Battery Size for Outage Calculator
Enter the kWh your essential loads use per day and how many days you want covered, and get the nominal battery kWh, the number of units and an estimated installed cost.
Battery size for outages
D05Scenario A
Battery units needed
About $27,000.00 installed. Without solar, a battery only lasts as long as what it stored.
- Nominal kWh needed
- 22.2
- Battery units
- 2
- Estimated installed cost
- $27,000.00
Estimate. Formula and sources are under “How it works”. Change any input; the result updates as you type.
2026 note: The federal 25C and 25D tax credits ended for installations after Dec 31, 2025. Our numbers use 2026 rules. State rebates, HEAR/HOMES programs and utility programs may still apply.
Formula
How it works
- Nominal kWh needed = daily kWh × days ÷ 0.9 ÷ depth of discharge
- Units = nominal kWh ÷ kWh per unit, rounded up
- Estimated cost = units × kWh per unit × installed $ per kWh
The 0.9 covers inverter and round-trip losses. Without solar, a battery only lasts as long as what's stored in it: nothing refills it during a multi-day outage. With panels, sunny days recharge it.
No federal credit in the cost. The federal 25D credit ended for systems placed in service after Dec 31, 2025, so the estimate is the installed price you enter. Some states run battery rebates, such as California's SGIP; subtract an approved rebate from the total yourself. The cost is an estimate; every input is editable.
Sources: installed cost of $900–$1,300 per kWh in 2026 from EnergySage; ~90% round-trip efficiency; California SGIP as an example of a state battery program.
Worked example
A worked example
Covering 30 kWh for one day after 10% losses takes 33.3 kWh of nominal capacity. At 13.5 kWh per unit that rounds up to 3 units, or 40.5 kWh installed, for an estimated $38,475.00 at $950 per kWh.
Questions
FAQ
How much battery for 3 days off grid?
At 10 kWh a day, 3 days need 33.3 kWh nominal: 3 units of 13.5 kWh, about $40,500.00 at $1,000 per kWh. Without solar recharging, cutting the daily load is cheaper than adding units.
Battery without solar: does it make sense?
For short outages, yes: one 13.5 kWh unit covers 10 kWh of critical use for one day (11.1 kWh nominal needed). For outages of several days, the stored energy runs out. A standby generator or solar recharging is what extends it. Compare costs in the Battery vs Generator calculator.
Why does the unit count jump?
Units are whole. 22.2 kWh needs 2 units of 13.5 kWh (27 kWh) but 5 units of 5 kWh (25 kWh), which is $25,000.00 at $1,000 per kWh.
Lithium vs lead acid?
This calculator assumes lithium home batteries, which are rated for deep discharge. Lead-acid banks usually shouldn't be discharged as deeply; enter a lower depth of discharge if you're sizing one.
Related