How do you calculate solar battery backup size?

Battery size is calculated from load power multiplied by backup hours, then adjusted for inverter efficiency, battery depth of discharge and reserve margin.

What is the difference between Ah and kWh for batteries?

Amp-hours depend on battery voltage, while kWh is total stored energy. Battery kWh equals voltage multiplied by amp-hours divided by 1000.

Solar Battery Backup Calculator | Battery Size & Runtime

Solar Battery Backup Calculator

Calculate solar battery backup size, runtime, Ah, kWh and autonomy days for home loads, inverters, off-grid systems and solar pump backup.

☀️ Field Rule: Battery backup sizing starts with load watts × backup hours, then adjusts for inverter efficiency, battery depth of discharge and reserve margin. Use this with the Solar Panel Output Calculator, Solar Inverter Size Calculator, Solar Pump Sizing Calculator, Peak Sun Hours Calculator and Solar Panel Cleaning Loss Calculator.

🔋 Solar Panels → Charge Controller → Battery Bank → Loads

Total Load Power

Backup Runtime Needed

Battery Bank Voltage

Battery Type / DOD

Depth of Discharge

Inverter Efficiency

Reserve Margin

Daily Cycles

Result Mode

For lead-acid batteries, avoid deep discharge. Lithium batteries can usually use a higher depth of discharge, but always follow the battery datasheet.

Presets:home backuprouter + lightspump backup

Battery Capacity

Battery Voltage

Load Power

Usable DOD

Inverter Efficiency

Reserve Margin

Runtime depends on usable energy, not just Ah. A 100Ah battery stores different energy at 12V, 24V and 48V.

Battery Energy to Recharge

Peak Sun Hours

Charge Controller Efficiency

Solar Loss Margin

Recharge Target

Panel Wattage

Recharge calculation estimates extra solar array needed for battery charging. Also size panels for normal daytime loads.

📐 Formula Reference

Load Energy

Wh = load watts × backup hours

Battery kWh

Battery kWh = load Wh ÷ inverter efficiency ÷ DOD × reserve

Battery Ah

Ah = battery Wh ÷ battery voltage

Recharge Solar

PV Wp = battery kWh ÷ peak sun hours ÷ efficiency × loss margin

📋 Quick Reference

Typical DOD

Lead-acid50%

AGM / Gel60%

LiFePO₄80–90%

System Voltage

Small backup12/24V

Home backup48V

Large systemsHV battery

Must Check

Surge loadinverter

Charge currentbattery

Cable sizehigh current

📚 Engineering Notes

Ah alone can misleadA 200Ah battery at 12V stores much less energy than a 200Ah battery at 48V. Compare batteries in kWh.

Use realistic DODLead-acid life drops fast with deep discharge. Lithium systems usually allow deeper discharge but still need BMS and datasheet limits.

Inverter size is separateUse the Solar Inverter Size Calculator for surge and continuous AC load checks.

Recharge solar must be enoughUse the Peak Sun Hours Calculator, Solar Panel Output Calculator and Solar Panel Cleaning Loss Calculator for practical recharge estimates.

What is a Solar Battery Backup Calculator?

A solar battery backup calculator estimates the battery capacity needed to run loads during power cuts, nighttime or cloudy periods. It calculates battery kWh, Ah, usable energy, backup runtime and solar recharge requirement.

How to calculate battery backup size

Multiply load power by backup hours to get load energy. Then divide by inverter efficiency and usable depth of discharge. Add reserve margin to avoid undersizing the battery bank.

Solar battery design workflow

Use the Solar Inverter Size Calculator for inverter power, the Solar Panel Output Calculator for panel production, the Peak Sun Hours Calculator for sun availability and the Solar Cable Loss Calculator for wiring losses.

Important limitation

This calculator is a planning aid. Final battery selection must follow battery datasheet, BMS current limit, inverter surge rating, charge controller rating, temperature limits, cable sizing and local electrical rules.

❓ Frequently Asked Questions

It depends on your load power, backup hours, battery voltage, depth of discharge and inverter efficiency. This calculator gives kWh and Ah requirements.

kWh is better for comparing energy. Ah is useful only when battery voltage is also known.

Only if the solar array has enough production after losses and peak sun hours. Use the solar recharge tab to estimate panel wattage.