What size battery backup do I need?

Sizing whole home battery backup comes down to two decisions: which circuits you want to keep running, and how long you want them to last. Everything else follows from those. The mistake worth avoiding is picking a number of kilowatt-hours first and working backwards.

The short answer

A 20 kWh system covering essentials — fridge and freezer, lights, Wi-Fi, a window air conditioner — runs roughly 10 to 14 hours. Add central air conditioning and that can drop to a few hours. If you want central air through a long outage, you are looking at a substantially larger system, or a generator alongside the battery.

Two numbers, not one: kWh and kW

Most sizing advice talks only about capacity, which leaves out half the problem.

kWh is storage. It sets how long the system runs. A 20 kWh battery supplying a steady 2 kW load lasts about ten hours.

kW is output. It sets how much you can run at the same time. A battery with a 3.6 kW inverter cannot supply a 5 kW load no matter how full it is — the load simply will not run.

These are sized separately, and a mismatch is a common and expensive mistake. A large battery behind a small inverter runs a few things for a long time. A small battery behind a large inverter runs plenty of things briefly. You want both matched to what you actually plan to power.

Step one: pick the loads

Rough running figures, useful for building a total. Actual draw varies by appliance and age.

Add up what you want running simultaneously and you have your kW requirement. Multiply a realistic average draw by the hours you want and you have your kWh requirement.

The surge figure most guides skip

Anything with a motor draws several times its running wattage for a moment when it starts. A sump pump running at 800 watts can spike past 2,000. A fridge compressor does the same on a smaller scale. Central air conditioning is the worst offender.

If your system cannot supply that surge, the appliance will not start — and stored capacity is irrelevant to that failure. This is why a well-sized battery backup setup is specified on peak power, not just energy, and why two systems with identical kWh ratings can behave very differently in the same house.

Sizing for outages vs. sizing for savings

These are different calculations and they produce different answers.

Backup sizing follows your essential loads and how long you want them to survive. It is driven by the worst night you can imagine.

Savings sizing on a time-of-use rate follows how much electricity you use during the expensive window. The battery needs to carry your household through that window without buying from the grid. Sizing to cover a Dominion Rate 7 peak works through that math specifically.

Households often find the savings number is the larger of the two, and that backup capability comes along for free once they have sized for the bill.

Leave yourself headroom

Sizing exactly to your calculated minimum leaves no slack for a hotter-than-usual day, an appliance you did not account for, or a rate change that widens the peak window. Undersized battery backup fails precisely when it matters, and adding capacity after the fact is more expensive than buying it up front.

This is one argument for modular systems — you can start smaller and add batteries later as your usage or budget changes.

For a starting range built from your own usage rather than averages, run your bill through the analyzer. You set the final size around what you want to keep running.