Solar panel size calculator
Turn daily energy use into the solar array watts and number of panels you need, using peak sun hours and realistic losses. For RVs, vans, cabins and power stations.
Results
| Solar array needed | 593 W |
|---|---|
| Panels needed | 3 × 200 W |
| Expected daily output of that array | 2,025 Wh/day |
| Real-world output factor | 75 % |
- Size for the worst month you plan to use the system, not the annual average.
About this calculator
A panel's wattage is measured under lab conditions: 1,000 W/m² of sunlight with the cells at 25 °C. On a real roof or van, panels run hotter, the sun moves, dust builds up, and wires, charge controllers and batteries all take a cut. A 400 W array rarely delivers 400 W, and it delivers that level only for the handful of "peak sun hours" each day.
This calculator starts from the energy you use each day, divides by your local peak sun hours and applies a realistic loss factor. That gives an array size that refills your battery on a typical day, rather than only on a perfect one.
How to use it
- Enter the daily watt-hours you want solar to cover.
- Enter peak sun hours for your location and season. The NREL solar resource maps and PVWatts give location-specific values.
- Keep losses at 25% for fixed or flat-mounted panels. Use 30-35% for flat RV roofs in hot climates and 15-20% for well-ventilated, tilted ground mounts.
- Choose your panel size to see how many panels it takes.
The formula
Array watts = daily Wh ÷ (peak sun hours × (1 − losses)).
Panels = array watts ÷ panel watts, rounded up.
Peak sun hours turn a day of changing sunlight into an equivalent number of hours at full lab intensity. For example, 5 peak sun hours means the day delivered 5 kWh per m² of panel area.
Worked example (default values)
Inputs
| Daily energy to replace | 2000 Wh/day |
|---|---|
| Peak sun hours | 4.5 h/day |
| System losses | 25 % |
| Wattage of one panel | 200 W |
Results
| Solar array needed | 593 W |
|---|---|
| Panels needed | 3 × 200 W |
| Expected daily output of that array | 2,025 Wh/day |
| Real-world output factor | 75 % |
Tips and common mistakes
Flat-mounted RV and van panels lose a lot in winter because the sun is low. Tilting them toward the sun can add 20-40% in winter months, but the effect is small in summer.
Portable folding panels often deliver 60-75% of their rating in practice. Use 30-40% losses for them, and see the solar charging time calculator for power stations.
Frequently asked questions
How many solar panels do I need for an RV?
A typical RV using 1.5-3 kWh a day needs roughly 400-900 W of panels with 4-5 peak sun hours. Enter your own daily use above for a firm number.
How many watts of solar do I need to charge a 100Ah battery?
To refill a 100 Ah LiFePO4 battery (1,280 Wh) from 20% to 100% in one 5-sun-hour day with 25% losses, you need about 270-300 W of panels.
What are peak sun hours?
The number of hours of sunlight at an intensity of 1,000 W/m² that would deliver the same energy as the whole day. It's not the same as daylight hours.
Related calculators
- Solar charge controller size calculator
Find the amp rating your MPPT or PWM solar charge controller needs from array watts, battery voltage or panel Isc, with the standard 25% safety margin.
- Solar string voltage calculator (series & parallel)
Check that your solar panels wired in series and parallel stay under your charge controller's max PV voltage on the coldest day, and see array voltage, current and watts.
- Solar charging time calculator
How long will a solar panel take to charge your power station or battery? Includes real-world panel output, solar input limits and peak sun hours.
- What size power station do I need?
List your devices and how many hours a day you run them to get the battery capacity (Wh), continuous output and surge rating your power station needs.
More solar calculators
Off-grid solar sizing, step by step
- Rough out the whole system · Off-grid solar system calculator
- Size the inverter from your loads · Inverter size calculator
- Plan the battery bank · Battery bank series & parallel calculator
- Check battery runtime · Battery bank runtime calculator
- Size the solar array (you are here)
- Check panel string voltage · Solar string voltage calculator (series & parallel)
- Choose the charge controller · Solar charge controller size calculator
- Size the DC cables · DC wire size calculator (12 V, 24 V, 48 V)
- Pick the fuses · DC fuse & breaker size calculator
Estimates for planning only, based on the assumptions shown. Real-world performance varies with equipment, temperature and installation. Updated October 19, 2026. See our methodology.