RV Solar Panel Calculator & Cabin Off-Grid Panel Sizing | SolarRatio
Calculate how many solar panels you need for your RV, cabin, or off-grid home. Enter daily consumption, peak sun hours, and system efficiency to find required panel capacity.
Solar panel sizing converts your daily energy demand into the minimum array wattage that will reliably charge the battery bank under real-world sky conditions. The calculation balances peak sun hours (PSH) for your specific location, end-to-end system efficiency, seasonal worst-case derating, and battery charge acceptance. Undersizing leads to chronic battery deficit and short cycle life; oversizing wastes capital and may exceed charge-controller MPPT input limits. This tool produces an honest array wattage that matches climate, latitude, and load profile. In the US, PSH varies widely — Phoenix, AZ averages 6.5 h/day while Seattle, WA averages just 3.7 h/day — meaning the same 1,000 Wh/day cabin needs roughly 215 W of panels in Phoenix but nearly 380 W in Seattle under identical efficiency assumptions.
How it Works
Take total daily consumption (Wh/day), divide by peak sun hours for your location (PSH ranges from 1.5 in cloudy temperate winters to 6.5 in desert summer), then divide by overall system efficiency (typically 0.7–0.85 accounting for inverter losses, wire voltage drop, dust, temperature derate, and battery charge efficiency). The result is required STC panel watts. For grid-tie systems, add 10–20% headroom for module degradation over 25 years (0.5–0.7%/year). For off-grid, use the worst-month PSH (typically December for Northern Hemisphere) so the system never starves the batteries. The calculator also reports recommended panels per series string based on MPPT voltage window and Voc temperature coefficient. For US residential installations, NEC 2023 Article 690 governs PV system design and your AHJ will verify that string Voc at minimum site temperature does not exceed equipment ratings.
Usage Scenarios
Residential rooftop installers feed annual kWh consumption from utility bills into the tool to spec a 6–10 kW grid-tie array with proper Voc string voltage at minimum site temperature. A San Diego homeowner averaging 5.8 h/day PSH and paying roughly $0.30/kWh finds that a 7 kW array meets 90% of annual demand and delivers strong ROI under California NEM 3.0 export rules. Off-grid cabin owners use the worst-month PSH approach to determine that a 1,200 W array is required to deliver 2,400 Wh/day in December at 45° N latitude. Agricultural irrigation engineers compute panel wattage for a 1.5 kW submersible pump running 5 hours/day on direct PV without batteries. Remote telecom and SCADA designers oversize the array by 2x to guarantee uptime through extended overcast periods. Marine installations balance limited deck space against PSH-limited tropical mooring to determine the maximum harvest per square meter.
Frequently Asked Questions
How many solar panels do I need for a 3,000 Wh/day system?
With 5 peak sun hours and 80% efficiency: 3,000 / (5 × 0.8) = 750W required. That's 3 × 250W panels or 2 × 400W panels. Always round up for safety margin.
What are peak sun hours and how do I find mine?
Peak sun hours represent the equivalent hours of full 1,000 W/m² sunlight per day. Tropical regions average 5–6h, temperate zones 3.5–5h, northern regions 2.5–3.5h. Use NASA's PVGIS tool for your exact location.
Why is system efficiency less than 100%?
Losses come from inverter inefficiency (5–10%), wiring resistance (2–3%), battery charging losses (5–15%), and temperature derating (5–15% in hot climates). A realistic efficiency factor is 70–85%.
Should I oversize my solar array?
Yes, by 20–30%. Oversizing compensates for cloudy days, panel degradation (0.5%/year), and seasonal variation. A larger array also charges batteries faster on good days.
What is the difference between panel wattage and actual output?
Panel wattage (STC rating) is measured under ideal lab conditions. Real-world output is typically 75–85% of rated wattage due to heat, angle, and atmospheric conditions.
How to Use the Solar Panel Sizing Calculator
Enter daily consumption (Wh), peak sun hours for your location, and system efficiency. Formula: Required capacity (W) = daily consumption / (sun hours × efficiency).
Peak sun hours vary by location: tropical regions ~6h, temperate ~4.5h, northern regions ~3.5h. Use the regional presets as a starting point.
System efficiency accounts for inverter losses, wiring losses, and temperature effects. A default of 80% is typical for most systems. Size up if you have high loads or limited sun.