Solar Battery Backup Size Calculator
Calculates the total battery capacity (kWh) needed to cover your home's energy use for a set number of backup days. Essential when sizing a solar-plus-storage system or planning for grid outages.
Last updated: May 2026
About this calculator
Battery banks are rated in amp-hours (Ah) at the system voltage, so the required capacity is: Required Capacity (Ah) = (dailyUsage_kWh × 1000 × backupDays) / (depthOfDischarge × systemVoltage). The daily energy need in kWh is converted to watt-hours (× 1000) and multiplied by the days of autonomy you want. Discharging a battery too deeply shortens its life, so only the usable fraction counts — the depth of discharge (DoD) is typically 0.80–0.95 for lithium and 0.50 for lead-acid. Dividing by the system voltage converts watt-hours to amp-hours, which is why a 48V bank needs roughly a quarter of the Ah of a 12V bank for the same stored energy.
How to use
A home uses 25 kWh/day, wants 2 days of backup, uses lithium-ion at 80% depth of discharge, on a 48V system. Step 1 — Energy needed: 25 kWh × 1000 × 2 days = 50,000 Wh. Step 2 — Divide by usable fraction × voltage: 50,000 / (0.8 × 48) = 50,000 / 38.4 ≈ 1,302 Ah. You would size a 48V bank of at least 1,302 Ah.
Frequently asked questions
What depth of discharge should I use for solar battery sizing?
Depth of discharge (DoD) depends on battery chemistry. Lithium iron phosphate (LFP) batteries support 80–100% DoD, making them the most space-efficient choice. Standard lithium-ion (NMC) batteries are typically rated at 80–90% DoD. Lead-acid batteries should only be discharged to 50% DoD to avoid drastically shortening their cycle life. Always use the manufacturer's recommended DoD, not the theoretical maximum, when sizing your system.
How many days of battery backup do I need for my home?
The right number of backup days depends on your local grid reliability and your energy goals. For brief outages, 1 day of storage is usually sufficient and is the most cost-effective starting point. Households in areas with frequent multi-day outages or those aiming for full off-grid independence typically plan for 3–5 days. Keep in mind that your solar panels will continue to recharge the batteries during a sunny backup period, so the required capacity may be less than a purely offline calculation suggests.
Why does system voltage matter when sizing a solar battery bank?
System voltage — commonly 12V, 24V, or 48V — determines how batteries must be configured in series and parallel to achieve your target capacity. Higher voltage systems (48V) are more efficient because they carry the same power at lower current, reducing resistive losses and heat in wiring. While system voltage does not change the total kWh you need, it does affect which battery products are compatible and how many cells must be wired together. Most modern home solar storage systems operate at 48V for this efficiency advantage.