Generator backup sizing for an off-grid battery bank
A backup generator has to do two things at once on a cloudy stretch: recharge your battery bank in a reasonable window and run your loads while it does. This sizes the generator for both, and estimates the fuel per recharge so you can price it with your own local rate.
How the generator size is worked out
The generator feeds a battery charger, which converts AC to DC at about 90% efficiency. To put your chosen energy back over the recharge window, the charger draws roughly that energy divided by the hours, divided by efficiency. The generator then has to supply that charger draw plus whatever loads are running at the same time, with about 25% headroom for surge and derating. A shorter recharge window needs a bigger generator; a longer window lets a smaller one do the job.
Why fuel quantity, not cost
Fuel prices vary by region and change constantly, so this tool estimates the quantity of fuel a recharge run uses — based on typical generator fuel consumption per kWh produced — and leaves the price to you. Multiply the quantity by your local rate for a cost you can trust, rather than a stale built-in number.
Sizing sensibly
Generators are most efficient at around 50–75% of rated load, so a wildly oversized generator just burns fuel at idle. A unit sized to recharge the bank while carrying your real loads, run in solid blocks rather than trickling, is the most fuel-efficient backup. To size the bank and array this generator supports, use the full system calculator.
Frequently asked questions
What size generator do I need to charge my off-grid batteries?
Size it for the charger draw plus any loads running at the same time, with headroom. For example, replacing 6 kWh in a 5-hour window needs about 1,300 W of charger draw; add a 500 W load and 25% headroom and a 2.5 kW generator is appropriate. A shorter recharge window needs a larger generator.
How much fuel does it take to recharge an off-grid battery bank?
It depends on the generator and fuel, but a rough guide is about 0.08 gallons of gasoline, 0.06 of diesel, or 0.11 of propane per kWh of electricity produced. Multiply by the energy your run produces, then by your local fuel price for the cost.
Is a bigger backup generator always better?
No. Generators are most efficient at 50–75% load, so an oversized unit wastes fuel running lightly loaded. Size it to recharge the bank while carrying your loads, and run it in solid blocks rather than at idle.
Looking for a definition or a quick answer? See the solar & battery glossary for terms used on this page, or the FAQ for common sizing questions. For the full explanation, read the generator backup sizing guide.
Sources & standards
- NFPA 70 — NEC Article 702, Optional Standby Systems — Transfer-equipment and load-calculation rules for optional standby/backup power, relevant to whole-home battery backup.
- NFPA 70 — National Electrical Code (NEC), Articles 690 & 705 — Governs PV source/output circuits (Art. 690) and interconnected power production sources (Art. 705) in the US.
Educational estimate. Actual sizing depends on your charger's real output, altitude and temperature derating, and the generator's quality. Follow the manufacturer's ratings and safe outdoor operation. See the methodology.