Inverter size calculator (with surge)

Add up what runs at the same time, allow for start-up surge, and see how many amps the inverter pulls from the battery.

System voltage

Lowest battery voltage (V)≈ low-voltage cutoff; worst case current

Inverter efficiency %

Headroom %don't run at 100% continuously

Inverter you're considering (W)blank = suggested size

AC devices

Surge watts = brief start-up draw (motors, compressors, blenders). Leave blank if none. Untick devices that never run at the same time as the others.

Device	Running W	Surge W	Qty	At once?

Enter values above.

The spreadsheet adds: no inverter or surge sizing (that's free here only), but it applies inverter efficiency to every AC load in the Load Budget and sizes the inverter cable and fuse on the Wire & Fuse tab. Free vs. spreadsheet

Safety: this is a planning estimate, not an engineering sign-off. DC systems can start fires and lithium batteries can deliver very high fault current. Check every result against the manufacturer's instructions and current ABYC/NEC or local rules, put a correctly rated fuse or breaker close to the battery on every positive cable, and have a qualified installer check work you are not sure about. We are not responsible for how results are used.

How this is calculated

Continuous. Add the running watts of everything that can be on at once, then add headroom (25% default) so the inverter isn't run flat out. We round up to the next common size. A pure sine wave inverter is the safe choice for laptops, chargers and anything with a motor.

Surge. Motors and compressors briefly draw several times their running power. Surge needed = running total + the largest single start-up excess. Inverter datasheets list a short-term peak rating (Victron's, for example, are roughly twice continuous); make sure it exceeds this.

DC side. Battery current = AC watts ÷ (lowest battery voltage × efficiency). We size the fuse at 125% of the current at the inverter's rated output, because the cable must survive the inverter running flat out. If the inverter manual specifies a fuse and cable, use those. Then size the battery cable with the wire-gauge tool at 3% drop.

Planning a whole build?

The paid spreadsheets run these same tested formulas for a whole build in one saved file. Compare free vs. spreadsheet.

12V Load & Wire Gauge Calculator spreadsheet on Etsy, $9.00

Overland Rig Planning Bundle (12V + Payload + Trip spreadsheets) on Etsy, $18.99

Sources

Victron Energy: inverter range and datasheets (continuous vs peak ratings). www.victronenergy.com
Victron Energy: Wiring Unlimited (cable, fuse and inverter wiring guidance). www.victronenergy.com
Blue Sea Systems: DC main circuit protection (fuse placement and sizing). www.arrid.com.au