Pick the circuits you want alive during an outage and get a recommended generator size, battery bank, and transfer-switch rating — computed live as you go.
How this is calculated
Each load has running watts (steady draw) and surge watts (the spike a motor pulls the instant it starts, often 2–4× running). Fridges, freezers, pumps and furnace blowers surge; LEDs, routers and most electronics don’t.
Peak load isn’t the sum of every surge number — loads don’t all start at once. It’s total running watts plus the single biggest surge-minus-running gap (usually the well pump) — that’s the worst instant a generator or inverter actually has to survive.
Battery bank size is total running watts × hours, divided by usable depth of discharge — about 80% for lithium (LiFePO4), 50% for lead-acid, so a lead-acid bank needs roughly 60% more capacity for the same runtime.
Transfer-switch amps is total running watts divided by panel voltage (240V split-phase in the US, 230V single-phase in Europe) — the continuous current the switch and its wiring must be rated for.
- All selected loads are treated as running the entire backup window — a worst-case estimate. Pumps and furnace fans actually cycle, so real battery runtime is usually longer than shown.
- Generator and switch sizes round up to the nearest common off-the-shelf size, not the raw math — you can’t buy a 5.28kW generator.
- Defaults are typical values, not your house — edit any wattage field to match your own appliance labels.
Want the full walkthrough first? Read what size generator to back up a whole house.
What we’d actually buy
Generator kW on one side, battery kWh on the other — these are the pairings we’d pick from. Picked on merit from what’s on the market right now — street prices, checked July 2026.
- Honda EU2200i — 2.2kW inverter generator, 48 dB — the essentials-only benchmark that starts every time. $1,000–1,200 · View
- Westinghouse iGen8200TFc — 8.2kW-peak tri-fuel (gas / propane / natural gas) inverter unit — panel-scale backup with clean output. $1,100–1,500 · View
- EcoFlow DELTA Pro 3 — the silent half of a hybrid setup — battery carries the night, generator only runs to recharge. $2,700–3,000 · View
Some links above may earn Grid CEO a referral fee at no cost to you — disclosure.
- Honda EU22i — 2.2kW inverter generator, the quiet-start benchmark. €1,300–1,600 · View
- EcoFlow DELTA Pro 3 — the silent half of a hybrid setup — battery carries the night, generator only recharges it. €2,800–3,500 · View
Some links above may earn Grid CEO a referral fee at no cost to you — disclosure.
Keep these numbers
Email me my spec
Your results above, in your inbox — plus a heads-up when the hardware in it moves in price.
The next step
Size it first. Then buy it once.
Run your loads through the free calculators, then take the result to the gear lists — makers linked direct, no forms, no sales calls.
Open the calculators See the gear
