Work out how many kWh of home battery you need to keep chosen appliances running through an outage, and how many battery units that takes. Edit the appliance list, choose days of backup and see the capacity, unit count and peak load.
Results update as you type. Without JavaScript you see the worked example below.
A solar battery size calculator adds up the daily kWh of the appliances you want during an outage, multiplies by the days of backup, then allows for usable capacity and losses. With the defaults, essentials use 5.7 kWh a day and need about 6.7 kWh of battery, so one 13.5 kWh unit covers a one-day outage.
You get the capacity needed, the number of battery units, backed-up use per day and peak load. A donut chart shows each appliance's share of daily energy, and a load-list schedule (watts, hours, kWh per day, with totals) can be downloaded as CSV. The scenario table reruns your list for 1, 2 and 3 days of backup.
Daily kWh = sum of (watts × hours) ÷ 1,000 for each appliance
Peak kW = sum of watts of every appliance you included ÷ 1,000
Energy needed = daily kWh + (daily kWh − solar recharge) × (days − 1)
Capacity needed = energy needed ÷ usable share ÷ √(round-trip efficiency)
Units for energy = round up (capacity needed ÷ unit kWh)
Units for power = round up (peak kW ÷ unit kW)
Battery units = the larger of the two (at least 1)Assumptions that match the code:
Default inputs: the default appliance list, 1 day, no solar recharge, 90% usable, 90% round trip, 13.5 kWh and 5 kW per unit.
The default scenario table shows 6.7 kWh (1 unit) for one day, 13.4 kWh (1 unit) for two days and 20 kWh (2 units) for three.
Second example, a storm-season home: the default list plus a CPAP (60 W for 8 hours), a gas furnace blower (500 W for 8 hours) and a sump pump (800 W for 2 hours), with 2 days of backup.
| Backup plan (example 2 loads) | Capacity needed | 13.5 kWh units |
|---|---|---|
| 1 day | 13.8 kWh | 2 |
| 2 days, no solar recharge | 27.6 kWh | 3 |
| 2 days, 8 kWh/day solar recharge | 18.2 kWh | 2 |
| 3 days, no solar recharge | 41.4 kWh | 4 |
Note how 13.8 kWh just misses one 13.5 kWh unit. Trimming a load, such as fewer TV hours, can save a whole unit.
The result is a target for usable storage, not a shopping list. Check it against quotes: a proposal for one battery when your list needs 27 kWh, or three batteries when you need 7, deserves questions. To see how long a specific battery lasts at a given load, use the battery backup runtime calculator. Our home battery backup guide covers whole-home versus partial-home backup, and battery vs generator compares costs.
This is an educational estimate. It uses average watts, ignores motor starting surges and battery aging, and assumes the battery is full when the outage starts. Real usable capacity, inverter limits and backup wiring vary by product and installer, so confirm sizing with a load calculation in a written proposal.
It depends on what you keep running. With the default list (refrigerator, lights, internet, phone charging, TV and a little microwave use), you need about 6.7 kWh for one day, which one 13.5 kWh unit covers. Adding a CPAP, gas furnace blower and sump pump for two days raises it to about 27.6 kWh, or three units.
Sometimes, but it takes a lot of capacity. Running a 3,500 W central AC or heat pump for 8 hours a day on top of the default loads uses about 33.7 kWh a day, which needs about 39.5 kWh of battery, or three 13.5 kWh units, for a single day. Motor starting surges may also need a soft starter or a larger inverter.
Two adjustments. Most batteries cannot use 100 percent of their rated capacity, so the tool divides by the usable share, 90 percent by default. It also divides by the square root of round-trip efficiency, about 0.95 at 90 percent, to cover losses on the discharge side.
Peak load is the total watts if everything you picked runs at once. Each battery unit can only deliver so much continuous power, 5 kW by default. If peak load exceeds what your units can deliver, the tool adds units even when energy alone would not require them.
Only if the system is designed for it. Many grid-tied systems shut down during an outage for safety unless the inverter and battery can form an island and keep the panels running. If yours can, enter a realistic daily recharge; the first day is always covered from storage alone. Use 0 if unsure.
LBNL reports the median home battery installed with solar in 2025 was 13.5 kWh, which is the calculator's default unit size. About 37 percent of new home solar systems in 2025 included a battery, up from 25 percent in 2024.
Most are measured in hours, but hurricanes, ice storms and wildfire-prevention shutoffs can last days. Choose days of backup based on your area's history and whether anyone at home depends on powered medical equipment.
Look for a battery system listed to UL 9540, installed according to NFPA 855 and your local fire and building codes. Inverters that connect to the grid are typically listed to UL 1741. Your installer and permit inspection should confirm these.
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