Battery Bank Size Calculator

Estimate battery bank size in watt-hours and amp-hours based on daily energy use, days of backup, battery voltage, usable capacity, and system losses.

Enter Your Battery Setup


Presets are planning examples. Use your actual daily energy use, battery specs, and system voltage when possible.


Use known daily watt-hours if you already estimated your loads. Otherwise use watts and hours per day.


Example: a 60W load running 10 hours uses 600Wh per day.


Use 1 for one day of storage, 2 for two days, and so on.


Higher-voltage battery banks need fewer amps for the same wattage.


This is the usable percentage of the battery bank, not the total label capacity.


Accounts for inverter losses, wiring losses, and real-world inefficiency.


Adds headroom for aging, cold weather, startup loads, and imperfect estimates.


Used to estimate how many batteries of this size are needed.


Optional. Enter 0 if you do not want a battery cost estimate.


Your Results

Nominal bank size

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Enter values and calculate.

Required amp-hours

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Based on selected battery voltage.

Batteries needed

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Rounded up to whole batteries.

Usable energy

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Estimated usable output energy.

This calculator gives a planning estimate only. Real battery performance depends on battery chemistry, age, temperature, discharge rate, inverter losses, wiring, charger limits, fuses, and manufacturer specifications.

How to Use This Battery Bank Size Calculator

Enter your daily energy use, desired days of backup, battery bank voltage, usable capacity, system efficiency, and planning buffer. The calculator estimates the nominal battery bank size, required amp-hours, battery count, and usable stored energy.

This calculator is mainly for early planning on small off-grid, shed, workshop, backup, camper, and low-voltage battery systems. It is not a full electrical design.

Formula Used

Daily energy use = load watts × hours per day
Usable energy needed = daily watt-hours × days of backup ÷ system efficiency
Nominal battery bank Wh = usable energy needed ÷ usable battery capacity
Adjusted nominal Wh = nominal battery bank Wh × (1 + planning buffer %)
Required Ah = adjusted nominal Wh ÷ battery bank voltage
Batteries needed = required Ah ÷ single battery Ah, rounded up

Example Calculation

A setup using 600Wh per day with 1 day of backup, 90% system efficiency, 80% usable battery capacity, and a 20% buffer needs about:

600Wh ÷ 0.90 = 667Wh usable energy needed
667Wh ÷ 0.80 = 834Wh nominal battery capacity
834Wh × 1.20 = 1,000Wh adjusted nominal capacity

On a 12V battery bank, that is about 83Ah, so a common planning choice may be around one 12V 100Ah battery, depending on the actual battery specifications.

Common Battery Bank Sizing Mistakes

  • Confusing total battery capacity with usable battery capacity.
  • Ignoring inverter losses, wiring losses, and cold-weather performance.
  • Planning around one perfect day instead of normal usage variation.
  • Using amp-hours without checking battery voltage.
  • Mixing old and new batteries or different battery types in the same bank.
  • Forgetting fuse size, wire size, charger limits, ventilation, and manufacturer instructions.

Important Safety Note

Use this calculator as a planning tool only. Battery banks can create fire, shock, burn, explosion, acid, gas, and short-circuit hazards if installed incorrectly.

For lithium battery banks, permanent wiring, inverter systems, solar charging, vehicle installations, generator charging, high-current DC wiring, permits, or code-sensitive work, get qualified professional guidance.