Home › Generators & home backup
Home backup battery size calculator
Size a whole-home or critical-loads backup battery: kWh needed for your outage length, how many battery units, continuous kW and surge, with optional solar recharge.
Results
| Usable battery capacity needed | 11.4 kWh |
|---|---|
| Battery units needed | 2 × 10 kWh |
| Critical-load energy per day | 10.23 kWh |
| Continuous power if all on | 1.93 kW |
| Peak power incl. largest motor start | 3.4 kW |
- Check each battery's continuous and peak kW ratings. Several units in parallel usually add their power ratings. Motor starts such as well pumps and AC compressors may need a soft-starter.
- Home batteries must be installed by a licensed installer with a critical-loads panel or whole-home transfer equipment, under local code and utility rules.
About this calculator
Whole-home batteries are sold in units of roughly 10-15 kWh, and the real question is how many you need. That depends on which circuits you back up. A fridge, freezer, internet, lights and a furnace fan might use 10 kWh a day. Add central air conditioning or an electric range and it can easily be 30-60 kWh.
This calculator sizes a backup battery from a list of critical loads. It accounts for conversion losses and optional solar recharging during the outage, and gives the continuous and peak power the system must deliver, which matters as much as storage.
How to use it
- List the loads you'd want during an outage, with realistic hours per day. Cycling appliances use their 24-hour average watts.
- Choose how long an outage you want to ride through on battery alone.
- If you have solar that keeps running in an outage, enter its realistic daily output for the season.
- Enter the usable kWh of the battery model you're considering to see how many units you need.
The formula
Daily energy = Σ watts × quantity × hours ÷ 1000 (kWh).
Battery energy = daily energy ÷ efficiency × days. With solar: (daily ÷ efficiency − solar per day) × days, but never less than 60% of one day's need.
Units = battery energy ÷ usable kWh per unit, rounded up.
Worked example (default values)
Inputs
| Critical loads and hours per day | Refrigerator (full size, average draw): 60 W, 24 h/day; Chest freezer (average draw): 40 W, 24 h/day; Wi-Fi router + modem: 15 W, 24 h/day; LED light bulb: 9 W × 4, 6 h/day; TV (50-inch LED): 100 W, 4 h/day; Sump pump (1/3 HP): 800 W, 2 h/day; Furnace blower (1/2 HP): 875 W, 6 h/day |
|---|---|
| Outage length to cover | 1 days |
| Solar recharge during the outage (optional) | 0 kWh/day |
| Usable capacity of one battery unit | 10 kWh |
| Battery-to-AC efficiency | 90 % |
Results
| Usable battery capacity needed | 11.4 kWh |
|---|---|
| Battery units needed | 2 × 10 kWh |
| Critical-load energy per day | 10.23 kWh |
| Continuous power if all on | 1.93 kW |
| Peak power incl. largest motor start | 3.4 kW |
Tips and common mistakes
Trim the list before buying more batteries. Replacing an old fridge, using LED lighting and leaving electric resistance heat off the backup panel can halve the battery you need. The appliance energy cost calculator shows which loads dominate.
Gas furnaces still need electricity for the blower and controls. Include them, especially in cold climates.
Frequently asked questions
How many kWh do I need to power a house for a day?
The average US home uses roughly 25-30 kWh a day, but essential loads only (fridge, freezer, lights, internet, furnace fan) are often 5-12 kWh.
How long will a 13.5 kWh battery last?
Running essential loads of about 10 kWh/day, roughly 1-1.3 days. With solar recharging in sunny weather, it can last indefinitely for those loads.
Battery or generator for home backup?
Batteries are silent, switch over instantly and need no fuel, but they're costly to size for multi-day outages. Generators handle long outages and big loads cheaply. Many homes combine a small battery for short outages with a generator for long ones.
Related calculators
- Generator size calculator
Add up running and starting watts for your fridge, sump pump, furnace, well pump and more to find the generator size you need for a power outage.
- What size power station do I need?
List your devices and how many hours a day you run them to get the battery capacity (Wh), continuous output and surge rating your power station needs.
- Home solar payback calculator
Estimate home solar payback years, 25-year savings and ROI with production, rate increases, degradation and incentives. Updated for the 2026 tax-credit changes.
- Generator vs power station cost calculator
Compare the multi-year cost of a gas generator against a battery power station for your outage or camping use, including fuel, electricity, maintenance and cost per kWh.
More generators & home backup calculators
Estimates for planning only, based on the assumptions shown. Real-world performance varies with equipment, temperature and installation. Updated October 19, 2026. See our methodology.