Battery Bank Calculator — Size Your Off-Grid Battery Bank (Free)

How the Battery Bank Calculator Works

A battery bank is the reservoir of your off-grid system. Solar panels fill it during the day; your loads drain it at night and through cloudy stretches. Size it too small and you will be sitting in the dark or running a generator constantly. Size it too large and you have spent thousands of dollars on capacity you will never cycle. The goal is a bank matched precisely to your daily load, your tolerance for cloudy days, and the usable depth of your chosen chemistry.

The calculator uses the same four-step method that professional system designers use. The core formula is:

Required bank (Wh) = Daily load (Wh) × Days of autonomy
                     ÷ Usable DoD

Required Ah        = Required bank (Wh) ÷ System voltage

From there, the tool converts your requirement into real hardware. Because each battery has a fixed voltage, the number wired in series is set by your system voltage: a 48V bank built from 12V batteries needs four in series (48 ÷ 12 = 4). The tool then adds parallel strings — each a complete series set — until the combined amp-hours meet or exceed your requirement. The result is reported as an arrangement like 4S3P (four in series, three strings in parallel).

Finally, the calculator reports the practical numbers you need to plan and buy: total nameplate capacity, usable capacity after depth-of-discharge limits, estimated bank weight, an estimated 2026 cost range, and the safe charge-current window. It also assembles a bill of materials so you can wire the bank correctly the first time.

Understanding Each Input

Daily load (Wh or Ah per day)

This is the energy your home pulls in a typical 24 hours. Most people work in watt-hours (Wh): add up each device's wattage multiplied by hours of use. If you already track consumption in amp-hours (Ah) at your battery voltage — common for RV and marine owners with a battery monitor — flip the unit toggle and enter Ah directly; the tool converts using your system voltage. If you do not know your load yet, run the numbers through the off-grid solar calculator first, or measure each appliance with a plug-in energy meter for a week.

System voltage (12V / 24V / 48V)

Higher voltage means lower current for the same power, which means thinner, cheaper wire and lower losses. Use 12V for small RV and weekend setups under about 1 kW, 24V for 1–3 kW cabins and tiny homes, and 48V for anything 3 kW and up. Nearly every modern hybrid inverter is 48V. The full trade-off is covered in our 12V vs 24V vs 48V guide.

Days of autonomy (1–5)

Autonomy is how many days the bank can carry your loads with zero solar input. One day suits a sunny climate with a generator on standby. Two to three days is the sweet spot for most full-time off-grid homes. Four to five days is for cloudy regions like the Pacific Northwest. Beyond five days you are usually better off adding a generator than buying more batteries.

Chemistry and usable depth of discharge

Depth of discharge (DoD) is the fraction of nameplate capacity you can safely draw. LiFePO4 tolerates 80–100% (this tool uses a conservative 80% for daily cycling longevity). AGM and flooded lead-acid should stay near 50% to avoid premature death. That single number roughly doubles the nameplate capacity — and the cost — needed for lead-acid versus lithium.

A note on inverter efficiency

This calculator sizes the bank to the stored (DC) watt-hours you enter as the daily load. Your inverter loses a slice of every watt-hour to conversion heat — a typical pure sine inverter runs 88–94% efficient under load — so if your daily-load figure was measured on the AC side of the inverter, add roughly 10% to it before sizing (a 90% efficient inverter means dividing the AC figure by 0.90). DC loads and panel-measured loads need no adjustment.

Battery unit (Ah and voltage per battery)

Pick the actual battery you intend to buy. Presets cover the most common formats: 100Ah 12V and 200Ah 12V LiFePO4 (the workhorses of DIY builds), the high-density 280Ah cell-based build, the popular 5 kWh 48V server-rack module, and a 100Ah 12V AGM for budget or lead-acid comparisons. The tool reads each unit's amp-hours and voltage to compute the wiring.

Worked Example: A 48V LiFePO4 Bank

Suppose a small off-grid cabin uses 3,000 Wh per day, wants 3 days of autonomy, runs a 48V system on LiFePO4 (80% DoD), and plans to buy 100Ah 12V LiFePO4 batteries. Here is the math the calculator runs:

Required Wh = 3,000 × 3 ÷ 0.80
            = 9,000 ÷ 0.80
            = 11,250 Wh (nameplate)

Required Ah = 11,250 ÷ 48
            = 234.4 Ah at 48V  (rounds up to 240 Ah)

Series count = 48V ÷ 12V = 4 batteries per string
Parallel strings = ceil(234.4 ÷ 100) = 3 strings

Arrangement = 4S3P  →  12 × 12V 100Ah batteries
Total capacity = 12 × 100Ah × 12V = 14,400 Wh
Usable capacity = 14,400 × 0.80 = 11,520 Wh
Bank Ah at 48V = 300 Ah  (3 strings × 100Ah)
Charge current = 0.2C–0.5C of 300Ah = 60 A to 150 A

So this cabin needs twelve 12V 100Ah LiFePO4 batteries wired 4S3P, giving 14.4 kWh nameplate and 11.5 kWh usable — comfortably above the 9 kWh of true load energy required across three sunless days. The recommended charge current is 60–150A, and for longest life you would aim near the low end (around 60–90A). Try these exact inputs in the calculator above and you will see the same readout.

