Solar Battery Bank Calculator
Size off grid battery kWh, Ah, and battery count from daily load.
What it does
A battery bank bridges the gap between sunshine and night loads. Panels charge by day, the bank carries the evening, and the size of that bank sets how many cloudy days a cabin can ride through.
The tool above turns daily use into three linked numbers. Nominal kWh is the nameplate size before discharge limits, amp hours restate it at 12, 24, or 48 volts, and the 100 Ah count tells how many common blocks to buy.
The default case is a small cabin at 1.5 kWh per day with 2 days of backup on a 12 V bank at 50 % usable share. The math gives 6.0 kWh nominal, which is 500 Ah at 12 V, or 5 units of 100 Ah.
Depth of discharge is the key judgment. A lead acid bank held near 50 % lasts far longer than one drawn to empty each night, while lithium chemistries allow deeper draws within maker limits. Volts matter too, since the same energy is fewer amp hours at 48 V than at 12 V.
Bank sizing is only one half of an off grid design. Panels must still refill the bank, the controller must fit the current, and the inverter must carry peak watts. Size the bank here first, then match the array to local sun hours.
How to use
- Enter daily use in kWh from bills or a meter.
- Enter backup days, often 1 to 3 for cabins.
- Pick system volts: 12, 24, or 48.
- Enter allowed discharge: near 50 for lead acid, higher for lithium per maker sheet.
- Read nominal kWh first, then Ah at your volts.
- Read the 100 Ah unit count and round up for wiring spares.
Formula
- daily kWh
- Energy used per day, in kWh.
- days
- Days of backup with no sun.
- DOD
- Allowed discharge share, in percent.
- volts
- System volts: 12, 24, or 48.
DOD is the allowed discharge share as a percent. The page counts 100 Ah units, a common building block size, and rounds up so partial banks still cover the load.
Worked example
Example: 1.5 kWh per day, 2 days, 12 V
A cabin uses 1.5 kWh per day, needs 2 days of backup, runs a 12 V bank, and allows 50 % discharge. These are also the default values, so the page opens on this answer.
- Daily load: 1.5 kWh per day.
- Backup: 2 days, so stored energy before limits is 1.5 x 2 = 3.0 kWh.
- Usable share: 50 %, so nominal size is 3.0 / 0.50 = 6.0 kWh.
- At 12 V: 6.0 x 1000 / 12 = 500 Ah.
- 100 Ah units: ceil(500 / 100) = 5 batteries.
- Tool shows 6.0 kWh, 500 Ah at 12 V, 5 batteries.
Reference table
| Daily use | Days | Nominal | 12 V Ah |
|---|---|---|---|
| 1.0 kWh | 1 | 2.0 kWh | 167 Ah |
| 1.5 kWh | 2 | 6.0 kWh | 500 Ah |
| 3.0 kWh | 1 | 6.0 kWh | 500 Ah |
| 5.0 kWh | 2 | 20.0 kWh | 1667 Ah |
Limits
- Plans energy only. Peak watts, surge, and wire size need separate checks.
- Uses 100 Ah blocks. Other block sizes change the count.
- Ignores temperature loss, aging, and inverter waste.
- Assumes steady daily use. Weekend cabins with spiky loads need margin.
Common mistakes
- Sizing from nameplate kWh while forgetting discharge limits.
- Mixing amp hours across volts. 500 Ah at 12 V is not 500 Ah at 48 V.
- Planning zero cloudy days, then running the bank flat each storm.
- Forgetting the inverter cutoff, which strands the last usable slice.
- Buying exact units with no spare for a weak cell.
Try it yourself
Quick checks. Answers sit tucked under each question.
2 kWh per day, 1 day, 24 V, 50 %?
Nominal is 2.0 x 1 / 0.50 = 4.0 kWh. At 24 V that is 4000 / 24 = 167 Ah, so 2 units of 100 Ah.
1 kWh per day, 3 days, 48 V, 60 %?
Nominal is 1.0 x 3 / 0.60 = 5.0 kWh. At 48 V that is 5000 / 48 = 104 Ah, so 2 units of 100 Ah.
Questions
How do I size a solar battery bank?
Multiply daily kWh by backup days, then divide by the usable share. A 1.5 kWh load for 2 days at 50 % needs 6.0 kWh nominal.
How do kWh convert to amp hours?
Amp hours equal nominal watt hours divided by system volts. Here 6000 Wh / 12 V = 500 Ah.
What depth of discharge should I plan?
Lead acid banks often plan near 50 % to protect cycle life, while lithium iron phosphate plans near 80 to 90 %. Check the maker sheet for the exact limit.
12 V, 24 V, or 48 V?
Use 12 V for small cabins, 24 V for mid size systems, and 48 V for large homes. Higher volts cut current and wire size for the same power.
Does this replace panel sizing?
No. Panels, controller, inverter draw, and wire loss all shift the result. Treat this page as bank sizing only, then size the array separately.
Does cold change the answer?
Cold cuts usable capacity, mainly for lead acid. If the bank sits in an unheated shed, add margin or pick a chemistry rated for the low end.