Solar battery storage calculator for sizing an off-grid or backup battery system, plus ROI versus a diesel generator.
Four tabs: backup battery sizing (DOD, system voltage), complete off-grid system (panels + battery + inverter), discharge runtime, and ROI vs. diesel genset.
Disclaimer: Initial estimate. Actual sizing requires a professional load analysis.
Calculator information
📋 How to use this calculator
- Backup Battery Sizing tab: enter critical load (watts), backup duration (hours), Depth of Discharge (DOD 50% for lead-acid or 80-90% for LiFePO4), and system voltage (12/24/48 V).
- Off-Grid System tab: enter daily consumption (kWh/day), local peak sun hours (most of the U.S. 4-5.5 hours; Southwest 5.5-6.5), inverter efficiency 90-95%, and days of autonomy (2-3).
- Discharge Runtime tab: enter battery capacity (Ah), voltage, and load (watts) for runtime estimates with Peukert correction for lead-acid.
- ROI vs Generator tab: enter generator cost ($1,000-$2,500 for 5 kW), gasoline consumption (~0.4 L/kWh), fuel price (~$3.50/gal), and battery system cost to find break-even.
- Always oversize by 20-30% to account for consecutive cloudy days and ~0.5%/year panel degradation.
- Tip: LiFePO4 costs more upfront but delivers 2,000-6,000 cycles vs 300-500 for lead-acid, giving a lower lifetime cost per kWh.
🧮 Battery Sizing and Solar Panel Sizing
Battery_Ah = (Load_Wh x Autonomy) / (V_system x DOD x Inverter_eff); Panel_Wp = (Load_kWh x 1000) / (PSH x System_eff)
- Load_Wh: total daily consumption in watt-hours
- Autonomy: number of backup days without charging (2-3 days standard off-grid)
- V_system: battery system voltage (12/24/48 V)
- DOD: maximum Depth of Discharge (50% lead-acid, 80-90% LiFePO4)
- Inverter_eff: inverter efficiency 0.9-0.95
- PSH: Peak Sun Hours, U.S. average 4-5.5 hours/day (NREL)
- System_eff: total system efficiency 0.7-0.8 (panels + MPPT + battery + inverter)
Peukert exponent for lead-acid is 1.1-1.3 (capacity drops at high discharge rates). LiFePO4 is near 1.0 (capacity is essentially linear).
💡 Worked example: Off-grid cabin in upstate New York (5 kWh/day load)
Given:- Daily load 5 kWh (LED lights, mini fridge, charging, fan)
- Peak Sun Hours: 4.2 h/day
- 48 V system, LiFePO4 batteries at 80% DOD
- Inverter efficiency 92%, system efficiency 75%
- 2 days of autonomy
Steps:- Battery capacity = (5,000 Wh x 2 days) / (48 V x 0.8 x 0.92) = 10,000 / 35.33 = 283 Ah
- Choose a 48 V 300 Ah LiFePO4 bank (~14.4 kWh)
- Panel capacity = 5,000 / (4.2 x 0.75) = 5,000 / 3.15 = 1,587 Wp
- Choose 6 x 300 Wp panels = 1,800 Wp (13% oversizing)
- Estimated 2026 costs: panels at ~$1,000/kWp x 1.8 = $1,800; LiFePO4 batteries at ~$400/kWh x 14.4 = $5,760; 3 kW hybrid inverter ~$1,500; wiring + MPPT + racking ~$1,200
- Total investment: ~$10,260
- 5 kW gasoline generator: 0.4 L/kWh x 5 kWh x 365 days = 730 L (~193 gal) x $3.50/gal = $676/year fuel + ~$150 maintenance = $826/year
- Payback vs generator (assuming free solar): 10,260 / 826 = ~12.4 years (worthwhile for sites off the utility grid; the generator wins on simple cost where grid power is available, but loses on reliability and noise)
Result: Off-grid system of 1.8 kWp + 14.4 kWh battery for about $10,260. Economical if the site is beyond utility lines; not economical vs a generator where the grid is reachable.
❓ Frequently asked questions
What are typical Peak Sun Hours in the U.S.?
Per NREL data, Peak Sun Hours (PSH) in the U.S. average 4-5.5 kWh/m²/day. The Southwest (Arizona, New Mexico, Nevada) is highest at 5.5-6.5 PSH. The Sun Belt and California sit around 5.0-5.5 PSH. The Northeast and Pacific Northwest are 3.5-4.5 PSH due to more cloud cover. For conservative design, use the worst-month PSH (typically December-January), not the annual average.
Lead-acid vs LiFePO4 for solar storage - which is better?
LiFePO4 (lithium iron phosphate) wins in the long run: 2,000-6,000 cycles vs 300-500 for lead-acid, 80-90% DOD vs 50%, and 95-98% round-trip efficiency vs 80-85%. LiFePO4 costs 2-3x more upfront (~$400-600/kWh vs $150-250/kWh for lead-acid), but cost per kWh-cycle over the lifetime is lower. Lead-acid is still used for tight budgets, small systems, or infrequent backup.
What is DOD and why does it matter?
Depth of Discharge (DOD) is the percentage of battery capacity you can use before recharging. Lead-acid should stay at 50% DOD for long life (80% DOD roughly halves cycle life). LiFePO4 is safe at 80-90% DOD with no major degradation. A 100 Ah lead-acid battery used at 50% DOD only delivers 50 Ah of usable capacity. Always size based on usable capacity, not nameplate.
How do I size the inverter for a solar system?
Size the inverter for simultaneous peak load, not total daily consumption. Inductive loads (AC, fridge, pump) need a 2-3x surge factor for startup. Typical residential systems run 3-5 kW continuous. Use a pure sine wave inverter for sensitive electronics. Hybrid inverters (combined inverter + charger + MPPT) save space but cost 30-50% more. Aim for at least 92% efficiency.
When is solar plus storage economical vs the grid?
In the U.S., residential electricity averages about $0.16/kWh (EIA), with higher rates in California, Hawaii, and the Northeast. Grid-tied solar without storage typically pays back in 7-12 years; add batteries and payback stretches to 10-15 years depending on incentives. Solar plus storage is most economical for: off-grid sites, areas with high time-of-use rates (e.g., California NEM 3.0), frequent outages (hurricane/wildfire zones), or homeowners taking advantage of the federal 30% ITC and state rebates.
📚 Sources & references
Last updated: May 11, 2026