Battery Bank Sizing Calculator
Sizing a battery bank is one multiplication, one division, and one honest margin — and the margin is the part people skip, which is why so many banks come up short on day three.
The calculator
Bank size: 133 Ah at 12V
800 Wh rated capacity
The result is rated capacity — what the label must say. The usable energy is the rated figure times the depth setting.
How does the formula work?
Four steps, each a decision rather than an arithmetic detail:
- Energy = load × hours. This is the demand side — what the system must deliver between charges. Add autonomy days to cover the worst-case gap.
- Amp-hours = energy ÷ voltage. The same 800 Wh is ~67 Ah at 12V, ~33 Ah at 24V, ~17 Ah at 48V — the voltage choice changes the number, not the energy.
- Rated capacity = amp-hours ÷ usable depth. Lead-acid's 50% window doubles the arithmetic answer; LFP's 80% adds a quarter.
- Round up to real batteries. The calculator's answer lands between catalogue sizes — the nearest group up is the purchase.
Why the margin is the design
The depth-of-discharge margin is where sizing succeeds or fails. Lead-acid survives longest at 50%; LFP allows 80–90% (see the cycle-life numbers). Skip the margin to save money and the bank cycles too deep and ages early — the margin is the difference between the arithmetic answer and the battery that actually lasts.
The autonomy decision
Autonomy days are the sizing question nobody can answer for you: how many days must the bank cover if the sun or the grid is unavailable? One day is the default; two days doubles the bank; off-grid homes often plan three or more — and each day multiplies the rated capacity, which is why the number must be decided before the first component is bought.
Worked examples
| Scenario | Load | Hours | Voltage / chemistry | Autonomy | Bank |
|---|---|---|---|---|---|
| RV house bank | 100 W | 8 h | 12V lead-acid | 1 day | ~133 Ah |
| Off-grid cabin | 300 W | 6 h | 24V LFP | 2 days | ~250 Ah |
| Solar shed | 50 W | 12 h | 12V LFP | 1 day | ~94 Ah |
When does this calculator mislead?
- Loads with peaks — motor starts and surges draw more than the average; a bank sized to the average may sag on the peaks.
- Cold — available capacity falls with temperature (see cold weather).
- Partial recharging — the model assumes a full recharge each cycle; chronic undercharging ages lead-acid faster than the depth table assumes.
What This Means
My take: Sizing is a margin problem wearing arithmetic's clothes — the formula is two lines, and the failures are almost always the depth-of-discharge or autonomy decision that was skipped to save money, paid back in early replacement.
Why: The arithmetic gives a floor; the margins give the design. A bank built on the usable window, the worst-case gap between charges, and a glance at the load's peaks matches its math in year three as well as day one — which is the actual definition of the right size.
My editorial view, not a purchasing guarantee.
Frequently asked questions
How do I size a battery bank?
Multiply the load in watts by the hours needed, divide by the system voltage, then divide by the usable depth of discharge. 100 W for 8 hours is 800 Wh; at 12V with a 50% lead-acid window that is about 133 Ah.
Should I add extra capacity for cloudy days?
Yes — autonomy days multiply the bank. Each extra day of backup doubles the rated capacity, so decide the worst-case gap between charges before sizing, not after.
Sources
Cross-verified from battery engineering references and off-grid system documentation.
- Off-grid system documentation — load tables and sizing practice.
- Battery engineering references — depth of discharge and cycle life.
More tools: runtime calculator · charging time calculator · Last reviewed: 2026-09-17