Sizing a backup generator for an off-grid solar system
A generator paired with a solar-battery system has a different job than a generator meant to run a house directly, and sizing it like the latter usually means an oversized, inefficient, unnecessarily loud generator for the job it's actually doing.
Size the generator to comfortably exceed your charger's rated charging current plus any household load running at the same time — its job is usually recharging the battery bank, not running the house directly, so fuel consumption per recharge cycle matters more than peak wattage alone.
The generator's real job: recharge the bank, not run everything directly
In a solar-plus-battery system, the generator's primary role during a bad stretch is usually recharging the battery bank through the charger/inverter, with the battery still handling instantaneous load swings and motor-starting surges the way it does normally. This is a fundamentally different sizing target than "big enough to directly run the whole house," which is how standalone backup generators (with no battery buffer) have to be sized.
Sizing around charger capacity
The generator needs to comfortably exceed the combined draw of your charger(s) at their rated charging current, plus whatever household load will realistically run at the same time the generator is on. Oversizing the generator relative to charger capacity doesn't charge the battery any faster — the charger's rated current is the real ceiling — it just means a bigger, thirstier generator running under-loaded, which is also harder on the generator itself over time.
Fuel-per-recharge, not just wattage
Because the generator's job is usually "put a specific amount of energy back into the bank," the more useful sizing question is often fuel consumption per recharge cycle, not peak wattage alone — a generator that's technically powerful enough but burns fuel inefficiently at partial load defeats some of the point of pairing it with solar in the first place. The generator backup sizing calculator estimates fuel per recharge from your bank size and charger rate directly.
When a bigger generator does make sense
If the generator will also need to run substantial household load directly during an extended outage (not just recharge the battery), size it against that combined charger-plus-household figure instead — effectively treating it as a standalone backup generator that happens to also recharge the bank when it runs.
How this changes your autonomy sizing
A reliable generator backup is also a legitimate reason to size fewer battery autonomy days than a system with no fallback at all — the generator covers the tail of a bad stretch instead of the battery having to carry all of it alone.
Related tools
Sources & standards
- NFPA 70 (NEC) Articles 690 & 706 — PV source circuits and energy storage systems.
- IEEE 1013 — Recommended Practice for Sizing Lead-Acid Batteries for Stand-Alone Photovoltaic Systems — general sizing-practice reference.
- UL 1741 — inverter safety and interconnection standard.
Educational content, not an electrical design. Verify with your equipment's datasheet and a licensed installer for anything permanently wired. See the methodology.