Solar + Battery System Sizing Calculator
Enter your daily power load, peak sun hours, and how many cloudy days you want covered. Get the minimum solar panel wattage and battery bank capacity needed for your off-grid or backup system.
Size your solar + battery system
All inputs are for your total system, not per panel or per battery.
375W panels · 5,625 Wh battery
For 1,500 Wh/day load, 5 sun hrs, 3-day autonomy.
All three bars are on the same Wh scale. Daily load is what you need to replace each day. Panel generation should match or exceed the load, more is better for cloudy days. Battery bank should be a multiple of daily load equal to your autonomy days.
The math explained
- Panel sizing: Daily load ÷ (peak sun hours × system efficiency) = minimum panel watts
- Battery sizing: Daily load × autonomy days ÷ battery DoD = usable Wh required
- Amp-hours: Wh ÷ system voltage = Ah needed at your system voltage
How to use this calculator
Estimate your daily watt-hour load (list each appliance × hours of daily use, then sum). Enter your location's peak sun hours, 5 is a good default for most of the US. Choose how many cloudy days your battery bank must cover without recharging. The calculator outputs the minimum solar panel wattage plus battery bank size in Wh and Ah at your chosen system voltage.
Tips for success
- Always oversize panels by 20–25% to compensate for real-world efficiency losses, shading, and panel degradation over time.
- LiFePO4 (lithium iron phosphate) batteries are the preferred choice for most home backup systems, longer cycle life, safer chemistry, no maintenance.
- A 48V system is more efficient than 12V for large loads because lower current means less heat and smaller wire gauge requirements.
- For emergency backup only (not daily off-grid), 1–2 days of autonomy may be sufficient if you have a generator as a fallback.
- Add a charge controller sized at least 25% above your calculated panel array current to allow for future expansion.
This calculator gives minimum sizing targets. Real installations should add 20–30% overhead to all figures. Solar output varies day-to-day with weather, seasons, and panel angle. Do not size your system to the absolute minimum or you will run out of power on average days, not just worst-case days.
FAQ
What are peak sun hours and how do I find mine?
Peak sun hours are not hours of daylight, they represent the equivalent number of hours per day when sunlight intensity equals 1,000 W/m². The US average is 4–6 hours. Southern California and Arizona get 6–7. The Pacific Northwest and Northeast get 3–4. NREL's PVWatts tool has location-specific data.
Why does battery type matter so much?
A lead-acid battery rated at 100 Ah should only be discharged to 50% (50 Ah usable) to preserve battery life. A LiFePO4 battery can safely discharge to 80–90%. This means you need nearly twice as many lead-acid batteries as lithium to get the same usable capacity, lead-acid is cheaper upfront but much larger and heavier.
What size inverter do I need?
Your inverter should be sized to handle your peak simultaneous wattage, not your daily total. If you might run a 1,500W microwave and a 500W fridge at the same time, you need at least a 2,000W inverter. Add 20–25% headroom. This calculator focuses on energy (Wh), not peak power, size your inverter separately.
How many panels and batteries does this translate to?
Divide the minimum panel wattage by the wattage of a single panel (typically 300–400W for home panels, 100–200W for portable). For batteries, divide the required Ah by the Ah rating of a single battery at your system voltage. Round up in both cases.
Is this calculation suitable for grid-tied systems?
This calculator is designed for off-grid and battery-backup scenarios. Grid-tied solar without batteries works differently, you size panels to offset your annual utility bill rather than to cover your daily load. Use a utility's solar estimator or PVWatts for grid-tied design.