Solar & battery
Solar Battery Size Calculator
Battery capacity to carry a given load for a given time, in nameplate and usable kWh.
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. Cycle life figures are manufacturer ratings at favourable temperature and discharge rate; heat and deep fast cycling both shorten them considerably.
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About this calculator
Why is the battery bigger than the energy I need?
Because you cannot use all of it. Between depth of discharge, a reserve and round-trip losses, roughly two thirds to three quarters of a lithium nameplate is actually deliverable, and about a third of lead acid.
How many days of autonomy?
One is normal for a grid-tied home with backup. Off grid, three is the usual design figure, because a generator or a very large bank is the only alternative during a long cloudy spell.
Is lead acid still worth considering?
Only where the up-front price dominates and the cycling is light. Per usable kWh over its life, lithium iron phosphate is usually cheaper despite costing more to buy.
The formula
nameplate = energy needed / (depth of discharge x (1 - reserve) x round-trip efficiency)
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