

• LED lights, electronics, resistance heaters: 1× (no surge)
• Universal motors (drills, vacuums, blenders): 2-3×
• Refrigerator compressor: 3-4×
• Window AC (5,000-15,000 BTU): 3-5×
• Well pump (1/2 to 1 HP): 5-7×
• Submersible pump (deep well): 6-9×
• Air compressor: 5-8×
• Power tools with hard start: 4-6×

A worked example.
RV setup with: 80 W TV (running), 40 W fan (running), 200 W mini-fridge running 600 W on startup, 1,500 W microwave (intermittent, not concurrent with fridge startup).
Continuous load with everything except microwave: 80 + 40 + 200 = 320 W.
Surge load when fridge starts: 600 + 80 + 40 = 720 W.
A 1,000 W continuous, 2,000 W surge inverter handles this comfortably.
For the microwave (1,500 W continuous, no significant surge), the inverter must handle that running on its own — so the continuous rating needs to be 1,500 W minimum, not 1,000.

Pure sine wave versus modified sine wave matters.
Modified sine inverters (cheaper) are fine for resistive loads (heaters, lights) and most motors will run on them, but they cause humming, overheating, and reduced efficiency in induction motors and many electronics.
Anything with a microprocessor (CPAP, sensitive lab gear, modern TVs, variable-speed motors) needs pure sine wave.
Pure sine costs 30-60% more for the same wattage but is the only correct choice for a backup or RV inverter that has to handle mixed loads.

Three details people miss.
Battery sag at high current: a 12V battery bank delivering high peak current drops well below 12V, so a 2,000 W inverter may shut down on low-battery alarm at the moment a pump starts even if the pump is within rated surge.
Cable size between battery and inverter must be sized for surge current, not continuous: undersized cable creates voltage drop that triggers low-input shutoffs.
And inverter efficiency is 85-92% at typical loads but can drop to 60-70% at very low loads — running a 3,000 W inverter to power a 50 W laptop wastes more energy in idle losses than the laptop uses.

For most RV and small off-grid setups, a 2,000-3,000 W pure sine inverter with 4,000-6,000 W surge is the sweet spot.
For homes running well pumps and AC, 5,000-8,000 W continuous with 12,000+ surge is more typical.