Battery Bank Calculator

Size your battery bank from daily consumption and days of autonomy. Get Ah, kWh, and series/parallel configuration.

Your consumption

Daily consumption?

Wh/day

Days of autonomy?

Depth of discharge?

System voltage?

Battery bank required

781 Ah at 48V (37.5 kWh total)

Usable energy30.0 kWh

100Ah batteries32

200Ah batteries16

Series groups (48V)4S × 8P

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How to Use This Calculator

Enter your daily consumption in Wh

The daily consumption field takes watt-hours per day (1 kWh = 1,000 Wh). Add up your appliances: a fridge draws ~150W for ~8 hours = 1,200 Wh/day; LED lighting 10W × 5 hours = 50 Wh; a laptop charger 65W × 4 hours = 260 Wh. For a whole home, take your monthly kWh from the utility bill, multiply by 1,000, and divide by 30.

Set days of autonomy

Days of autonomy is the number of consecutive days your battery bank can power your loads with zero solar input — think overcast winter days or heavy cloud cover. Grid-tied homes use 1–2 days for backup. True off-grid systems in cloudy climates (Pacific Northwest, UK) need 3–5 days. Desert locations often get away with 1–2 days.

Depth of discharge

Set depth of discharge to match your battery chemistry. LiFePO4 (lithium iron phosphate): 80%. AGM lead-acid: 50%. Flooded lead-acid: 50%. The calculator sizes your bank so you never have to exceed this DoD limit — protecting battery longevity.

System voltage

The system voltage directly affects how many batteries you wire in series to reach that voltage. A 48V bank uses four 12V batteries in series (4S). Adding parallel strings increases amp-hours. The result shows series × parallel configuration (e.g., 4S3P = 4 batteries in series, 3 strings in parallel = 12 batteries total).

The Formula

Total Wh needed = Daily Wh × Days of autonomy Gross kWh (with DoD) = Total Wh ÷ (DoD × 1000) Total Ah at voltage = (Gross kWh × 1000) ÷ System voltage Series count = System voltage ÷ 12V (per battery) Parallel strings = Total Ah ÷ Battery Ah rating Total batteries = Series × Parallel

The series/parallel configuration result uses 12V batteries as the base unit. For a 48V system, you need 4 batteries wired in series per string (4 × 12V = 48V). Each parallel string adds amp-hours. Three parallel strings of 4 batteries each gives you a 4S3P bank — 12 batteries total.

Example

Off-grid cabin in Vermont

A Vermont off-grid cabin uses 5,000 Wh/day and needs 3 days of autonomy for winter cloudy periods. The owner chooses LiFePO4 batteries at 80% DoD and a 48V system.

Daily consumption5,000 Wh

Days of autonomy3 days

Depth of discharge80%

System voltage48V

Result

Total energy needed15,000 Wh (15 kWh)

Gross bank size needed18.75 kWh

Total Ah at 48V391 Ah

Series group (48V)4 batteries in series

Parallel strings4 strings

Total 100Ah batteries16 batteries (4S4P)

Sixteen 100Ah LiFePO4 batteries in a 4S4P configuration creates a 48V, 400Ah (19.2 kWh) bank. At 80% DoD, 15.36 kWh is usable — covering the 3-day requirement. At roughly $200 per 100Ah 12V LiFePO4 battery, the bank costs about $3,200.

FAQ

What does series vs. parallel wiring do? ▼

Wiring batteries in series adds their voltages (2 × 12V = 24V) while keeping amp-hours the same. Wiring in parallel adds their amp-hours (2 × 100Ah = 200Ah) while keeping voltage the same. To build a 48V bank with 100Ah batteries, wire 4 in series for 48V, then add parallel strings to increase Ah. A 4S2P configuration gives 48V at 200Ah.

How many days of autonomy do I really need? ▼

It depends on your solar resource and risk tolerance. In Phoenix, Arizona, you rarely have more than 1 consecutive cloudy day — 1–2 days of autonomy is enough. In Seattle or Vermont, winter storms can mean 3–5 days with minimal solar. Grid-tied systems with batteries for backup usually size for 1 day. Full off-grid systems in cloudy climates should target 3–5 days. Check historical solar irradiance data for your location at NREL's PVWatts database.

Can I mix different battery brands or capacities? ▼

Generally, no. Mixing batteries of different brands, capacities, ages, or chemistries in the same bank causes imbalance — the weaker battery pulls down the whole bank and degrades faster. If you must mix, use identical batteries (same brand, model, and age). Modern LiFePO4 batteries with built-in BMS (battery management systems) can sometimes be mixed within the same brand, but check manufacturer guidance. Lead-acid batteries are much more sensitive to mixing.

What size charge controller do I need for my battery bank? ▼

Your MPPT charge controller is sized based on the solar array size, not the battery bank. The controller's output current rating must handle the maximum charge current: divide solar watts by battery bank voltage. For a 3,000W solar array and 48V bank: 3,000 ÷ 48 = 62.5A — use a 60A or 80A MPPT controller. Also check that the controller's maximum input voltage exceeds your solar string's open-circuit voltage (Voc) by 25%.

What's the difference between a battery bank and a battery storage system? ▼

A battery bank is the collection of individual batteries wired together — it's the storage hardware. A battery storage system includes the bank plus the battery management system (BMS), inverter/charger, charge controller, and monitoring. All-in-one products like the Tesla Powerwall or Enphase IQ Battery bundle everything together. DIY systems use separate components, which costs less but requires more expertise to install correctly.