How Many Solar Batteries Do I Need?
Built by Jeremy Panasuk. Published October 3, 2026.
Both counts use Battle Born's 2,400 Wh and 12 V. They disagree because one method stops at watt-hours ÷ voltage and the other divides by the depth of discharge selected in the calculator.
Why the article uses fewer batteries
The BB10012i manual (PMI_BB10012i, revision 015, dated 8/12/2026) rates that model at 12 V, 100 Ah, and 100% usable depth of discharge, with 3,000–5,000 cycles, 99% efficiency, and self-discharge of 2–3% per month. Their sizing article never divides by a depth of discharge below 100%, which matches a battery they publish as fully usable. The calculator's LiFePO4 option is 80%, and its AGM / lead-acid option is 50%. Those are selections in the form. They are not printed in the BB10012i specifications table.
Using 80% on a battery the manufacturer rates at 100% usable makes the bank larger than the manufacturer's own article. That is what the 5-versus-4 gap is. Five batteries is the calculator being conservative relative to a 100% rating. Four batteries is the article, doubled for two days, on a 100% rating. Pick the input on purpose. The chemistry comparison, including this manual's weight of 31 lb, is on AGM vs lithium.
| Method | Inputs | 12 V amp-hours | 100 Ah batteries |
|---|---|---|---|
| Battle Born article, one day | 2,400 ÷ 12 | 200 Ah | 2 |
| Battle Born article, two days | double the one-day capacity | 400 Ah | 4 |
| This calculator, 2 days, 80% | 2,400 × 2 ÷ 0.8 ÷ 12 | 500 Ah | 5 |
| This calculator, 2 days, 50% | 2,400 × 2 ÷ 0.5 ÷ 12 | 800 Ah | 8 |
The 50% row is the calculator's AGM / lead-acid option applied to the same 2,400 Wh. It is not an AGM datasheet. The array line does not change with that option: 2,400 ÷ 4 ÷ 0.75 = 800 W at 4 sun hours in every row.
The form opens at 24 V
Leave voltage at 24 V and the 80%, 2-day bank is still 6,000 Wh, which is 250 Ah. The calculator then rounds the "100 Ah units to parallel" line up to the next 100 Ah, and 250 Ah becomes 300 Ah on that line. This model's series rule makes the hardware match: two BB10012i batteries in series are a 24 V 100 Ah bank, current capacity unchanged, voltage added. The manual's charge setting for that pair is 28.8 V bulk/absorption and 27.2 V float. Two pairs are 200 Ah at 24 V, which is short of 250 Ah. Three pairs are 300 Ah at 24 V, six batteries, which covers the rounded 300 Ah line and the un-rounded 250 Ah. Series stops at 48 V. Four batteries in series are the manual's 48 V 100 Ah example. Exceeding 48 V voids the warranty.
Charge each battery to 14.2 V–14.6 V before you series them, so the state of charge matches. Do not mix this battery with another type. Torque the terminals to 9–11 ft-lb. The charge-current check, including the 50 A maximum on one battery, is on how to charge LiFePO4 batteries with solar.
The 500 Wh refrigerator, counted the same way
Battle Born's refrigerator example is 100 W × 5 h = 500 Wh. Their method: 500 ÷ 12 = 41.7 Ah for one day, and two days doubles that to 83.3 Ah, which is still one 100 Ah battery. The calculator at 2 days and 80% does 500 × 2 ÷ 0.8 = 1,250 Wh, then 1,250 ÷ 12 = 104.17 Ah, which rounds up to 110 Ah. The 100 Ah-unit line then rounds 110 Ah up to 200 Ah, so the screen asks for two of these batteries even though 110 is only a little over one. That second battery is the rounding step plus the 80% input. The full refrigerator writeup is on what size solar system for a refrigerator. System-level bank watt-hours, before the battery count, are on what size solar system.
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One common mistake
Counting batteries from watt-hours and skipping voltage. 2,400 Wh is not 2,400 Ah, and it is not 24 batteries. The article's first division is watt-hours ÷ system voltage. The other mistake is taking the one-day count of 2 batteries and typing 2 days into the calculator without noticing that the calculator also divides by 0.80. Two days in the article is 4 batteries. Two days at 80% in the calculator is 5.
Limits
The manual's cover warning stands: risk of fire and burns; do not open, crush, heat above 150°F, or incinerate. Do not series past 48 V. Do not parallel or series this model with a different battery type. The BMS will not charge under 32°F (0°C). A licensed electrician sizes the overcurrent protection on the battery cables the manual says must carry the bank's maximum current. This count is a capacity count. It is not a wiring diagram.
FAQ
How many 100 Ah batteries does 2,400 Wh need for two days?
Battle Born divides 2,400 by 12 V to get 200 Ah, then doubles it for two days, which is 400 Ah and 4 batteries of 100 Ah. This calculator at 2 days and 80% depth of discharge produces 6,000 Wh and 500 Ah at 12 V, which is 5 batteries. Their manual rates the BB10012i at 100% usable depth of discharge.
What changes at the form's default 24 V?
The bank is still 6,000 Wh, now 250 Ah. The calculator's 100 Ah-unit line rounds that up to 300 Ah. Two BB10012i batteries in series are a 24 V 100 Ah bank, so three such pairs are 300 Ah and six batteries. Two pairs are 200 Ah, which is short of 250 Ah.
How many batteries does the 500 Wh refrigerator example need?
Battle Born's method gives 41.7 Ah for one day and 83.3 Ah for two days, one 100 Ah battery. This calculator at 2 days and 80% gives 1,250 Wh and 110 Ah at 12 V, and the 100 Ah-unit line rounds that to 200 Ah, two batteries.
Compiled with AI assistance from the sources below. Spot an error? Email [email protected].
Sources
- Battle Born Batteries, How to Size a Deep Cycle Battery Bank, May 22, 2026 — accessed Oct 3, 2026.
- Battle Born / Dragonfly Energy, BB10012i and BB10012iH manual, PMI_BB10012i Rev 015, 8/12/2026 — accessed Oct 3, 2026.
- SolarSizer calculator, assets/app.js (sizeSystem) — accessed Oct 3, 2026.