Home Battery Backup Size Calculator
Enter the essential electrical load you want to keep running during an outage and how many hours of backup you want, and this calculator estimates the battery capacity you need, how many battery units that works out to, and how long the installed pack would last at different load levels. It sizes essential-load backup, not whole-home backup.
Backup goal
A common home wall battery is 13.5 kWh.
Battery units needed
2 × 13.50 kWh
24.00 kWh needed · 27.00 kWh installed
Installed capacity
27.00 kWh
Usable capacity
27.00 kWh
Runtime at different loads
Backup runtime vs load
How it works
The energy a battery must store is your continuous load multiplied by the hours you want to run it. Backing up a 2 kW essential load for 12 hours needs 24 kWh delivered. Because batteries cannot always be drained to empty, that figure is divided by the usable depth of discharge to get the nameplate capacity required — modern lithium iron phosphate (LFP) home batteries are often rated for close to 100% usable, while some chemistries are limited to around 80%. So needed capacity = load × hours / depth of discharge.
Battery count is the needed capacity divided by the capacity of a single unit, rounded up, since you can only buy whole batteries. A common residential wall battery holds about 13.5 kWh, so a 24 kWh need rounds up to two units and 27 kWh of installed capacity. The calculator then reports usable capacity (installed × depth of discharge) and a runtime table showing how many hours the pack lasts at half your load, your load, and heavier loads — useful because real outage draw fluctuates as appliances cycle.
This is essential-load sizing: it assumes you back up a chosen subset of circuits (refrigerator, lights, internet, a few outlets, maybe a furnace fan) rather than the entire home. Whole-home backup during a multi-day outage needs far more capacity, and surge loads — well pumps, air conditioners, and anything with a motor that spikes at startup — require enough instantaneous power output, not just energy, which is a separate spec an installer sizes. Treat this as a planning baseline before a professional load assessment.
Frequently asked questions
What counts as my essential load?+
Essential load is the set of circuits you decide must stay powered during an outage, expressed as the average continuous kilowatts they draw. A typical essentials list — refrigerator, some LED lighting, phone and laptop charging, internet router, and perhaps a gas furnace's blower — often averages somewhere between 0.5 and 2 kW. Add up the running wattage of the specific devices you want backed up and convert to kilowatts (divide watts by 1,000). Leave out big intermittent loads like electric ranges, dryers, and central AC unless you specifically intend to back them up, since they dramatically increase the battery size required.
Why does depth of discharge matter?+
Depth of discharge (DoD) is the fraction of a battery's rated capacity you can actually use without harming its lifespan. If a 13.5 kWh battery has a 100% usable rating, you get the full 13.5 kWh; at 80% DoD you would only get 10.8 kWh of usable energy from the same nameplate. Because you need a certain amount of usable energy to cover your load, a lower DoD means you must install more nameplate capacity to deliver it. This calculator raises the required nameplate capacity by dividing by DoD, so the installed pack actually meets your backup goal rather than falling short.
Can a battery back up my whole house?+
It can, but usually not for long, and it costs a lot more. Whole-home backup means the battery must cover every load your home might draw, including heavy and surge loads, for the full duration of an outage — which for a multi-day event can require many battery units plus a way to recharge them (solar or a generator). Most homeowners instead back up an essentials subpanel, which this calculator sizes. If you want whole-home backup, size the battery to your home's actual peak and daily energy use, and have an installer verify the inverter's continuous and surge power ratings can handle your largest motor loads.