Size a DC-to-AC inverter from your continuous load and largest single starting surge — not the sum of every surge, which oversizes badly.
Your numbers
Leave blank if same as running watts
For the battery-bank estimate
Result
Continuous load is the sum of everything running at once, plus a safety margin. The surge requirement is different: it's the single largest starting surge that can occur while everything else keeps running — not the sum of every appliance's surge. Motors and compressors can draw 2–4x their running watts for a fraction of a second at startup; sizing for the sum of all surges at once oversizes the inverter for no reason, since appliances essentially never all start in the same instant.
continuous = Σ(running watts × qty, for continuously-running appliances) × (1 + margin)
surge = max over appliances of (starting watts + everything else still running)
A 12V RV with a fridge (150 W running, 600 W starting), LED lighting (60 W), and an occasional-use well pump (750 W running, 2,200 W starting):
In practice, appliances rarely all start in the exact same instant — and even if two motors did start together, most real-world loads are staggered by a user turning things on one at a time. Sizing for the sum of every possible surge at once produces a wildly oversized (and expensive) inverter for a scenario that essentially never happens. Sizing for the worst single simultaneous start — the largest surge occurring while everything else you're already running keeps running — is the actual worst case your inverter has to survive, and the standard, defensible way to size for it.
Some sizing guides and calculators add up every appliance's starting watts as if a fridge, a well pump, and every other motor on the circuit could all surge at the exact same instant. That's a real number, but not a real scenario — it produces an inverter sized for a moment that essentially never happens in practice, at real added cost. FigureDesk instead finds the single largest surge that can occur while everything else you're already running stays running, which is the actual worst case the inverter needs to survive.
If you don't know it, enter the same value as running watts — the calculator will treat it as a purely resistive load with no surge. Motors, compressors, and pumps typically have a meaningfully higher starting than running wattage; check the appliance's nameplate or manufacturer spec sheet if precision matters.
No. This calculates the inverter size only. Conductor sizing and overcurrent protection depend on ampacity and code requirements this calculator does not check — consult the Wire Gauge Calculator for conductor properties, and a qualified electrician for a code-compliant installation.
Size the battery bank that will feed this inverter.
Check how long your battery bank will run this inverter's load.
Convert your inverter's AC output current requirements.
Once you know the DC input current, look up conductor sizing properties.
Compare against a fuel-powered generator sized for the same loads.
FigureDesk results are advisory and do not replace a licensed electrician, a qualified engineer, or the applicable local electrical code. Verify any electrical work against your local code and with a qualified professional before relying on it.