How Long Will a Home Battery Last in an Outage?

“How long will it last?” has no single answer, and any installer who gives you one without asking what you intend to run is guessing. But you can work it out yourself in about five minutes, and it is worth doing before you are quoted rather than after.

The calculation

Runtime, in hours, is roughly the battery’s usable capacity in kWh divided by your average draw in kW while the grid is down.

Note usable. Most batteries reserve a portion of their nameplate capacity and do not discharge to zero, so the usable figure is lower than the badge. Ask for the usable number specifically.

And note average draw, not peak. Your fridge cycles; your furnace fan cycles; your lights are on for part of the evening. What matters is the average over the outage.

Typical loads, so you can add up your own

Rough figures for a New York house, which you should replace with your own appliances’ actual ratings:

• Refrigerator — small, but it runs continuously and it is the single most common reason people want backup.
• LED lighting — negligible. Lighting is almost never what drains a battery.
• Internet and phone charging — negligible.
• Gas or oil boiler controls and circulator pump — modest, and the difference between a warm house and a frozen one in a New York January.
• Well pump — modest energy, but a large starting surge. This is a power-output problem, not a capacity one.
• Sump pump — same profile as a well pump.
• Central air conditioning — very large. Do not assume it is backed up.
• Electric range, electric dryer, electric water heater — very large. Usually excluded from a backup panel deliberately.
• Electric vehicle charging — larger than everything else combined. Never on a backup panel.

A critical-loads backup — fridge, some lights, boiler controls, internet, a few outlets — is a modest continuous draw, and that is why most New York installations are designed that way. A whole-house backup including air conditioning or an electric range is a different order of magnitude entirely.

Solar changes the answer completely

This is the part people miss. A battery on its own runs down and stops. A battery paired with solar and the right switchgear recharges every day the sun comes up, and can carry a house through a multi-day outage almost indefinitely provided the daily generation covers the daily draw.

That is a genuinely different proposition from a battery alone, and it is the strongest argument for buying storage together with solar rather than separately.

Two caveats that matter in New York:

• Winter generation is a fraction of summer generation. A system that easily carries a house through a July outage may barely dent a January one, and January is when the outages happen.
• Snow-covered panels generate nothing. The outage that follows an ice storm is precisely the outage during which your array is under ice.

Cold and battery performance

Lithium batteries lose capacity in the cold, and how much depends on whether the unit is installed indoors, in an unheated garage, or outdoors, and on whether it has active thermal management.

Ask two specific questions: what is the rated operating temperature range, and does the unit heat itself in cold conditions? An outdoor installation in Watertown and one in a heated basement in Brooklyn are not the same product decision.

How to size it honestly

1. Decide what must stay on. Write the actual list — this is the whole exercise.
2. Add up the continuous draw from the appliances’ own rating plates.
3. Identify the surges — well pump, sump pump, furnace blower — and check them against the battery’s power output, not its capacity.
4. Decide the target duration. Overnight is a different battery from three days.
5. Ask whether solar will recharge it, and what that looks like in December rather than June.

An installer who works through those five points with you is designing a system. One who quotes a model number is selling one.

Frequently Asked Questions

How do I calculate home battery runtime?

Divide the battery's usable capacity in kWh by your average draw in kW during the outage. Ask for the usable capacity specifically, because most batteries reserve part of their nameplate figure and do not discharge to zero.

Can solar recharge a battery during a power cut?

Yes, with the right switchgear to island the house — and it changes the proposition entirely, because the battery refills each day rather than simply running down. In New York, note that winter generation is a fraction of summer, and snow-covered panels generate nothing.

Why will my battery not run the air conditioning?

Central air conditioning, electric ranges, electric dryers, electric water heaters and EV charging are very large loads, usually excluded from a backup panel deliberately. What limits them is often the battery's power output rather than its capacity.

Does cold weather reduce home battery capacity?

Yes, and how much depends on where the unit is installed and whether it has active thermal management. Ask for the rated operating temperature range and whether the unit heats itself — an outdoor installation upstate is a different decision from a heated basement downstate.

This article is general information, not tax, legal or financial advice. Incentive amounts, eligibility rules and deadlines change and depend on your utility, your tax situation and your property. Confirm current terms with NYSERDA, your utility and a qualified tax professional before making a decision.