Battery bank size calculator (LiFePO4 vs AGM)

List what you run, for how long, and get the amp-hours you use per day plus the battery size for lithium or AGM.

System voltage

Inverter efficiency %for AC devices only

Days without charging

Safety margin %

LiFePO4 usable %

AGM usable %

Daily loads

Pick a preset or "Custom". Leave Watts/Amps blank to use the preset's typical watts. Presets are rough placeholders: enter your device's label rating where you can. Duty % is the share of time it actually draws power (a fridge compressor cycles).

Device	Qty	Watts / amps	Unit	AC/DC	Hours/day	Duty %	Ah/day

Enter values above.

The spreadsheet adds: a 25-row Load Budget that feeds Battery Sizing and Charging automatically, so changing one device updates the whole system. 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

Load budget. For each device: amps = watts ÷ system volts (÷ inverter efficiency if it runs on AC), then Ah/day = amps × quantity × hours × duty cycle. If you enter amps instead of watts, watts = amps × system volts.

Battery size. Minimum rated capacity = daily Ah × days of autonomy × (1 + margin) ÷ usable depth of discharge, rounded up to the next 100 Ah. The defaults (80% usable for LiFePO4, 50% for AGM) are common planning figures: Victron describes lead-acid lasting far longer when kept above 50% and lithium tolerating deep discharge. Use your battery's datasheet values if they differ. Cold weather, high-current loads and battery age all reduce real capacity.

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: Lithium-ion vs AGM batteries (usable depth of discharge). www.victronenergy.com
Victron Energy: Upgrading an RV from lead-acid to lithium. www.victronenergy.com
Victron Energy: Wiring Unlimited (cable, fuse and inverter wiring guidance). www.victronenergy.com