Why nameplate exceeds the obvious number. You might expect a 3-day, 3,000 Wh load to need just 9,000 Wh. But you can only safely use 80% of a LiFePO4 bank, so you must install 11,250 Wh of nameplate capacity to deliver 9,000 Wh of usable energy. This calculator sizes to that stored (DC) energy. If your 3,000 Wh figure was measured on the AC side of an inverter, add roughly 10% to the daily load first (≈3,300 Wh) to cover the inverter's conversion loss — forgetting the depth-of-discharge headroom is the single most common sizing error.

Reading the Series/Parallel Arrangement

The arrangement string — 2S2P, 4S3P, and so on — is the heart of a correct build.

• Series (the "S" number) adds voltage while keeping amp-hours the same. Four 12V batteries in series make 48V at the same Ah. The series count is forced by your system voltage divided by the battery voltage.
• Parallel (the "P" number) adds capacity while keeping voltage the same. Three 48V strings in parallel triple the amp-hours at 48V.

A 4 × 12V 100Ah → 2S2P result means: buy four 100Ah 12V batteries, wire two in series to reach 24V, then put two of those pairs in parallel for 24V at 200Ah. Always confirm every parallel string reads the same voltage with a multimeter before you connect them, and use equal-length cables so the strings share current evenly. The full wiring walkthrough lives in our DIY solar battery bank guide.

Charge Current and the C-Rate

Charge current is expressed as a C-rate — a multiple of the bank's amp-hour capacity at the battery voltage. A 300Ah string charged at 0.2C accepts 60A; at 0.5C it accepts 150A. The calculator reports the 0.2C–0.5C window for your bank, measured at the battery (string) Ah, not the system voltage.

• 0.2C–0.3C is the long-life sweet spot for LiFePO4 and the safe ceiling for most lead-acid. Gentler charging means cooler cells and more cycles.
• 0.5C is a typical maximum for quality LiFePO4 — fast, but confirm your battery's datasheet maximum first.
• Lead-acid generally wants 0.1C–0.2C; pushing harder causes gassing and heat.

This charge-current window tells you how much solar and charge-controller output your bank can absorb. Match it to the controller you choose from our best solar charge controllers roundup.

Bill of Materials (What to Actually Buy)

A battery bank is more than batteries. Below is the complete parts list the calculator's BOM section generates, with the quantities scaling to your arrangement. Prices are 2026 ballpark ranges.

As an Amazon Associate, Off Grid Authority earns from qualifying purchases (tag onamzashl042b-20) at no extra cost to you. Links are category searches, not specific endorsements — verify the exact spec and current price before buying.

Chemistry Cheat Sheet: LiFePO4 vs AGM vs Flooded

For nearly every off-grid build in 2026, LiFePO4 wins on lifetime cost despite the higher upfront price, because you install half the nameplate capacity and replace it a third as often. Lead-acid still has a niche for very low-use, low-budget, or freeze-prone installations. Compare specific cells in our best LiFePO4 batteries guide.

Common Battery Bank Mistakes

1. Forgetting depth of discharge and inverter losses

Sizing the bank to the raw load number leaves you 20–40% short. Always divide by usable DoD — the calculator does this automatically — and, if your daily load was measured AC-side, add ~10% first to cover inverter conversion loss.

2. Mixing old and new batteries

An aged battery has higher internal resistance. Paralleled with fresh cells, it forces the new ones to overwork, dragging the whole bank's life down and creating a heat risk. Build with matched, same-batch units, and expand only with complete new strings.

3. Undersizing or skipping fusing

Every parallel string and the main positive lead must be fused. On a lithium bank the fault current is high enough that only a DC-rated class-T fuse can safely clear it. This is non-negotiable.

4. Unequal cable lengths between parallel strings

If one string's cables are shorter, it carries more current and ages faster. Use equal-length "home-run" cables to a common busbar so every string shares load evenly.

5. Charging LiFePO4 below freezing

Charging lithium below 32°F (0°C) plates the anode and permanently destroys capacity. Use a bank with low-temperature cutoff or add heating pads in cold climates.

Frequently Asked Questions

Keep building

With your bank sized, the natural next steps are choosing the cells and wiring it safely:

• DIY Solar Battery Bank Guide — full wiring, fusing, and step-by-step build.
• Best LiFePO4 Batteries 2026 — the specific cells worth your money.
• 12V vs 24V vs 48V — lock in the right bank voltage before you buy.