Solar Battery Backup Runtime Calculator

Estimate usable battery energy, backup runtime, and recommended battery-bank size from critical load, target duration, depth of discharge, efficiency, and solar recharge.

  1. Enter critical load Input expected backup load in watts from your critical-loads panel estimate.
  2. Set target outage duration Enter required backup hours for your design scenario.
  3. Enter battery characteristics Provide nameplate kWh, allowable depth of discharge, and round-trip efficiency.
  4. Add solar recharge and calculate Enter expected solar recharge kW during outage and calculate runtime plus recommended bank size.

How long will a 13.5 kWh battery run my house?

It depends on your actual backup load and usable energy after depth-of-discharge and efficiency losses. This tool calculates runtime from those real inputs.

What is usable battery capacity versus nameplate capacity?

Usable capacity is nameplate kWh multiplied by allowed depth of discharge and round-trip efficiency assumptions, which is always lower than nameplate.

Can rooftop solar recharge batteries during an outage?

Yes when the system is configured for islanding and backup operation. The calculator subtracts recharge power from load to estimate net discharge rate.

Should I size for whole-home backup or critical loads only?

Most projects size to a critical-loads panel first because whole-home backup can require much larger inverter and battery capacity.

Why does motor startup still trip backup systems?

Starting current for compressors and pumps can be several times running watts. Include surge capability checks in addition to steady-state energy sizing.

Is daily cycling hard on backup batteries?

Cycle life depends on chemistry, depth of discharge, and thermal conditions. The daily-cycle estimate helps gauge how aggressively the bank is being used.

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