List what you'll run at the same time and get the continuous watts, the peak surge for motor startup, and the DC current your battery has to supply.
Load presets
Appliances running at the same time
Appliance
Running W
Qty
Startup surge
A planning estimate. Real startup surges vary by model — use the nameplate locked-rotor amps (LRA) when you have it. Size for the worst case that can actually happen at once, and prefer a pure sine wave inverter for motors and electronics.
Sizing an inverter is not about adding up every device you own. It's about two numbers for the worst moment that can realistically happen at once:
Continuous rating — the running watts of everything on at the same time, plus a 20–30% margin so the inverter isn't pinned at 100% (which shortens its life and runs it hot).
Surge rating — enough headroom for the biggest motor to start while everything else is already running.
The surge that catches people out
Motors and compressors draw a brief inrush of current at startup — often 3 to 7 times their running watts — to overcome inertia. A fridge that runs at 150 W can demand 600 W for the first second. If your inverter's surge rating can't cover that spike, it trips every time the compressor kicks in, even though the running load is tiny.
The realistic worst case isn't "everything starts at once." It's everything running normally while your single largest motor starts:
Peak surge = (running watts of all loads) + (largest single startup surge above its running watts)
This calculator applies a surge multiplier per appliance, finds the single largest startup, and adds it on top of the running total — the same way installers size for the worst case without absurdly oversizing.
Typical startup surge by load type
Load
Surge (× running watts)
Resistive — heater, kettle, lights, electronics
1× (no surge)
Light motor, power drill
~2×
Washing machine, dishwasher, furnace blower
~2.5×
Refrigerator / freezer compressor
~3×
Large fridge, small window AC
~4×
Well pump, deep freezer
~5×
Air conditioner, air compressor
~6×
Don't forget the DC side
The inverter pulls all that power from your battery as DC current:
DC amps = AC watts ÷ (battery volts × inverter efficiency)
A 2,000 W inverter on a 12 V bank draws around 185 A — which needs very thick cable (about 1/0 AWG) and a large fuse. The same inverter on 48 V draws about 46 A. This is the main reason larger systems move to 24 V or 48 V.
A worked example
A backup setup runs a fridge (150 W, ~3× surge), some lights (40 W) and a wifi router and laptop (80 W) — all resistive except the fridge:
Continuous load: 150 + 40 + 80 = 270 W
With 25% margin → recommended continuous ≈ 340 W → a 600 W inverter
Surge: fridge startup adds (3 − 1) × 150 = 300 W on top of 270 W running = 570 W peak
A 600 W pure sine inverter (≈1,200 W surge) handles both comfortably
Add a well pump and the surge — not the running watts — is what forces a much bigger inverter.
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Frequently asked questions
What size inverter do I need?
Add up the running watts of everything you'll run at once, then add 20–30% — that's your continuous rating. Separately, check the peak surge: running watts plus the extra startup surge of your single largest motor. The inverter's surge rating must cover that peak. Size for the worst simultaneous case, not every appliance you own.
What's the difference between continuous and surge watts?
Continuous watts is the power an inverter delivers indefinitely. Surge (peak) watts is the much higher power it can supply for a few seconds to start motors and compressors. A motor can draw 3–7× its running watts at startup, so an inverter that's fine on continuous power can still trip if its surge rating is too low.
How much does a motor surge at startup?
It depends on the load. Resistive loads (heaters, kettles, lights) have essentially no surge. A fridge or freezer compressor surges to roughly 3× its running watts; a well pump about 5×; an air conditioner or air compressor up to 6×. Use the nameplate locked-rotor amps (LRA) when you have it for the most accurate figure.
Why does my inverter draw so many DC amps?
Because DC amps = AC watts ÷ (battery volts × inverter efficiency). A 2,000 W inverter on a 12 V bank pulls roughly 185 A, which needs thick cable and a large fuse. The same inverter on 48 V draws about 46 A. High-power systems run at 24 V or 48 V largely to keep this current manageable.
Should I get a pure sine wave inverter?
For any permanent installation, and for motors, compressors, microwaves and sensitive electronics, use a pure sine wave inverter. Modified sine units are cheaper but can buzz, run motors hot, and confuse some chargers and medical devices. Pure sine is the safe default.