Solar Battery Size Calculator
Calculates the minimum battery bank capacity (in kWh) needed to cover your home's energy use during an outage or off-grid period. Use it when designing a solar-plus-storage system or selecting a home battery like the Tesla Powerwall.
About this calculator
Battery capacity must account for both your energy needs and the usable fraction of the battery, since discharging a lithium battery below its minimum state of charge degrades it prematurely. The formula is: Required Capacity (kWh) = (dailyUsage × backupDays) / (depthOfDischarge / 100). Daily usage is your average consumption per day in kWh (from your utility bill). Backup days is how many days you need the battery to supply power without solar recharging. Depth of discharge (DoD) is the percentage of total battery capacity that is safely usable — lithium-ion batteries like the LG Chem RESU are typically rated at 90–95% DoD, while lead-acid batteries are limited to 50%. Dividing by the DoD fraction scales up the required nameplate capacity so the usable portion still covers your needs.
How to use
Example: Your home uses 30 kWh per day, you want 2 days of backup power, and your lithium battery has a 90% depth of discharge. Step 1 — Enter Daily Energy Usage: 30 kWh. Step 2 — Enter Backup Days: 2. Step 3 — Enter Depth of Discharge: 90%. Step 4 — The calculator computes: (30 × 2) / (90 / 100) = 60 / 0.90 = 66.67 kWh. You would need approximately 66.7 kWh of total nameplate battery capacity — for example, four Tesla Powerwalls (each 13.5 kWh usable, 27 kWh total for two days of backup for critical loads).
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 discharged before it needs recharging. For example, a 10 kWh battery with 80% DoD delivers 8 kWh of usable energy per cycle. Repeatedly discharging beyond the rated DoD shortens battery lifespan significantly by stressing the cells. Lithium iron phosphate (LFP) batteries, used in many home storage systems, typically allow 80–100% DoD with thousands of cycles. Traditional lead-acid batteries degrade quickly if discharged below 50%, making them much less space-efficient. Always use the manufacturer's rated DoD, not 100%, to calculate the nameplate capacity you actually need.
How many days of backup power should I plan for when sizing a solar battery?
The right number of backup days depends on your risk tolerance and typical outage patterns in your area. Most homeowners choose 1–2 days of backup for grid outages caused by storms, which covers the majority of events. If you live in an area prone to extended outages (hurricanes, wildfires, ice storms) or are fully off-grid, 3–5 days is a more prudent design target. Keep in mind that during those backup days your solar panels may still be generating power, effectively extending your backup duration beyond what the calculator shows. For critical medical equipment or a fully off-grid cabin, consult a qualified solar designer for a site-specific analysis.
What is the difference between a battery's nameplate capacity and its usable capacity?
Nameplate capacity is the total energy a battery can theoretically store, stated in kWh on the product label. Usable capacity is the amount you can actually draw out per cycle without harming the battery, determined by the depth of discharge rating. For example, the Tesla Powerwall 2 has a 13.5 kWh nameplate capacity and a 13.5 kWh usable capacity at 100% DoD — an unusually high figure made possible by its LFP chemistry. In contrast, a 10 kWh lead-acid bank at 50% DoD provides only 5 kWh usable. When comparing batteries, always compare usable capacities and cycle-life warranties rather than nameplate figures alone.