Solar & battery
Solar Battery Runtime Calculator
How long a battery runs a given load, after depth of discharge and losses.
Result
Depth of discharge is how much of the nameplate you may actually use. Lithium iron phosphate tolerates 80% to 90%; lead acid should not go past 50% if you want it to last. Round-trip efficiency then takes another 5% to 15% on the way in and out. Real loads are not constant, and a fridge cycling on and off averages far less than its nameplate. Runtime also falls in the cold, and lithium batteries may refuse to charge below freezing without a heater.
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About this calculator
Why is real runtime shorter than the nameplate suggests?
Four deductions stack up: depth of discharge, the reserve you keep, round-trip efficiency and inverter losses. Together they often remove a third of the nameplate before anything runs.
What actually uses the power in an outage?
Usually the fridge, the well pump and anything with a heating element. Air conditioning and electric water heating will empty most home batteries in an hour or two.
The formula
runtime = nameplate x depth x (1 - reserve) x efficiency x (1 - inverter loss) / load
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