Solar & battery
Solar Battery Charge Time Calculator
How long solar takes to refill a battery, from the array size and how empty it is.
Result
Charging slows as a battery fills, because the final stage tapers current to protect the cells, so real time to 100% is longer than this linear estimate. The house load matters more than people expect: whatever the house is using comes off the charging rate first.
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About this calculator
Why does the last 10% take so long?
Lithium and lead acid both taper the charge current near full to avoid damage. Getting to 90% is fast; the final stretch can take as long again.
Should I size the array for charging or for use?
Both, and the battery is the harder one. An array sized to cover daily consumption has nothing spare to refill a battery after an outage.
The formula
hours = energy needed / (array output - house load)
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