Can the Battery Sizer design my final backup system?

No. It gives a planning estimate. An installer still needs to verify circuits, inverter output, surge loads, code requirements, and battery compatibility.

Should I include air conditioning?

Only if you truly need it backed up. HVAC loads can make storage systems much larger and more expensive.

What is reserve margin?

Reserve margin is extra capacity added so the estimate is not sized exactly to the minimum calculated load.

Interactive solar tool

Battery Sizer

Estimate battery storage capacity for critical loads, backup runtime, reserve margin, and usable depth of discharge.

Inputs

Backup load assumptions

Size storage around the loads you actually need during an outage.

Methodology

How this calculator works

The Battery Sizer helps turn a backup wish list into a practical storage estimate. Instead of starting with a battery brand, it starts with critical loads, runtime, autonomy days, depth of discharge, inverter efficiency, and reserve margin.

What the calculator estimates

Usable battery capacity needed for the entered critical loads.

Reserve-adjusted storage so the estimate is closer to real-world capacity.

Runtime sensitivity when load size or autonomy days change.

Inputs to verify before buying storage

Large motor loads and HVAC equipment may need surge-power checks beyond kWh capacity.

Whole-home backup usually requires a more complex design than selected critical circuits.

Solar recharge during an outage depends on inverter and battery configuration.

Use the estimate with the Solar Battery Backup Planning Guide to prepare better questions for storage quotes.

Formula used

Critical-load energy = critical load kW x runtime hours.
Autonomy energy = critical-load energy x backup days.
Required nominal storage = (autonomy energy + reserve margin) / depth of discharge / inverter efficiency.

Key inputs defined

Critical load: The appliances and circuits you want to keep running during an outage, such as refrigeration, lighting, internet, or medical devices.
Runtime hours: The number of hours those selected loads need to run before grid power returns or solar recharge is available.
Autonomy days: The number of outage days you want the battery system to support before needing a full recharge.
Depth of discharge: The share of a battery's nameplate capacity that can be used without exceeding the recommended discharge limit.
Inverter efficiency: The conversion loss between stored DC battery energy and usable AC power for household loads.

Model the storage economics

Once you know the battery capacity range, compare how storage changes your solar payback and long-term project economics.

Calculate total savings

Plan storage around real critical loads

Use the storage guide to decide which loads deserve backup power and where installer design checks are still required.

Read battery buying guide

How this calculator works

What the calculator estimates

Usable battery capacity needed for the entered critical loads.

Reserve-adjusted storage so the estimate is closer to real-world capacity.

Runtime sensitivity when load size or autonomy days change.

Inputs to verify before buying storage

Large motor loads and HVAC equipment may need surge-power checks beyond kWh capacity.

Whole-home backup usually requires a more complex design than selected critical circuits.

Solar recharge during an outage depends on inverter and battery configuration.

Use the estimate with the Solar Battery Backup Planning Guide to prepare better questions for storage quotes.

Formula used

Critical-load energy = critical load kW x runtime hours.
Autonomy energy = critical-load energy x backup days.
Required nominal storage = (autonomy energy + reserve margin) / depth of discharge / inverter efficiency.

Key inputs defined

Critical load: The appliances and circuits you want to keep running during an outage, such as refrigeration, lighting, internet, or medical devices.
Runtime hours: The number of hours those selected loads need to run before grid power returns or solar recharge is available.
Autonomy days: The number of outage days you want the battery system to support before needing a full recharge.
Depth of discharge: The share of a battery's nameplate capacity that can be used without exceeding the recommended discharge limit.
Inverter efficiency: The conversion loss between stored DC battery energy and usable AC power for household loads.

Model the storage economics

Once you know the battery capacity range, compare how storage changes your solar payback and long-term project economics.

Calculate total savings

Plan storage around real critical loads

Use the storage guide to decide which loads deserve backup power and where installer design checks are still required.

Read battery buying guide