Introduction #

How to size a solar battery bank: start from daily energy (kWh/day), choose days of autonomy, divide by usable depth of discharge (DoD), and add margin for inefficiencies. For PV/BESS planning on CalcPanel use the Solar Battery Autonomy Calculator; for industrial BESS energy architecture see Industrial BESS kWh Sizing. Do not use UPS runtime tools for PV storage.

Best for: off-grid or hybrid commercial screens needing autonomy days.

Not ideal for: UPS IT backup minutes, EV battery packs, or consumer power-station shopping.

Core formula #

Conclusion: Usable bank energy must cover autonomy days at your DoD.

Required bank kWh ≈ (Daily kWh × Autonomy days) ÷ DoD

Optionally divide by round-trip efficiency (e.g. 0.90–0.95) for a more conservative bank.

Input Source
Daily kWh Loads / Off Grid Load
Autonomy days Outage policy (e.g. 1–3 days)
DoD Chemistry limit (e.g. 0.80 LiFePO4 planning)

CTA: Solve both directions in the Battery Autonomy Calculator on the Solar calculator hub.

Worked example (numbers) #

Given: 20 kWh/day critical load, 2 days autonomy, DoD 0.80, η_rt 0.92.

Bank ≈ (20 × 2) ÷ 0.80 = 50 kWh usable-nameplate screen
With round-trip: 50 ÷ 0.92 ≈ 54.3 kWh planning bank

Then pair recharge with array size (How to Size a Commercial Solar Array) and charge time (Solar Panel Charge Time).

PV bank vs UPS vs industrial BESS #

Conclusion: Same “kWh” word, different products and tools.

  • PV autonomy bank — days of load; this page + autonomy tool
  • UPS — minutes of IT load; keep UPS calculators separate
  • Industrial BESS — peak shaving / PCS — Industrial BESS kWh Sizing

Common mistakes #

  1. Using 100% DoD in planning.
  2. Ignoring inverter/charge efficiency.
  3. Sizing from array kW only without daily kWh.
  4. Mixing UPS Ah calculators into solar autonomy.

Next steps #

  1. Battery Autonomy Calculator.
  2. Industrial BESS kWh Sizing for facility BESS.
  3. Array: Solar Panel Sizing.
  4. Charge path: MPPT Sizing.

Assumptions and disclaimer #

Chemistry limits, temperature, and C-rates change usable energy. Planning estimate only.

FAQ #

How do I size a solar battery bank?

Use bank kWh ≈ (daily kWh × autonomy days) ÷ DoD, then add efficiency margin. Example: (20 × 2) ÷ 0.80 = 50 kWh.

What is days of autonomy?

How many days the bank should carry the load with little or no PV input—set by outage policy, not by module count.

Is this the same as UPS battery sizing?

No. UPS tools target backup minutes for IT loads. Keep them separate from PV/BESS autonomy.

Which CalcPanel pages should I use?

Solar Battery Autonomy Calculator for days/DoD screens; Industrial BESS kWh Sizing for facility storage architecture.