How the inverter size is calculated
Add up everything that can realistically run at the same time to get the continuous load, and estimate the worst momentary peak — the largest motor start on top of whatever else is running. Both figures get a safety margin, giving the continuous rating the inverter needs and the surge it must survive.
For example, 800 W of simultaneous loads with a compressor start peaking at 2,000 W and a 25% margin needs 1,000 W continuous and 2,500 W surge. A common 1,500 W inverter with typical 2× surge capability (3,000 W) covers both.
Why the datasheet still matters
Surge capability varies widely — some inverters hold 2× their rating for seconds, others only briefly or not at all, and high-frequency models tolerate motor starts worse than low-frequency ones. Ambient temperature also derates continuous output. Treat the suggestion as a shortlist filter, then verify the actual surge specification and duration.
The DC side is the dangerous side
Inverter loads look small on the AC side but large on the battery side: watts ÷ (voltage × efficiency) means a 1,000 W load on 12 V pulls roughly 93 A. That is why higher system voltages are used for bigger inverters, and why cable cross-section and fusing are sized from this current — the calculator shows it for the recommended rating.
Methodology
The calculator UI, the appliance table and the displayed formula all call the same calculation function. Suggested sizes come from a fixed list of common ratings and assume roughly 2× surge capability. To size the battery feeding the inverter, see the Battery Bank Sizing calculator; to check how long it lasts under load, see Battery Runtime.
- Motor start-up draw of 2–6× running power — NEMA MG-1 motor locked-rotor code letters and appliance manufacturer start-up specifications
- Surge capability ≈ 2× continuous rating — Typical pure-sine inverter datasheets (e.g. Victron Phoenix, Renogy, Growatt) — always verify the model's own surge spec and duration
- 20–25% sizing safety margin — Common electrical design practice, mirroring the ~125% continuous-load factor used in wiring codes such as the US NEC
- Battery-side current = watts ÷ (voltage × efficiency) — Ohm's law and inverter datasheet efficiency figures
Assumptions last reviewed on 26 August 2026. If a source disagrees with your battery or equipment datasheet, trust the datasheet.
Frequently asked questions
Why do motor loads need surge capacity?
Compressors, pumps and power tools draw 2–6 times their running power for a moment at start-up, and the inverter must survive that peak.
Why not simply buy the biggest inverter available?
Oversized inverters waste more idle power around the clock and cost more. Sizing for real loads with a modest margin is more efficient.
Why does the calculator show a battery-side current?
A 1,000 W inverter on a 12 V system can draw over 90 A from the battery, and DC cables and fuses must be rated for that current.