Battery backup duration calculator

Two questions decide what backup feels like, and most calculators only answer one. This one tells you whether the battery can start your loads at all, and then how long it runs them.

Battery backup duration

Choose what you want running and adjust the wattages to match your own appliances. The defaults are typical figures, not measurements of your home.

Loads to keep running
Your battery

Can it run these loads?

Yes — within continuous output

Your selection draws up to 380 W at once. This battery configuration delivers 11,500 W continuously.

Approximate runtime

68.2 hours

Based on an average draw of 198 W across 13.5 kWh usable. Solar recharging during daylight extends this, and is not included.

Duty cycles are applied to each load, since appliances such as refrigerators and air conditioners cycle rather than drawing continuously. Starting surges are not modelled here and can exceed continuous output even when the running total does not. This is a planning tool, not a load calculation.

How to read the result

The first panel is an output question. If your selected loads draw more at once than the battery can deliver, adding capacity will not help — you need more units, fewer loads, or a soft starter on the largest motor.

The second is a capacity question. It uses your average draw, applying a duty cycle to loads that cycle rather than run continuously, which is why a refrigerator does not consume its nameplate rating all night.

Neither models the surge when a compressor starts. That surge is handled by a battery’s peak rating and is the moment undersized systems typically fail, so treat a marginal result as a reason for a load calculation rather than as a pass.

Sources

What each source supports on this page, and when we last verified it. Every source we use, with its freshness policy, is listed in the source register.

  1. Powerwall 3 Datasheet

    Tesla

    Powerwall 3 energy capacity, continuous output, and warranty term.

    Verified August 16, 2026

  2. IQ Battery 5P data sheet

    Enphase Energy

    IQ Battery 5P usable capacity, continuous and peak output, chemistry and warranty.

    Verified August 16, 2026

  3. Helio41 estimate methodology, version 0.1

    Helio41

    How Helio41 sizes an indicative system, what it models, and what it deliberately does not claim.

    Verified August 16, 2026

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