Battery bank for 7 kWh a day with 5 days of autonomy, 12 V LiFePO₄
| Usable energy needed7 kWh a day × 5 days | 35kWh |
|---|---|
| Nominal capacity to buyusable ÷ (80% DoD × 95% round-trip) | 46.05kWh |
| Capacity at 12 V | 3,838Ah |
| 100 Ah blockswired in parallel | 39blocks |
| Charge current0.2C — the charger and the array both have to deliver this | 768A |
| Depth of discharge usedmanufacturer figure for rated cycle life, not the physical limit | 80% |
| Round-trip efficiencyenergy lost on the way in and out again | 95% |
| Rated cycle lifeat 80% DoD | 3.000–6.000 cycles |
Notes
- Two derates, not one. Dividing by depth of discharge alone is the usual mistake — the pack also loses 5% round-trip on LiFePO₄, and that comes off the top of every cycle.
- LiFePO₄ tolerates deeper discharge than lead, which is why a 46.05 kWh lithium bank replaces roughly 82.35 kWh of AGM for the same job. The upfront price gap narrows a long way once you size both honestly.
- 12 V is the practical floor for this size. Higher system voltage means proportionally lower current for the same power — 3,838 Ah at 12 V would be 3,838 Ah at 12 V, and the cable to carry it gets expensive fast.
- 5 days of autonomy covers 5× 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.
Two derates, not one
Pulling 7 kWh a day for 5 days means 35 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 — 80% here. Round-trip efficiency takes another 95% off, lost as heat on the way in and out.
Divide by both and the bank you actually buy is 46.05 kWh, or 3,838 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 39 × 100 Ah blocks. The charger and the array both have to be able to push 768 A into it, which is the constraint people find after the batteries arrive rather than before.
Voltage is the cheap lever
3,838 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.
Nearby sizes
- Battery bank for 6 kWh a day with 5 days of autonomy, 12 V LiFePO₄
- Battery bank for 8 kWh a day with 5 days of autonomy, 12 V LiFePO₄
- Battery bank for 5 kWh a day with 5 days of autonomy, 12 V LiFePO₄
- Battery bank for 9 kWh a day with 5 days of autonomy, 12 V LiFePO₄
- Battery bank for 7 kWh a day with 4 days of autonomy, 12 V LiFePO₄
- Battery bank for 7 kWh a day with 3 days of autonomy, 12 V LiFePO₄
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