PowerCalcLab

Solar calculator

Solar Battery Bank Calculator

Calculate the nominal kWh and amp-hours needed to carry your loads through the chosen number of low-solar days, with every derating visible.

Advanced assumptions

Results update as you type. The calculator runs entirely in your browser — nothing you enter is sent anywhere.

Required solar battery bank10.02 kWh · 195.7 Ah

Calculation breakdown

Load energy for autonomy6 kWh
With design margin6.9 kWh
Usable depth of discharge90%
Bank Wh = daily Wh × days × margin ÷ system efficiency ÷ DoD ÷ health
= 3000 × 2 × 1.15 ÷ 0.85 ÷ 0.9 ÷ 0.9
= 10.02 kWh = 195.7 Ah at 51.2 V
  • This sizes storage for a period without useful solar input; panel sizing is a separate energy-production task.

Before you rely on this

Confirm battery/BMS current limits, protection, cabling and manufacturer topology rules before installation.

Method

How the number is reached

Autonomy means the period the bank must carry loads without meaningful charging. Daily load energy is increased by a design margin, then divided by conversion efficiency, usable depth and remaining health.

This page deliberately does not credit expected sunshine during the autonomy period. Use the panel and charging-time calculators to model production separately.

Nominal Wh = daily Wh × autonomy × margin ÷ efficiency ÷ DoD ÷ health
Ah = nominal Wh ÷ bank voltage

Symbols

DoD
Allowed depth of discharge

Worked examples

The same maths, applied

Example

3 kWh/day for two low-solar days

A 51.2 V LiFePO4 bank with 90% DoD, 85% system efficiency, 15% margin and 90% end-of-design health.

Design load = 3,000 × 2 × 1.15 = 6,900 Wh
Bank = 6,900 ÷ 0.85 ÷ 0.90 ÷ 0.90 = 10,021.8 Wh
Ah = 10,021.8 ÷ 51.2 = 195.74 Ah

Result: 10.02 kWh · 195.7 Ah.

Limits of the model

What it assumes, and where it stops

Assumptions

Daily energy is representative and no useful recharge occurs during autonomy.

Not covered

Does not size inverter power, array watts or seasonal solar resource.

Sources

Where these figures come from

  • How to Size a Solar System for Your RVVictron EnergyManufacturer sizing walkthrough connecting daily energy, days of autonomy, system voltage and chemistry-dependent usable capacity, with an explicit reminder to include inverter losses.
  • ESS design — Battery bank capacityVictron EnergyExplains that backup battery capacity is determined by required autonomy and why larger banks reduce cycling stress and extend outage coverage.

Next steps

Related calculators

  • Solar Panel Size Calculator

    Turn a daily energy target and local peak-sun resource into array watts, then round up to whole panels without hiding system losses.

  • Solar Battery Charging Time Calculator

    Estimate how many design-solar days an array needs to move a battery from its starting SOC to the target, with array and charging losses shown separately.

  • Battery Bank Size Calculator

    Turn daily energy use into the nominal kWh and amp-hours your bank needs. The model accounts for days without charging, conversion loss, battery health, usable depth of discharge and whole-battery rounding.

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Last meaningful update: 2026-08-19. This date changes only when the model, the sources or the guidance change.