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.
About this calculator
Battery capacity is measured in kilowatt-hours (kWh), but not all of that energy is usable. Discharging a battery too deeply shortens its life, so manufacturers specify a Depth of Discharge (DoD) — typically 80–90% for lithium-ion and 50% for lead-acid. The formula accounts for this: Required Capacity (kWh) = (dailyUsage × backupDays) / (depthOfDischarge / 100). For example, if you want to cover 30 kWh of usage over 3 days using a battery with 80% DoD, you need 30 × 3 / 0.8 = 112.5 kWh of installed capacity. System voltage affects how many battery cells must be wired in series but does not change the total energy calculation. Always add a 10–20% safety margin for real-world efficiency losses.
How to use
Assume your home uses 25 kWh per day, you want 2 days of backup, and you are using lithium-ion batteries with an 80% depth of discharge. Step 1 — calculate total energy needed: 25 × 2 = 50 kWh. Step 2 — divide by DoD as a decimal: 50 / (80/100) = 50 / 0.8 = 62.5 kWh. You need at least 62.5 kWh of installed battery capacity. If using 10 kWh battery modules, you would purchase 7 modules (70 kWh total) to cover the shortfall with a small buffer.
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.