Inverter Sizing Calculator

Size an off-grid or backup inverter from the running watts it must carry plus the surge watts of your largest motor start.

Loads to run
W
W
Results update as you type — no submit
ResultsLIVE
Minimum continuous rating
1,800 W
suggested class: 2,000 W
Suggested inverter class2,000 W
Surge capability needed4,500 W
Formula
continuous = 1,500 W × 1.2 = 1,800 W min
surge ≥ 4,500 W (largest motor start)
Size the inverter’s own surge rating to clear your largest motor start, not just the running watts. Confirm appliance nameplates and use a pure sine-wave inverter for motors and electronics.
How it works

An inverter turns battery DC into the AC your appliances use, and it has to satisfy two very different demands. The first is continuous power: the sum of the running watts of everything that might be on at the same moment. You never want an inverter working at its rated limit around the clock, so the calculator adds headroom — `20%` by default — which both extends the inverter's life and leaves room to plug in one more thing.

The second demand is surge. Motors in well pumps, compressors, and power tools draw a brief inrush of several times their running watts as they start. The inverter's surge or peak rating must clear the largest single start on your system, because two motors rarely start on the same instant. This tool reports the running rating you need after headroom and the surge rating your inverter must tolerate, then points you at the next standard inverter class.

The numbers are a planning starting point. Real appliance surge varies by model, low battery voltage limits available surge, and a pure sine-wave inverter is worth it for electronics and motors. Check nameplate ratings on your actual equipment before you buy.

Worked example
Running load `1,500 W` · headroom `1.2` · largest surge `4,500 W`
continuous = `1,500 W` × `1.2` = `1,800 W` minimum
next inverter class = `2,000 W`
surge capability required = `4,500 W`
Questions about inverter sizing
Add up the running watts of everything on at once, multiply by about 1.2 for headroom, and pick the next standard inverter above that. A 1,500 W load becomes an 1,800 W minimum, so a 2,000 W inverter fits — provided its surge rating also clears your biggest motor start.
SOURCES — 20% headroom: common off-grid sizing guidance (NABCEP-aligned references, inverter maker manuals)
Estimates for planning only — confirm appliance nameplates before you buy
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