Solar Battery Bank Size Calculator

The Rolls figure is for off-grid systems in general, and the Renogy figures are for sunny and for cloudier regions. For a weekend cabin, a full-time RV, critical loads such as medical or communications gear, or a remote station with no quick resupply, size from your own local weather record and from whether a generator can carry you through a long overcast spell.

Each extra day adds one more day of storage, so on N days it adds 1/N to the bank: 1 to 2 days adds 100% (1/1), 2 to 3 adds 50% (1/2), 3 to 4 adds 33% (1/3) and 4 to 5 adds 25% (1/4). The step shrinks as the days rise, but the bank still grows in line with them: going from 5 to 10 days doubles it, for five extra days you may never use.

LiFePO4 (lithium iron phosphate) dominates new off-grid builds for good reason. You can use 80-90% of rated capacity per cycle, the batteries last 3,000-5,000 cycles, they weigh a third of equivalent lead-acid, and charge/discharge efficiency sits around 95-98%. A 200Ah LiFePO4 battery gives you 160-180Ah of usable power.

Lead-acid (flooded or AGM) costs less upfront but requires a larger bank because you can only safely use 50% of capacity. Cycle life is 300-500 cycles at 50% DoD. A 200Ah lead-acid battery only delivers 100Ah of usable power (55 to 63% of the usable capacity the same-rated LiFePO4 gives). Weight is roughly 3x higher per usable kWh.

On a cost-per-usable-kWh-per-cycle basis, LiFePO4 wins convincingly. As of early 2026, a $250 100Ah LiFePO4 battery delivering 80Ah at 12.8V nominal (80 x 12.8 = 1,024Wh, 1.02kWh) over 3,000 cycles costs about $250 / (1.024 x 3,000) = $0.08 per usable kWh cycle. A $120 100Ah AGM battery delivering 50Ah (0.6kWh) over 400 cycles costs about $0.50 per usable kWh cycle, about six times more expensive over its lifetime. Prices and rates on this page are our own estimates as of the date shown; they change often. Enter your own figures for a current result. Our LiFePO4 vs lead-acid runtime comparison covers the performance differences in more depth.

Scenario: an off-grid tiny house using 5,000Wh per day. The owner wants 3 days of autonomy and is choosing between LiFePO4 (80% DoD) and AGM (50% DoD) at 24V, both through an inverter taken at 92%, the page's default.

LiFePO4 at 24V: 5,000Wh x 3 days = 15,000Wh needed. A 24V LiFePO4 bank is 25.6V nominal, so each amp-hour delivers 25.6 x 0.80 x 0.92 = 18.84Wh, and 15,000 / 18.84 = 796Ah. Sixteen 100Ah 12V LiFePO4 batteries wired as 8 series pairs in parallel (a 24V system at 2 x 12.8 = 25.6V nominal, 8 x 100 = 800Ah) meets this target. Approximate cost as of early 2026, as a rough guide rather than a quoted price: $4,000-4,800.

AGM at 24V: A 24V AGM (lead-acid) bank is 24V nominal, so each amp-hour delivers 24 x 0.50 x 0.92 = 11.04Wh, and 15,000 / 11.04 = 1,359Ah. That is 28 x 100Ah 12V AGM batteries (14 series pairs) to reach 1,400Ah at 24V. Approximate cost as of early 2026, again as a rough guide: $2,600-3,900, and the 28 batteries make a much heavier bank than the LiFePO4 option. After 400 cycles (roughly 1 year of daily cycling), the entire bank needs replacing.

The LiFePO4 option costs more upfront, but weighs less, lasts 7.5 to 12.5 times as long (3,000 to 5,000 cycles against 400), and, with the two makers' footprints within a square inch of each other (Battle Born 12.76 by 6.86 in; Victron 110 Ah AGM 330 by 171 mm), takes up a little over half the floor space, 16 batteries against 28.