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Battery Bank Sizing

How big a battery bank has to be for a given daily load — with depth of discharge and round-trip efficiency both taken off, which is where most estimates come up short.

Two derates, not one. Dividing daily load by depth of discharge is the calculation most people do, and it comes out about 20% short on any lead-acid bank.

Energy also gets lost on the round trip — into the battery and back out. LiFePO₄ returns about 95% of what goes in; AGM about 85%; flooded lead-acid about 80%. That loss comes off the top of every single cycle, and it compounds with the depth-of-discharge limit rather than replacing it.

Nominal capacity = daily load × days ÷ (depth of discharge × round-trip efficiency).

Depth of discharge is a warranty number, not a physical limit

LiFePO₄ can be taken to 100%. It just will not last as long doing it. The 80% figure is what manufacturers rate cycle life at, and the 50% figure for lead-acid exists for the same reason. Using the physical limit instead of the rated one is how a bank that was supposed to last ten years lasts three.

Days of autonomy is a weather question

One day covers an evening and a night, and it is normal for grid-tied backup. Off-grid with no generator normally carries three to five, because that is what a run of overcast weather costs you — and the day you find out is the day it matters.

System voltage is the cheap lever

The same energy at 48 V draws a quarter of the current it would at 12 V. Current is what makes cable expensive, and it is what makes voltage drop bite. Every doubling of system voltage halves the current and quarters the loss in the wire.

Charge current is the constraint people discover late

A bank sized here needs a charger and an array capable of pushing the listed current into it. Finding out afterward that the controller cannot is a second purchase.

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Battery bank for 10 kWh a day with 3 days of autonomy, 24 V AGM

Battery bank
70.59kWh nominal
2,941 Ah at 24 V — AGM, 3 days of autonomy
Usable energy against nameplate capacity30 kWhenergy you useout of the bank70.59 kWhcapacity to buyAGM50% depth of discharge × 85% round-trip — two derates, not one
Usable energy needed10 kWh a day × 3 days30kWh
Nominal capacity to buyusable ÷ (50% DoD × 85% round-trip)70.59kWh
Capacity at 24 V2,941Ah
100 Ah blocks2 in series makes one 24 V string30blocks
Charge current0.15C — the charger and the array both have to deliver this441A
Depth of discharge usedmanufacturer figure for rated cycle life, not the physical limit50%
Round-trip efficiencyenergy lost on the way in and out again85%
Rated cycle lifeat 50% DoD400–800 cycles

Notes

  • Two derates, not one. Dividing by depth of discharge alone is the usual mistake — the pack also loses 15% round-trip on AGM, and that comes off the top of every cycle.
  • At 50% usable, AGM needs nearly twice the nameplate capacity of lithium for the same job. Compare on kWh delivered over the warranty, not on sticker price per kWh.
  • 24 V is the practical floor for this size. Higher system voltage means proportionally lower current for the same power — 2,941 Ah at 24 V would be 5,882 Ah at 12 V, and the cable to carry it gets expensive fast.
  • 3 days of autonomy covers 3× a normal day's use. Grid-tied backup normally carries one day; off-grid with no generator normally carries three to five, because that is what a run of overcast weather costs you.

Battery bank size

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