Solar Battery Bank Calculator — Size Your Battery Storage
Total Battery Capacity (Wh)
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Battery Bank (Ah)
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Approximate Batteries Needed
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Sizing a Battery Bank for Off-Grid Solar Systems
A properly sized battery bank is the backbone of any reliable off-grid solar power system. This calculator determines the total battery capacity needed based on your daily energy consumption, desired days of autonomy (backup during cloudy weather), and the battery depth of discharge (DoD). Undersizing the battery bank leads to power outages, while oversizing wastes money on unnecessary storage capacity.
The calculation follows the standard formula: Required Capacity (Wh) = Daily Energy Use (Wh) * Days of Autonomy / Depth of Discharge. Depth of discharge is the percentage of the battery that can be safely used before recharging. Lead-acid batteries should only be discharged to 50% (DoD = 0.5) for long cycle life, while lithium iron phosphate (LiFePO4) batteries can safely use 80-90% of their capacity. For example, with 5000 Wh daily use, 2 days of autonomy, and 50% DoD, you need 5000 * 2 / 0.5 = 20,000 Wh or 20 kWh of battery capacity.
Battery bank sizing must also account for temperature derating, as batteries lose capacity in cold weather. Lead-acid batteries at 0 degrees Celsius deliver only about 70% of their rated capacity. System voltage (12V, 24V, or 48V) determines how many batteries are wired in series and parallel. Higher system voltages reduce current flow and allow smaller wire sizes. Use this calculator as a starting point, then consult with a solar installer to finalize your design based on local climate, roof orientation, and seasonal energy needs.
Frequently Asked Questions
What is depth of discharge (DoD)?
DoD is the percentage of battery capacity you use before recharging. Lead-acid batteries should not exceed 50% DoD. Lithium batteries can handle 80-90% DoD, giving more usable capacity.
How many days of autonomy do I need?
2-3 days is typical for grid-tied backup. Off-grid systems may need 3-5 days depending on location and sunlight reliability. More autonomy days mean a larger (more expensive) battery bank.
Should I use 12V, 24V, or 48V?
Higher voltages mean lower current and smaller wire sizes. Systems under 2kW often use 12V or 24V. Systems above 2kW should use 48V for efficiency. Most modern inverters support 48V.