Whole-Home Battery Backup Sizing Calculator

Frequently Asked Questions

Do I need solar to qualify for the federal tax credit on batteries?

The 30% federal ITC applies to batteries charged at least 75% from solar. A standalone battery without solar does not qualify for the residential credit.

How long do home batteries last?

Most LFP batteries carry 10-year warranties for 70-80% capacity retention. Expected calendar life is 15-20 years. Daily cycling depletes the warranty faster than emergency-only use.

Can I back up my central AC with a battery?

Yes, but a 3-ton AC draws 2,000 W running and needs 3,500 W surge. Powering it for 8 hours requires about 19.2 kWh net - roughly 2 full Powerwall units. Most homeowners skip AC backup and cover essentials only.

What happens during a multi-day outage with solar?

Solar panels recharge the battery each day. With 4 kWh of critical loads per day and 8 kWh of solar production, you can sustain essential loads indefinitely even without the grid.

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Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How It Works

Select the critical loads you want to keep running, set your desired backup hours, and indicate whether you have or are adding solar. The tool sums running and surge wattages, sizes the battery capacity and inverter, matches likely products, and estimates cost including any federal tax credit.

Battery capacity needed (kWh) = running watts × hours ÷ 1,000 × 1.2, where the 1.2 is a 20% derate for inverter losses and depth-of-discharge reserve. The inverter is sized to exceed the surge load, rounded up to standard sizes: 5, 7.6, 11.4, or 15 kW.

Whole-Home vs. Critical-Loads Strategy

Backing up the entire home requires 20-40+ kWh for most US households - expensive and often unnecessary. A critical-loads approach instead identifies the 5-8 circuits that matter most (refrigerator, lights, modem/router, well pump, medical equipment) and backs only those up on a dedicated sub-panel. This can cut battery cost by 60-80% while still making an outage livable.

If budget allows whole-home backup, start with a 13.5 kWh system (one Powerwall equivalent) and plan for expansion - most systems accept additional battery modules. A second unit extends runtime or covers HVAC on hot days.

Battery Chemistry and Products

Lithium iron phosphate (LFP): The current standard for residential storage. Long cycle life (3,000-6,000 cycles), no thermal runaway risk, tolerates 80-100% depth of discharge. Products include Tesla Powerwall, Enphase IQ, and FranklinWH.

NMC (nickel manganese cobalt): Higher energy density, slightly lower cycle life and higher thermal management requirements. Used in some older battery products.

Roughly $400-$600 per kWh of installed LFP capacity is a current planning estimate. Get installer quotes - incentives, installation complexity, and regional labor rates vary significantly.

FAQ

Do I need solar to qualify for the federal tax credit on batteries?

The 30% federal ITC applies to batteries charged at least 75% from solar. A standalone battery without solar does not qualify for the residential credit, though it may qualify under separate commercial provisions for businesses.

How long do home batteries last?

Most LFP batteries carry 10-year warranties covering 70-80% capacity retention. Expected calendar life is 15-20 years with proper use. Cycles also matter - a battery cycled daily for backup arbitrage depletes its warranty faster than one used only for emergency backup.

Can I back up my central AC with a battery?

Technically yes, but a 3-ton AC draws 2,000 W running and 3,500 W surge. Powering it for 8 hours requires about 19.2 kWh net, meaning 2 full Powerwall units. Many homeowners skip AC backup and rely on the battery for essentials only.

What happens during an extended multi-day outage with solar?

If you have solar, the panels will recharge the battery each day, potentially sustaining indefinite operation of low-draw critical loads. With 4 kWh of critical loads per day and 8 kWh of solar production, you can run indefinitely even with no grid.

Whole-Home Battery Backup Sizing Calculator

Frequently Asked Questions

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