Solar Battery Storage Size Calculator
Right-sizing a solar battery storage system requires balancing your daily energy needs, desired backup duration, battery chemistry constraints (depth of discharge), and round-trip efficiency losses. This calculator applies the NREL (National Renewable Energy Laboratory) standard sizing formula to determine the minimum nameplate battery capacity you need. Enter your daily energy use, how many days of backup you want, and your battery specifications to get a recommended system size in kWh.
Battery storage sizing formula
Usable Capacity (kWh) = Daily kWh x Days of Backup
Nameplate Capacity (kWh) = Usable Capacity / (DoD / 100) / (RTE / 100)
Estimated Cost = Nameplate Capacity x Cost per kWh
This formula is consistent with NREL's methodology for residential battery storage sizing, as described in NREL Technical Report NREL/TP-6A20-53447. The round-trip efficiency accounts for energy lost during the charge and discharge cycle. The DoD factor ensures the battery is not stressed beyond its rated capacity limits.
Battery technology comparison
- Lithium Iron Phosphate (LFP): DoD 80-90%, RTE 92-98%, ~3,000-6,000 cycles, safest lithium chemistry. Example: Tesla Powerwall 3, LG RESU.
- Nickel Manganese Cobalt (NMC): DoD 80%, RTE 92-96%, ~1,500-3,000 cycles. Higher energy density than LFP.
- Lead-acid (AGM/flooded): DoD 50%, RTE 75-85%, ~500-1,200 cycles. Lower upfront cost but higher cost per cycle.
- Flow batteries (vanadium): DoD 80-100%, RTE 65-80%, very long cycle life (20,000+). Best for multi-day commercial storage.
Solar battery storage: frequently asked questions
How is solar battery storage capacity sized?
Battery storage capacity is sized based on: (1) daily energy consumption in kWh, (2) the number of days of autonomy (backup days) required, (3) the maximum depth of discharge (DoD) allowed by the battery chemistry, and (4) the round-trip efficiency of the battery system. Required capacity = (Daily kWh x Days) / (DoD x Round-trip efficiency).
What is depth of discharge (DoD)?
Depth of discharge is the percentage of the battery's total capacity that can be used before it must be recharged. Lithium iron phosphate (LFP) batteries can safely discharge to 80-90% DoD. Lead-acid batteries should be discharged no more than 50% DoD to maximize cycle life. Using a higher DoD than recommended significantly reduces battery lifespan.
What round-trip efficiency should I use?
Round-trip efficiency is the ratio of energy out to energy in for a full charge-discharge cycle. Modern lithium (LFP) batteries achieve 92-98% round-trip efficiency. Lead-acid batteries are typically 75-85%. The Tesla Powerwall 3 specifies 90% round-trip efficiency. NREL uses 85% as a conservative default for system modeling.
How many days of battery backup do I need?
The answer depends on your goals. For overnight storage of solar excess, 1 day of storage is sufficient. For outage resilience in areas with frequent multi-day outages (hurricanes, ice storms), 2-5 days may be appropriate. NREL research suggests 2 days covers over 95% of outage durations in most US regions.
What is the difference between nameplate capacity and usable capacity?
Nameplate capacity is the total kWh a battery stores. Usable capacity is the portion you can actually use (nameplate x DoD). A 10 kWh battery with 80% DoD has 8 kWh of usable capacity. Always size your system based on usable capacity, not nameplate capacity.
Official sources
- NREL: NREL/TP-6A20-53447: Cost and Performance Assumptions for Modeling Electricity Generation Technologies.
- U.S. DOE Office of Electricity: Energy Storage Technology.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.