How long will a Powerwall last in an outage?

A 13.5 kWh battery at a 20% reserve and 10% losses runs a 1 kW average load for about 9.7 hours. Your load is the deciding input; enter it above.

How many batteries do I need to run my whole house?

Divide the energy you want to cover (average kW × hours) by usable kWh per battery, then check the combined kW rating covers your largest loads starting together.

Can solar recharge my battery during an outage?

Only if the system is designed to island: the inverter must disconnect from the grid and keep the panels running. Ask your installer; a grid-tied system without that capability shuts down in an outage.

What uses the most battery in an outage?

Electric heat, central air, electric water heaters, ranges and well pumps. Leaving them off is usually the cheapest way to add hours.

HyreSolar tools

Home battery runtime calculator

For anyone asking how long a home battery will keep the lights on in an outage, with or without solar recharging it.

Checked September 3, 2026. Data as of October 22, 2026. HyreSolar sells and installs nothing.

This home battery runtime calculator gives an estimate from your own numbers in under a minute, with every assumption listed under the result.

A 13.5 kWh battery with a 20% reserve and 10% losses runs a 1 kW average load for about 9.7 hours.

Halve the load to essentials and it runs about 19.4 hours; add panels that recharge it and the outage can last days, or indefinitely in good sun.

13.5 kWh is the median battery capacity installed with residential solar in 2025, according to Lawrence Berkeley National Laboratory’s 2026 data update, so it is a fair starting point if you have not chosen a battery. Runtime depends far more on your load than on the battery.

A refrigerator, lights, internet and a gas furnace fan average well under 1 kW; electric heat, an electric range or central air can average several kilowatts and drain the same battery in a few hours.

The reserve matters too. Most batteries let you hold back a share of capacity for outages and use the rest daily for bill savings.

Only the share above the reserve at the moment the grid fails is available, which is why the reserve is an input.

If you set a high backup reserve, the battery has more left when the outage starts.

Outage runtime

How long the battery carries your load.

Example values The filled-in numbers are an example, not your home. Type over them with your own.

— Battery-only runtime

— With solar recharging

— Energy available

— Your load per day

How this result was worked out

Working it out from the numbers above.

Energy only. Confirm the battery’s kW rating covers your largest loads.

HyreSolar does not sell batteries and names no brand in this model.

What the result means

Runtime is energy ÷ average load. It assumes the load averages out over the outage; short spikes do not matter for runtime as long as the battery’s power rating can carry them.

The power rating is a separate check. A battery can hold enough energy and still trip if a well pump and an air conditioner start together. Ask the installer for the continuous and surge kW figures.

With solar, the result is the days until the battery is empty on days like the one you entered. Cloudy days produce far less.

How to use it

Where each input comes from, so nothing is a guess.

1. Enter usable battery capacity

kWh usable, not nameplate.

Where to find it: The battery datasheet. Our battery sizing calculator works it the other way round.

2. Enter the backup reserve

The share kept back for outages, or the charge you expect when the grid fails.

Where to find it: Your battery app settings.

3. Estimate your average load in kW

Daily kWh ÷ 24 for the circuits you will run.

Where to find it: A summer or winter bill: monthly kWh ÷ 730 gives whole-home average kW.

4. Add daily solar recharge

kWh your panels make on a typical day. Leave at 0 for battery only.

Where to find it: Your monitoring app, or annual production ÷ 365.

Assumptions

1
Available energy
Capacity × (1 − reserve) × (1 − losses). Losses default to 10%, an allowance for inverter and discharge losses, not a measured figure for any product.
2
Battery only
Available energy ÷ average load = hours.
3
With solar
Daily deficit = load × 24 − solar × (1 − losses). Days = available energy ÷ deficit. If solar covers the load, it lasts on days like that indefinitely.

The formula, in full. hours = kWh × (1 − reserve) × (1 − losses) ÷ load kW. With solar: hours = energy ÷ (load × 24 − solar kWh/day × (1 − losses)) × 24.

How the answer moves

One input changed at a time. Every figure is computed by the calculator on this page.

Situation	Battery only	With solar
Base case13.5 kWh, 20% reserve, 1 kW average	9.7 hours	No solar entered
Essentials only, 0.5 kW	19.4 hours	No solar entered
Heavy load, 2 kW	4.9 hours	No solar entered
Reserve set to 0%	12.2 hours	No solar entered
Two batteries, 27 kWh	19.4 hours	No solar entered
Solar recharges 12 kWh/day	9.7 hours	17.7 hours
Solar recharges 30 kWh/day	9.7 hours	Indefinitely on days like this: solar covers the load

Cutting the load does more than adding a second battery for the same money. Solar recharge is what turns hours into days.

Next step

Price the battery on solar battery cost or whole-home battery backup cost, and weigh it against the alternative on battery vs generator. If you export under net billing, the net billing calculator shows what the same battery earns day to day.

To have an installer size a battery for your circuits, send your result with the form under the calculator.

Embed this calculator

Free to use on any site, including news and installer pages. Paste this where you want the calculator to appear. Keep the credit line: it is the only condition.

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<p><a href="https://hyresolar.com/tools/backup-runtime-calculator/">Home battery runtime calculator</a> by HyreSolar</p>

What this calculator leaves out

The result is a planning estimate. A real proposal accounts for these; this page does not.

Does not check the battery’s kW power rating against motor starting loads.
Assumes a steady average load and a steady solar day.
Cold temperatures reduce usable capacity; not modeled.
Losses are a flat allowance, not a product specification.

Questions this calculator answers

How long will a Powerwall last in an outage?: A 13.5 kWh battery at a 20% reserve and 10% losses runs a 1 kW average load for about 9.7 hours. Your load is the deciding input; enter it above.
How many batteries do I need to run my whole house?: Divide the energy you want to cover (average kW × hours) by usable kWh per battery, then check the combined kW rating covers your largest loads starting together.
Can solar recharge my battery during an outage?: Only if the system is designed to island: the inverter must disconnect from the grid and keep the panels running. Ask your installer; a grid-tied system without that capability shuts down in an outage.
What uses the most battery in an outage?: Electric heat, central air, electric water heaters, ranges and well pumps. Leaving them off is usually the cheapest way to add hours.

Sources and methodology

Figures dated October 22, 2026. Last reviewed September 3, 2026 by the HyreSolar research desk, which writes and checks every tool on this site.

U.S. distributed solar and storage 2026 data update (reported) (Mercom, reporting Lawrence Berkeley National Laboratory, retrieved October 22, 2026). Median residential battery capacity 13.5 kWh in 2025. Secondary report; LBNL’s own page was not reachable when checked.

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