Solar Battery Sizing Calculator
Find the minimum battery bank capacity needed to power your home through cloudy days or outages. Use this when designing an off-grid or hybrid solar system and choosing between lithium or lead-acid batteries.
About this calculator
Battery bank sizing ensures you have enough stored energy to meet your daily needs for a chosen number of backup days without over-discharging the cells. The required capacity in watt-hours is: rawWh = (dailyUsage × backupDays) / (depthOfDischarge / 100) / (efficiency / 100). Depth of discharge (DoD) caps how deeply you can drain the battery—typically 50% for lead-acid and 80–90% for lithium—to protect cycle life. System efficiency accounts for inverter and wiring losses, usually 85–95%. Multiplying by system voltage converts watt-hours to amp-hours: capacity (Ah) = rawWh × systemVoltage, though the formula as implemented keeps the result in a combined watt-hour-voltage unit for direct battery bank comparison. Proper sizing prevents premature battery degradation and ensures reliable backup power.
How to use
Assume daily usage of 5 kWh, 2 backup days needed, a lithium battery with 80% DoD, 90% system efficiency, and a 48 V system. Step 1 — raw Wh needed: (5 × 2) / (80/100) / (90/100) = 10 / 0.8 / 0.9 = 13.89 kWh. Step 2 — multiply by system voltage: 13.89 × 48 = 666.7. This result guides you to select a battery bank of at least 666–700 Ah at 48 V, or approximately 14 kWh of usable capacity when accounting for all derating factors.
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 safely used before recharging. Exceeding the recommended DoD shortens the battery's cycle life dramatically. Lead-acid batteries typically allow 50% DoD, meaning a 200 Ah battery provides only 100 Ah usably. Lithium iron phosphate (LFP) batteries tolerate 80–90% DoD, delivering more usable energy per unit of rated capacity. Sizing your bank with the correct DoD ensures the battery lasts its rated number of cycles.
How many backup days should I plan for when sizing a solar battery system?
The right number of backup days depends on your climate and goals. In areas with frequent multi-day overcast periods, 2–3 days of storage is a common design target. Off-grid cabins in cloudy regions may need 4–5 days. For grid-tied homes seeking only outage protection, 1 day is often sufficient since the grid resumes quickly. Over-sizing for backup days significantly increases battery cost, so balance resilience against budget carefully.
What is the difference between lead-acid and lithium batteries for solar storage?
Lead-acid batteries are cheaper upfront but have lower DoD (around 50%), shorter cycle life (300–500 cycles), and require maintenance. Lithium batteries—especially LFP—cost more initially but offer higher DoD (80–90%), longer cycle life (2,000–5,000 cycles), and are virtually maintenance-free. Over a 10-year period, lithium often proves more cost-effective on a cost-per-kWh-cycled basis. Lithium batteries are also lighter and can be discharged and recharged faster, making them the preferred choice for most modern solar installations.