Interactive battery tool

Battery Sizer

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

Methodology

How this calculator works

Estimated storage starts with critical load in kilowatts, runtime hours, and desired autonomy days.
The result is adjusted for usable depth of discharge, inverter efficiency, and reserve margin so the number is closer to usable battery capacity.
Large motor loads, HVAC startup surge, grid recharge planning, and local code requirements should be checked by a qualified installer.

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.

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.

Once you know the battery capacity range, model installed cost, incentives, and lifetime electricity savings to complete your planning.

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

How this calculator works

Estimated storage starts with critical load in kilowatts, runtime hours, and desired autonomy days.
The result is adjusted for usable depth of discharge, inverter efficiency, and reserve margin so the number is closer to usable battery capacity.
Large motor loads, HVAC startup surge, grid recharge planning, and local code requirements should be checked by a qualified installer.

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.

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.

Once you know the battery capacity range, model installed cost, incentives, and lifetime electricity savings to complete your planning.

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