Solar Battery Bank Size Calculator — DIY Off-Grid Sizing
Calculate exactly how much battery capacity you need for an off-grid or backup solar system. Enter daily energy use and get capacity, cost and autonomy results.
How to use this calculator
- Enter your total daily energy consumption in watt-hours (Wh). Add up the wattage × hours used for each appliance.
- Enter your desired days of autonomy — how many cloudy days you need to run without solar input (typically 1–3 days).
- Select your battery type. Lithium (LiFePO4) typically allows 80–90% depth of discharge; lead-acid should stay above 50%.
- Your required battery bank size, number of units and estimated cost appear instantly.
Understanding your results
Battery bank size is calculated as: (daily Wh × days autonomy) ÷ depth of discharge. A 3,000Wh/day load with 2 days autonomy and 80% DoD requires 7,500Wh (7.5kWh) of capacity.
Depth of discharge (DoD) is how far you can drain the battery before damaging it. LiFePO4 lithium batteries handle 80–95% DoD routinely. Lead-acid batteries should not exceed 50% DoD or their lifespan drops sharply.
Days of autonomy is how many days you need the system to run without solar input. For most backup systems, 1–2 days is sufficient. True off-grid systems in cloudy climates often plan for 3–5 days.
Cost estimate uses current average installed prices per kWh. LiFePO4 typically costs $400–$700/kWh installed; lead-acid $150–$250/kWh but with shorter lifespan.
Frequently asked questions
Direct answers for US homeowners.
Related solar guides
In-depth sizing, cost, and payback articles — with state-by-state data.