Solar Battery Backup Calculator
Determines how many kilowatt-hours of battery capacity you need to keep essential appliances running during a grid outage. Use it when sizing a home battery bank paired with a solar array.
About this calculator
The battery capacity needed depends on four factors: how much power your essential loads consume (in watts), how long you want backup coverage (in hours), how efficiently the battery system converts stored energy to usable power, and how deeply you can discharge the battery without degrading it. The formula is: Capacity (kWh) = (essentialLoad × backupDuration) / (1000 × batteryEfficiency × depthOfDischarge). Dividing by 1000 converts watt-hours to kilowatt-hours. Battery efficiency (typically 0.90–0.95 for lithium) accounts for charge/discharge losses. Depth of discharge (DoD) — commonly 0.80 for lithium-ion — limits how much stored energy you actually use, protecting long-term cycle life. A higher DoD or lower efficiency both require a larger installed capacity for the same real-world backup runtime.
How to use
Suppose your essential loads total 800 W (refrigerator, lights, router) and you want 12 hours of backup. Your lithium battery system has 92% efficiency and a DoD of 0.80. Plugging into the formula: Capacity = (800 × 12) / (1000 × 0.92 × 0.80) = 9,600 / 736 ≈ 13.04 kWh. You would need at least a 13 kWh battery bank. Rounding up to the nearest available product size — say 15 kWh — gives you a comfortable safety margin for real-world variation in loads.
Frequently asked questions
How does depth of discharge affect the battery capacity I need for solar backup?
Depth of discharge (DoD) is the fraction of a battery's total capacity you can safely use before recharging. A DoD of 0.80 means only 80% of the rated capacity is usable. Because the formula divides by DoD, a lower DoD forces you to buy a larger battery bank to achieve the same backup hours. Lithium-ion batteries typically allow 80–90% DoD, while lead-acid is limited to around 50%, making lead-acid banks roughly twice as large for the same effective storage.
What is a typical battery system efficiency and why does it matter?
Battery round-trip efficiency describes how much energy comes back out relative to what went in. Modern lithium iron phosphate (LiFePO4) batteries achieve 95–98% efficiency, while older lead-acid types are closer to 80–85%. Lower efficiency means more energy is lost as heat during charging and discharging, so your solar panels and battery bank must be larger to deliver the same backup runtime. Always use the manufacturer's round-trip efficiency figure rather than assuming 100%.
How long can a solar battery backup system power a home during an outage?
Runtime depends on the installed battery capacity, your load size, efficiency, and DoD. Using the rearranged formula, Duration = (Capacity × 1000 × efficiency × DoD) / essentialLoad. A 10 kWh lithium battery (90% efficiency, 80% DoD) can power a 600 W essential load for about 12 hours. Reducing non-essential loads — turning off HVAC, water heaters, and EV chargers — is the quickest way to extend backup time without buying more batteries.