Battery Backup Calculator
Calculate the battery capacity a home backup needs for the loads you pick, or how long a battery you already have will run them, with the surge and inverter limits.
By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Battery needed
- 7.4 kWh
- 500 W for 12 hours, after 90% depth of discharge and 90% inverter efficiency
- Units to buy
- 1 x 13.5 kWh unit
- 13.5 kWh usable installed, and 5 kW of continuous output
- Continuous load
- 500 watts
- 6 kWh over the 12 hours you asked for
- Start surge
- 500 watts
- No motor load entered, so the surge equals the running load
- Planning estimate
- Electrician sizes it
- Always consult a professional here: a licensed electrician sizes the transfer equipment and the local inspector governs the install
Battery Backup formula
Continuous load (W) = Refrigeration + Pump or furnace + Lights and outlets + Other
Energy (kWh) = Load (W) x Hours / 1,000
Battery needed (kWh) = Energy / (Depth of discharge x Inverter efficiency)
Runtime (h) = Usable kWh x Depth x Efficiency x 1,000 / Load (W)Worked example
A house on a well wants the fridge, the pump, lights and a sump pump through a 24 hour outage: 150 + 900 + 200 + 400 = 1,650 running watts. Over 24 hours that is 39.6 kWh of energy.
At 90 percent depth of discharge and 90 percent inverter efficiency the battery has to hold 48.9 kWh, which is four 13.5 kWh units. The well pump is the other problem: it surges to about 2,700 W on start, so on top of 750 W of other loads the inverter sees roughly 3,450 W for a moment, past a single 5 kW unit only when several loads start together.
Quick reference
| Load | Typical setup | Runtime |
|---|---|---|
| 300 W | Fridge, LED lights, internet | 36.5 hours |
| 500 W | Essentials with a few outlets | 21.9 hours |
| 1,000 W | Essentials plus a furnace blower | 10.9 hours |
| 1,650 W | Essentials plus a well pump | 6.6 hours |
| 3,000 W | Most of a small house | 3.6 hours |
| 5,000 W | At the inverter's continuous limit | 2.2 hours |
Two numbers, not one
A battery is bought on two specifications and homeowners usually only look at one. Kilowatt hours decide how long it runs; kilowatts of continuous output decide what it can run at all. A 20 kWh pack behind a 5 kW inverter will keep a fridge going for two days and still refuse to start a well pump alongside a microwave.
Start from the loads. Write down the running watts of everything that has to stay on, using average draw rather than the nameplate, and add them. Multiply by the hours of outage you are planning for to get kilowatt hours of energy, then divide by the depth of discharge and about 90 percent for inverter losses. That is the pack size. Then check the surge separately.
Surge, and what not to back up
Anything with a motor pulls roughly three times its running watts for a second or two at start. A 900 watt submersible pump asks for about 2,700 at that instant, and if the fridge compressor happens to kick in at the same moment the inverter sees more still. A soft starter on the pump or the air conditioner cuts that surge substantially and is far cheaper than a second battery.
Resistance heating is the other trap. A water heater element at 4,500 watts, a range element, a clothes dryer or baseboard heat will empty any residential pack in a couple of hours. Leave them off the critical loads panel and back up the fridge, the freezer, lights, internet, the furnace blower and any medical equipment instead. That list usually lands between 400 and 900 watts.
What this calculator leaves out
The transfer equipment and the critical loads panel, which is often as much work as the battery itself, and the electrical service upgrade a large system may need. It does not model solar recharging during an outage, which depends on the inverter type and on rapid shutdown being wired correctly, or temperature derating in an unconditioned garage. Permits, utility interconnection agreements, and the specific installation clearances in NEC 706.10 also sit outside it.
Code limits that apply
- NEC Article 706
Energy storage systems: disconnects, listing, working space and marking
If you miss it: An unlisted DIY pack fails inspection and voids the homeowner's insurance after a fire
- NEC 706.10
Installation location limits; not in a habitable room or an unrated closet
If you miss it: A pack mounted in a bedroom closet is refused and has to be relocated to a garage or exterior wall
- NEC 706.15
A readily accessible disconnecting means for all ungrounded conductors
If you miss it: No way for a firefighter to isolate the pack, and the inspector red-tags the install
- NEC 702.4(B)
Optional standby capacity has to carry the loads the transfer equipment selects
If you miss it: A whole-home switch on an undersized battery trips as soon as two motors start together
- NEC 690.12
Rapid shutdown where a PV array charges the battery
If you miss it: A roof array that stays energized in an outage is a hazard and fails the PV inspection
Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.
Mistakes that cost money
- Sizing on kWh alone. A 20 kWh pack with a 5 kW inverter still cannot start a well pump and a range at the same time.
- Using nameplate watts off the appliance label instead of average running draw; a fridge cycles and averages far less.
- Backing up resistance heat. A 4,500 W water heater element empties a 13.5 kWh battery in under three hours.
- Forgetting the critical loads panel. Whole-home backup needs the transfer equipment and the panel work, not just the battery.
- Assuming solar refills the battery in an outage; without the right inverter and rapid shutdown setup the array shuts down with the grid.
- Placing a battery in a bedroom or an unrated closet. NEC 706.10 limits where energy storage can be installed.
Key facts
- Battery capacity is quoted as usable kWh, which is already net of the reserve the manufacturer holds back.
- Inverter conversion costs about 10 percent, so a 13.5 kWh unit delivers roughly 12.2 kWh of household energy at full depth.
- Motor loads draw roughly three times their running watts for a second or two at start, which is what trips an undersized inverter.
- Continuous output, typically 5 to 11.5 kW per unit, limits what can run at once no matter how many kilowatt hours the pack holds.
- Most US grid outages last under 8 hours, but storm and wildfire outages routinely run 24 to 72 hours.
Frequently asked questions
How big a battery do I need to back up my house?
For essentials at about 500 running watts through a 12 hour outage, roughly 7.4 kWh, which one 13.5 kWh unit covers with room to spare. Adding a well pump and stretching to 24 hours pushes it near 49 kWh, or four units.
How long will a 13.5 kWh battery run a house?
About 22 hours on 500 watts of essentials, 11 hours at 1,000 watts with a furnace blower, and under 4 hours at 3,000 watts. Divide the usable kWh by the load in kilowatts and subtract about 10 percent for inverter losses.
Can a home battery run an air conditioner or a well pump?
It depends on the inverter, not the capacity. Both surge to roughly three times their running watts on start, so a 900 W pump asks for about 2,700 W momentarily. A soft starter or a second unit solves it.
Should I back up the water heater?
No, unless the system is very large. A 4,500 watt element empties a 13.5 kWh pack in under three hours. Resistance heat, ranges and dryers belong off the critical loads panel.
Sources and references
Next steps for this project
The usual order for a electrical & plumbing project. See all electrical & plumbing calculators.
Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.