Solar Battery Calculator

A solar battery bank is sized from how much energy you use in a day and how many sunless days it has to carry, not from a load in watts. Enter the daily kilowatt hours, the days of autonomy, the chemistry and the system voltage, and the result gives the nameplate capacity to buy, the usable energy behind it, the number of batteries, the charge current the bank wants and the energy the array must harvest to refill it. A second action sizes the same job in flooded lead acid, AGM and lithium iron phosphate side by side, with the cost per kilowatt hour actually delivered over the life of each. Round trip efficiency is applied to the array and not to the bank, because a battery is rated by what it gives back rather than by what it swallowed.

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Daily energy use and the days you want to ride out without sun. The result is the nameplate capacity to buy, which is larger than the usable figure.

Measured at the AC outlets, which is what a bill or a plug meter shows. A fridge, lights and a laptop is roughly 2 to 4; a whole small house 10 to 30.
Consecutive days the bank must carry the load with no useful sun. Grid-tied backup often 1, an off-grid cabin 2 to 3, a system with no generator 4 or 5.
Leave at 0 to use the figure for the chemistry chosen above.
Leave at 0 to use the figure for the chemistry. This sizes the array, not the bank.
A decent inverter runs 90 to 95 percent at a sensible load and much worse at a very light one. Set it to 100 if your loads are all DC.
Lead acid at freezing gives about 80 percent of its room temperature rating. Lithium gives most of its capacity down to freezing but must not be charged below it.
The bank voltage as wired. 12 V for a van, 24 or 48 for a house. Higher voltage means thinner cable for the same power.
Used only to turn the answer into a number of batteries wired in parallel.
Leave at 0 to skip the array lines. Roughly 4 in the northern states in winter, 5 to 6 in the southwest.
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The same daily load sized three ways, so the nameplate difference and the cost per kilowatt hour actually delivered sit side by side.

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