Home Battery Backup Calculator
How long will a home battery power your house? Size it by the loads you want to keep on.
A 13.5 kWh home battery runs a typical essentials load (about 1 kW: fridge, lights, Wi-Fi, and phones) for roughly 13 hours. Running a whole-home load with AC or electric heat (~3.5 kW) drops that to about 4 hours. Backup time equals usable battery capacity divided by your load.
How long a battery lasts comes down to one ratio: usable kilowatt-hours divided by the kilowatts you draw. This calculator lets you pick an essentials, partial, or whole-home load, or enter your own, and shows the backup runtime for one or more batteries.
It uses usable capacity (not nameplate), since batteries hold back a little to protect their cells. Add daily solar recharge and a battery paired with panels can run essentials almost indefinitely.
How we calculate this
Divide usable battery capacity by your average load in kilowatts to get backup hours. Solar recharge each day offsets part of the load, extending runtime.
What affects your cost
Essentials vs whole-home
Backing up only a critical-loads subpanel, fridge, lights, outlets, internet, lasts far longer than powering the whole house through the main panel.
Usable capacity
Rated (nameplate) capacity is higher than what you can actually use. A 13.5 kWh battery delivers close to its full rating on modern LiFePO4 chemistry; older packs less.
Big surge loads
Central AC, electric heat, and well pumps spike instantaneous draw to 4–6 kW and can exceed a single battery’s continuous output. Two or three batteries handle them.
Solar recharge
Without solar, runtime is fixed by capacity. With panels that recharge each day, a battery can run essential loads through a multi-day outage.
Common questions
How long will a home battery power my house?+−
Divide usable capacity by your load. A 13.5 kWh battery running a 1 kW essentials load (fridge, lights, internet) lasts about 13 hours; running a 3.5 kW whole-home load, about 4 hours. Big surge loads like AC shorten it further.
How many kWh do I need to back up my house?+−
For essentials only, fridge, lights, phones, internet, most homes need about 5–10 kWh per day. For whole-home backup including HVAC, plan on 20–40+ kWh per day, which usually means two or more batteries.
How long does a Tesla Powerwall last during an outage?+−
A Powerwall holds about 13.5 kWh usable. On a light essentials load (~1 kW) it can run 13+ hours; on a heavy whole-home load (~4 kW), closer to 3 hours. Solar recharging can extend it much further.
What is the difference between whole-home and essentials backup?+−
Whole-home backup powers everything through the main panel and needs more capacity. Essentials (critical-loads) backup powers a chosen subset, fridge, lights, outlets, internet, through a small subpanel, so it lasts far longer on the same battery.
Can a home battery back up my house without solar?+−
Yes. A grid-charged battery stores electricity when the grid is up and discharges during an outage. Without solar it cannot recharge mid-outage, so its runtime is fixed by its kWh capacity and your load.
How many batteries do I need for whole-home backup?+−
A typical whole home draws 25–30 kWh per day, so two to three 13.5 kWh batteries give roughly a day of full backup, or several days if you run essentials only. Homes with AC or electric heat need more.
Is a home battery worth it just for backup power?+−
For frequent, long outages a battery beats a gas generator on noise, maintenance, and fumes, but it costs more upfront. If outages are rare, a smaller essentials battery or a portable generator is often the better value.
Runtime uses usable (not nameplate) capacity divided by your average continuous load. Load presets: essentials ≈ 1 kW, partial ≈ 2 kW, whole-home ≈ 3.5 kW, adjust to your appliances. Surge loads (AC, well pumps) can exceed a single battery’s output. Estimates only. Full methodology →