Battery bank for 18 kWh a day with 2 days of autonomy, 12 V AGM
| Usable energy needed18 kWh a day × 2 days | 36kWh |
|---|---|
| Nominal capacity to buyusable ÷ (50% DoD × 85% round-trip) | 84.71kWh |
| Capacity at 12 V | 7,059Ah |
| 100 Ah blockswired in parallel | 71blocks |
| Charge current0.15C — the charger and the array both have to deliver this | 1,059A |
| Depth of discharge usedmanufacturer figure for rated cycle life, not the physical limit | 50% |
| Round-trip efficiencyenergy lost on the way in and out again | 85% |
| Rated cycle lifeat 50% DoD | 400–800 cycles |
- Daily load
- 18 kWh
- Days of autonomy
- 2
- System voltage
- 12 V
- Chemistry
- AGM
Two derates, not one
Pulling 18 kWh a day for 2 days means 36 kWh has to come out of the bank. That is not the number you buy.
Two things stand between nameplate capacity and delivered energy. Depth of discharge caps how much of the pack you are allowed to use if you want the rated cycle life — 50% here. Round-trip efficiency takes another 85% off, lost as heat on the way in and out.
Divide by both and the bank you actually buy is 84.71 kWh, or 7,059 Ah at 12 V.
Dividing by depth of discharge alone is the common mistake, and it is why so many banks fall short in their first winter.
What that looks like on a shelf
About 71 × 100 Ah blocks. The charger and the array both have to be able to push 1,059 A into it, which is the constraint people find after the batteries arrive rather than before.
Voltage is the cheap lever
7,059 Ah at 12 V is the same energy as a much larger amp-hour figure at 12 V, and current is what makes cable expensive. Every doubling of system voltage halves the current for the same power and quarters the loss in the wire.
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.
- 12 V is the practical floor for this size. Higher system voltage means proportionally lower current for the same power — 7,059 Ah at 12 V would be 7,059 Ah at 12 V, and the cable to carry it gets expensive fast.
- 2 days of autonomy covers 2× 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.
Nearby sizes
- Battery bank for 17 kWh a day with 2 days of autonomy, 12 V AGM
- Battery bank for 19 kWh a day with 2 days of autonomy, 12 V AGM
- Battery bank for 16 kWh a day with 2 days of autonomy, 12 V AGM
- Battery bank for 20 kWh a day with 2 days of autonomy, 12 V AGM
- Battery bank for 18 kWh a day with 1 days of autonomy, 12 V AGM
- Battery bank for 18 kWh a day with 3 days of autonomy, 12 V AGM
- Battery bank for 18 kWh a day with 4 days of autonomy, 12 V AGM
- Battery bank for 18 kWh a day with 2 days of autonomy, 24 V AGM
- Battery bank for 18 kWh a day with 2 days of autonomy, 48 V AGM
- Battery bank for 18 kWh a day with 2 days of autonomy, 12 V LiFePO₄
- Battery bank for 18 kWh a day with 2 days of autonomy, 12 V flooded lead-acid
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