Solar Battery Size Calculator

Calculate the battery bank capacity you need for your solar energy system. Enter your daily energy usage, desired autonomy, and system parameters to get your required battery size in kWh and Ah, plus battery count recommendations.

Daily Energy Usage (kWh)

Autonomy Days (backup)

Depth of Discharge — DoD (%)

Battery Efficiency (%)

System Voltage (V)

Battery Cost ($/kWh, optional)

Battery Sizing Formula

Capacity (kWh) = (Daily kWh × Days) / (DoD% × Efficiency%)

Ah Capacity = Capacity (Wh) / System Voltage

DoD limits how deeply you cycle batteries to maximise lifespan. Efficiency accounts for charge/discharge losses. Both are expressed as decimals in the formula.

How to Use the Solar Battery Calculator

1

Enter Daily Energy Usage

Sum the watt-hours of all appliances you plan to run. Divide by 1000 to convert Wh to kWh. A typical off-grid cabin uses 2–5 kWh/day.
2

Choose Autonomy Days

How many consecutive cloudy days must your battery cover without solar charging? 2–3 days is common for most systems.
3

Set DoD and Efficiency

Lithium batteries handle 80–90% DoD; lead-acid should stay above 50% (50% DoD). Efficiency is typically 95% for lithium, 85% for lead-acid.
4

Select System Voltage

Higher voltages (48V) reduce current and cable losses for larger systems. 12V suits small portable setups; 24V–48V for home systems.

Example Calculation

Daily usage 5 kWh, 2-day autonomy, 80% DoD, 95% efficiency, 24V system:

Capacity = (5 × 2) / (0.80 × 0.95) = 10 / 0.76

Capacity = 13.16 kWh

Ah at 24V = 13,158 Wh / 24V = 548 Ah

100Ah batteries needed = ceil(548/100) = 6 batteries

Frequently Asked Questions

What is Depth of Discharge (DoD)?

DoD is the percentage of a battery's total capacity that can safely be used before recharging. Lithium iron phosphate (LiFePO4) batteries can handle 80–90% DoD with thousands of cycles. Lead-acid batteries should be kept above 50% DoD (50% max discharge) to avoid shortening lifespan significantly.

What system voltage should I choose?

12V systems are common for small RV and marine applications. 24V suits medium off-grid systems (1–5 kWh/day). 48V is preferred for larger home systems (5+ kWh/day) because higher voltage means lower current, reducing wire sizes and losses. Most modern all-in-one inverter-chargers run at 48V.

How many solar panels do I need to charge my battery bank?

Divide your daily energy usage (kWh) by your location's peak sun hours (typically 4–6 hours). For 5 kWh/day with 5 peak sun hours: 5 / 5 = 1 kW of solar panels needed, accounting for system losses. Add 20–25% margin for efficiency losses in wiring, charge controllers, and inverters.

Lithium vs lead-acid — which is better for solar?

Lithium batteries (LiFePO4) cost more upfront but offer longer cycle life (2,000–6,000 cycles vs 500–1,000 for AGM), higher DoD, faster charging, and consistent voltage. Over 10 years, lithium is typically cheaper on a cost-per-cycle basis and requires no maintenance. Lead-acid remains viable for budget-constrained or infrequently used systems.