Solar Battery Storage Calculator
Find the battery bank capacity (in amp-hours) needed to power your home for a set number of backup days. Use this when designing an off-grid system or adding storage to a grid-tied solar setup.
About this calculator
Battery storage capacity must cover your energy needs for the desired backup period without over-discharging, which shortens battery life. The formula is: Required Capacity (Ah) = (dailyUsage × backupDays × 1000) / ((depthOfDischarge / 100) × systemVoltage). Multiplying daily usage (kWh) by backup days gives total energy needed in kWh; multiplying by 1,000 converts to watt-hours. Depth of discharge (DoD) reflects how much of the battery's rated capacity can safely be used — lithium batteries typically allow 80–95% DoD, while lead-acid batteries should stay above 50% (DoD = 50). Dividing by system voltage converts watt-hours to amp-hours, the standard rating on battery spec sheets.
How to use
A cabin uses 3 kWh/day and wants 2 days of backup. The system runs on 48 V and uses lithium batteries with 80% DoD. Calculate: (3 × 2 × 1000) / ((80/100) × 48) = 6,000 / 38.4 ≈ 156 Ah. You would need a battery bank rated at least 156 Ah at 48 V. A common choice would be four 12 V / 200 Ah batteries wired in series, giving 192 Ah — providing a small safety margin above the calculated minimum.
Frequently asked questions
What is depth of discharge and why does it matter for battery sizing?
Depth of discharge (DoD) is the percentage of a battery's total capacity that can be used before recharging is needed. Discharging lead-acid batteries below 50% DoD significantly shortens their cycle life, sometimes halving it. Lithium iron phosphate (LFP) batteries tolerate 80–95% DoD with minimal degradation. Using a higher DoD means you need a smaller bank for the same usable energy, but you must match the DoD setting to the specific battery chemistry to avoid premature failure.
How do I choose between 12V, 24V, and 48V solar battery systems?
System voltage affects wire sizing, inverter options, and efficiency. Small systems under 1 kW often use 12 V for simplicity. Systems between 1–3 kW typically use 24 V to keep current (and thus wire losses) manageable. Larger residential systems above 3 kW almost always use 48 V because halving the current compared to 24 V dramatically reduces resistive losses and allows thinner, cheaper wiring. Most modern off-grid inverters are designed around 48 V.
How many days of battery backup do I actually need for a home solar system?
The right number of backup days depends on your climate and grid reliability goals. In areas with predictable sun, 1–2 days of storage often bridges overnight and cloudy periods adequately. Fully off-grid systems in cloudy climates may need 3–5 days of storage or a backup generator. Grid-tied systems with battery backup typically size for 1 day to handle outages while keeping costs reasonable. Adding more backup days increases battery cost roughly linearly, so most homeowners balance coverage against budget.