Home Battery Backup Sizing Calculator

Reviewed for 2026, updated September 8, 2026.

Estimates how many home battery units you need, and what they cost, to keep essential circuits running through an outage of a given length.

Your estimate

Battery units required...

Total installed cost...

Runtime at your average load...

Installed cost per usable kWh...

Estimates only. Assumptions are listed below, and you can change every input.

Battery quotes tend to arrive as a product name and a price, with little explanation of how the number of units was chosen. Too few and a two-day outage ends with a dark house on the second morning; too many and you have paid a five-figure premium for capacity that mostly sits idle. This calculator sizes the bank from the two things that actually matter in an outage: how much energy your essential circuits consume over time, and the largest load they draw at once.

Energy sets one requirement: the average draw of your backed-up circuits times the hours you want to ride through, divided by the usable capacity of one unit. Power sets another: your peak simultaneous load divided by the continuous output of one unit. The larger of the two, rounded up, is the number of units. Cost, total usable capacity and the runtime you actually get follow from that count. It is a planning figure to test against a quote, not an engineering design for your electrical panel.

How to use this tool

List the circuits you want backed up and estimate their average draw and their peak simultaneous draw in kilowatts.
Choose the outage length you want to cover, then enter the usable capacity, continuous output and installed price of the battery unit in your quote.
Compare the recommended unit count and cost per usable kWh with the proposal, and adjust the load or hours to see how sensitive the count is.

What the math assumes

The unit count is the larger of the energy-based requirement (average load times hours) and the power-based requirement (peak load divided by continuous output), rounded up to whole units.
No solar recharging during the outage is assumed, so runtime is the bank draining from full with nothing coming in.
Usable capacity is taken as fully deliverable; reserve settings, cold-weather derating and cell aging are not applied.
Motor start-up surges are not modeled; if a well pump or air conditioner starts hard, check the unit surge rating separately.
Cost per usable kWh divides total installed price by total usable capacity and includes no incentives or financing.

Frequently asked questions

How do I estimate the average load on my backup circuits?

Add the running wattage of each essential appliance and scale by how often it runs; a refrigerator cycles perhaps a third of the time, while lighting and networking run steadily. A plug-in energy monitor on the major loads for a week gives a far better number than guessing.

Why does the calculator want more units than my energy need suggests?

Because your peak load exceeds what one unit can deliver continuously. Batteries have a power limit as well as an energy limit, and a large well pump or central air can require a second unit purely for power even when one has enough stored energy.

Can I count on solar to recharge the battery during an outage?

In daylight with a compatible inverter, yes, and that can extend runtime substantially. The calculator leaves it out deliberately so the result covers the worst case of a storm outage with little sun; treat any solar contribution as a margin on top.

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