Free Tool

Solar Battery Bank Size Calculator

Determine exactly how many amp-hours (Ah) and watt-hours (Wh) of battery capacity your off-grid or backup solar system needs — accounting for depth of discharge, battery chemistry, and safety margins.

How to Use This Calculator

Enter your daily energy consumption in watt-hours (Wh). Check your utility bill or add up each appliance's wattage × hours used per day.
Set days of autonomy — how many cloudy or low-sun days your system must run without solar input. Two days is standard for most off-grid homes.
Choose your system voltage — 12V for small/RV systems, 24V for medium systems, 48V for large homes and commercial setups.
Select battery chemistry — Lithium LiFePO4 (90% usable, 95% efficient) or Lead-Acid AGM (50% usable, 85% efficient).
Review your results — the calculator outputs required capacity in Ah and Wh, with and without safety margin.

Battery Sizing Formula Explained

The core formula for solar battery bank sizing is:

Required Capacity (Wh) = (Daily Usage × Days of Autonomy) ÷ (DoD × Efficiency)

Then convert to Amp-Hours: Ah = Wh ÷ System Voltage

Chemistry Comparison

Parameter	Lithium LiFePO4	Lead-Acid AGM
Usable Depth of Discharge	90%	50%
Round-Trip Efficiency	95–98%	80–85%
Cycle Life	3,000–6,000+	500–1,200
Expected Lifespan	10–15 years	3–5 years
Upfront Cost	Higher	Lower
10-Year Total Cost	Lower	Higher (replacements)

System Voltage Selection Guide

System Size	Recommended Voltage	Typical Use
Under 1,000W	12V	RVs, boats, small cabins
1,000W – 3,000W	24V	Medium off-grid homes
Over 3,000W	48V	Full homes, commercial

Frequently Asked Questions

How many batteries do I need for a 2,000W solar system?

It depends on your daily usage and days of autonomy. A 2,000W system running 5 hours/day produces 10 kWh. To store 2 days of energy with LiFePO4 batteries at 48V: 20,000 Wh ÷ (0.90 × 0.95) ÷ 48V ≈ 488 Ah. Use the calculator above for your specific numbers.

What is depth of discharge (DoD) and why does it matter?

DoD is the percentage of a battery's total capacity you can safely use before recharging. Discharging lead-acid batteries beyond 50% permanently shortens their life. Lithium LiFePO4 batteries can safely discharge to 90% without damage, making them significantly more efficient per dollar.

Should I add a safety margin to my battery calculation?

Yes. A 20–25% safety margin accounts for battery aging (capacity degrades 20–30% over time), unexpected loads, and cold-weather performance reduction. The calculator includes this as an optional field.

What is the difference between Ah and Wh?

Watt-hours (Wh) measure total energy stored regardless of voltage. Amp-hours (Ah) measure capacity at a specific voltage. Convert with: Wh = Ah × Volts. Always compare batteries at the same voltage when using Ah ratings.

Can I mix Lithium and Lead-Acid batteries in the same bank?

No. Never mix battery chemistries, different capacities, or batteries of different ages in the same bank. Mismatched batteries charge and discharge unevenly, reducing performance and potentially causing damage or fire.