Battery Bank Calculator
Usable capacity, not nameplate. The gap is enormous.
Work out Battery Bank. Usable capacity, not nameplate. The gap is enormous. Shows the working, not just the answer.
Usable capacity
30.72 kWh
38.4 kWh nameplate at 80% usable depth
Nameplate capacity is not usable capacity, and the gap is enormous. Lead-acid should not go below 50% state of charge without shortening its life sharply; lithium iron phosphate takes 80% or more routinely. A 200 Ah lead bank and a 200 Ah lithium bank differ by a factor of 1.6 in what you can actually take out. That difference compounds with cycle life. Lithium iron phosphate typically manages four to five times as many cycles as flooded lead-acid, so the cost per usable kilowatt-hour over the bank's life usually favours lithium heavily despite the higher purchase price. Sizing on nameplate is why so many off-grid systems disappoint in their first winter. Size on usable energy, and add margin for cold — capacity falls with temperature, substantially so below freezing.
How the Battery Bank Calculator works
Usable capacity, runtime and expected life for a battery bank, by chemistry. Nameplate capacity is not usable capacity — lead-acid gives half of it and lithium iron phosphate gives 80% or more.
Also known as: how many batteries do i need off grid · usable capacity of a battery · lifepo4 vs lead acid capacity · battery runtime calculator
Frequently asked questions
What is depth of discharge?
How much of a battery's nameplate capacity you can use without shortening its life sharply. Lead-acid is limited to about 50%; lithium iron phosphate takes 80% or more routinely.
Why is my battery bank smaller than it says?
Because nameplate capacity assumes full discharge, which no chemistry tolerates repeatedly. A 200 Ah lead-acid bank holds 100 Ah you can actually use, which is why so many off-grid systems disappoint in their first winter.
Is lithium worth the extra cost?
Usually, on cost per usable kilowatt-hour over the bank's life. Lithium iron phosphate gives more usable depth and four to five times the cycles, which typically outweighs the higher purchase price.
How do I work out runtime?
Usable energy divided by the load. Add margin for inverter losses, which run 5 to 10%, and for cold — capacity falls with temperature and substantially so below freezing.
Should I wire batteries in series or parallel?
Series raises voltage and parallel raises capacity. Higher system voltage means lower current for the same power, which allows thinner cable and smaller losses, so larger systems almost always go to 48 V.
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