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Solar Battery Calculator

Size your off-grid solar battery bank in amp-hours by entering your daily energy use, system voltage, autonomy days, and depth of discharge.

Result

833 Ah

You need about 833 Ah of battery capacity.

Quick Answer

A solar battery calculator sizes your battery bank using: Battery Capacity (Ah) = (Daily kWh ÷ System Voltage × Days Autonomy) ÷ DoD. For a 3 kWh per day system at 24V with 2 days autonomy and 50% DoD, the required capacity is (3000 ÷ 24 × 2) ÷ 0.50 = 500 Ah.

Understanding Solar Battery Bank Sizing: What It Shows You

A solar battery calculator tells you how large a battery bank you need to power your off-grid load for a set number of days without solar charging. It converts your daily energy consumption and system voltage into amp-hours, then adjusts for your depth of discharge limit to protect the batteries. The result is the minimum usable capacity your bank must have. This is the first number you need before selecting battery modules for an off-grid or backup solar system.

The Solar Battery Bank Sizing Formula And How It Works

A solar battery calculator sizes your battery bank by converting daily kilowatt-hours and system voltage into amp-hours and scaling by days of autonomy and maximum depth of discharge.

  • Battery Capacity (Ah) = (Daily kWh ÷ System Voltage × Days Autonomy) ÷ Depth of Discharge
  • Daily kWh: daily total energy consumption in kilowatt-hours
  • System voltage: nominal voltage of the battery bank (12V, 24V, or 48V)
  • Days autonomy: cloudy days the bank covers without solar input
  • Depth of discharge: safe usable fraction (0.50 lead-acid, 0.80-0.90 lithium)

No temperature derating, inverter losses, or charge-controller losses are applied in this core formula. Real systems should add a 10 to 20% margin to the result to cover these system losses.

How To Use The Solar Battery Calculator In Four Steps

Inputs

  • Daily energy use: average daily energy consumption in kWh
  • System voltage: battery bank nominal voltage (commonly 12, 24, or 48V)
  • Days of autonomy: number of days the battery bank must cover with no solar input
  • Depth of discharge: maximum fraction of capacity usable without harming battery life (0.50 for lead-acid, 0.80-0.90 for lithium)

Steps

  1. Enter your daily energy consumption in kilowatt-hours.
  2. Select your system voltage from the available options.
  3. Enter the number of autonomy days you want the bank to cover.
  4. Enter your battery's depth of discharge limit as a decimal.
  5. Read the required bank capacity in amp-hours from the result field.
  6. Add a 10 to 20% margin (SEI and IEEE 1562 guideline) to account for temperature and inverter losses.

Solar Battery Bank Capacity Example, Worked In Full

A 24V off-grid system uses 3 kWh per day, requires 2 days of autonomy, and uses lead-acid batteries limited to 50% depth of discharge.

  1. Convert daily kWh to Wh: 3 kWh = 3,000 Wh.
  2. Divide daily Wh by system voltage: 3,000 / 24 = 125 Ah per day.
  3. Multiply by days of autonomy: 125 × 2 = 250 Ah.
  4. Divide by depth of discharge: 250 / 0.50 = 500 Ah.

Required battery bank: 500 Ah at 24V (provides 12 kWh total energy, 6 kWh usable).

Is A Solar Battery Calculator Right For Your Situation?

Use this calculator when you are designing an off-grid or backup solar system and need a starting-point battery bank size. It suits residential off-grid cabins, RV/marine solar systems, and grid-tied systems adding battery backup.

It is not the right tool for critical high-temperature loads requiring complex multi-array modeling. Check out our engineering calculators hub, including the battery capacity calculator, battery charge time calculator, and UPS battery calculator.

Assumptions

  • Daily kWh is a consistent average; seasonal variation is not modeled.
  • System voltage is constant across the full bank.
  • Depth of discharge is uniform across all cells in the bank.
  • No inverter losses, charge-controller losses, or temperature derating applied.
  • Battery self-discharge during the autonomy period is not included.

This calculator does not apply temperature derating, inverter efficiency losses, or charge controller losses; add margin for real-world system losses.

Limitations

  • Real losses from inverters and charge controllers typically add 10 to 20% to the required Ah (SEI and IEEE 1562 guideline).
  • Cold temperatures reduce usable capacity; a battery rated at 500 Ah at 25C may deliver only 400 Ah at 0C.
  • This formula does not size the solar array or the charge controller, only the battery bank.
  • Multi-chemistry or mixed battery banks require separate calculations for each chemistry.

In Practice

The most common mistake is entering daily kWh from a utility bill average without accounting for seasonal peaks. A home that uses 3 kWh per day in summer may use 6 kWh in winter when heating and lighting loads increase. Size your battery bank for the worst month, not the annual average, or you will run out of autonomy on the coldest, cloudiest days when you need storage most.

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Frequently Asked Questions About Solar Battery Sizing

What is depth of discharge?

Depth of discharge is the percentage of a battery's rated capacity that you can safely use before recharging. Lead-acid batteries should not go below 50% DoD without shortening their life. Lithium batteries can typically be discharged to 80 to 90% without lasting damage.

What system voltage should I choose?

For small systems under 1 kWh per day, 12V is common. For residential systems from 1 to 5 kWh per day, 24V reduces wiring losses. Systems above 5 kWh per day benefit from 48V, which further reduces current and allows lighter wiring at the same power level.

How many days of autonomy should I design for?

Two to three days is typical for residential off-grid systems in regions with reliable sun. Systems in cloudy climates or for critical loads often design for five to seven days, per IEEE 1562 guidance for non-critical loads in high-insolation areas.

Why does this calculator not include inverter losses?

The formula applies a simplified model based on SEI and IEEE 1562 sizing guidelines. Inverter and charge-controller losses are a known simplification. Add 10 to 20% to the result as a real margin before purchasing batteries.

How is this different from the battery capacity calculator?

The battery capacity calculator sizes a battery for a known watt-hour load and voltage, without the autonomy-days dimension. The solar battery calculator adds the autonomy layer: how many days of no solar input the bank must cover. Both use the same underlying Ah formula but with different inputs.

Sources

Last updated: . Reviewed for accuracy against the formula shown above.