Most battery calculators ask for your electric bill and hand back a number. This one answers the question that actually matters in an outage: can your battery start what you need — and for how long?
No email, no phone number. Every assumption is shown so you can check the math.
Tick everything you'd want powered during an outage. Start with the essentials, then add heavy loads one at a time — that's where batteries usually fail.
We list only models with published datasheet specs. Choose Custom if your quote is for something else — you can read the kWh and kW straight off the spec sheet.
Solar changes everything: a battery that lasts 3 hours at night can keep a house running indefinitely in sunny weather.
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| Item | Running | Startup | Share of battery |
|---|
A battery's headline number is usually its capacity in kilowatt-hours — but capacity tells you how long it lasts, not what it can start. Those are different specifications, and the second one is where batteries quietly fail to deliver.
A central air conditioner drawing 3.5 kW while running can pull 6–8 kW for a second or two when the compressor kicks on. Some batteries simply cannot supply that, no matter how much energy they store. If the surge exceeds the battery's peak output, one of two things happens: the battery shuts down to protect itself, or the A/C never starts.
This is why two batteries with identical capacity can behave completely differently in the same house. It's also why we show the surge check as its own result rather than burying it.
A soft-start kit is the usual fix — it can cut A/C inrush by 50–70%, which is often the difference between "doesn't work" and "works comfortably."
| Result | What it means |
|---|---|
| Runtime | How long the battery lasts with your selection running nonstop. Real runtime is usually longer because appliances cycle. |
| Startup surge ✅ | The battery can supply the worst-case inrush. Everything you ticked can start. |
| Startup surge ❌ | At least one load exceeds peak output. Either drop it, add a soft-start kit, or size up. |
| Continuous load ✅ | Steady-state draw stays within the battery's continuous rating. |
| Solar recharge | Whether daily production covers daily consumption — the difference between a 6-hour reserve and an indefinite one. |
Want the background? Our complete battery storage guide covers sizing, chemistry, and cost, and the model comparison puts specific products head to head.
This calculator is an educational estimate, not an engineering design. Before buying, have a qualified installer perform a load calculation on your actual panel and appliances — especially if you plan to back up a well pump, central A/C, or medical equipment.