Solar Panel Calculator

Prices are approximate for monocrystalline panels from major brands. This is our own estimate; no published source backs these figures. Check your results against your product data or a licensed professional before acting. Budget panels can cost 20-30% less but may have lower efficiency and shorter warranties. Prices and rates on this page are our own estimates as of the date shown; they change often. Enter your own figures for a current result.

Sizing happens in two steps. First find the total array wattage your load needs: divide your daily watt-hours by your peak sun hours and your derate factor. A 3,000Wh daily load at 5 sun hours, 14% losses (the PVWatts default), PVWatts' 96% inverter and the cell-temperature factor 302 / 320 = 0.944 needs 3,000 / (5 × 0.86 × 0.96 × 302 / 320) = 770W of panels. Second, divide that total by the wattage of the panel you want to use: 770 / 400 = 1.9, so two 400W panels. The total array figure drives your charge controller and wiring choices; the panel count is what you actually buy.

If you would rather start from a monthly utility bill than a daily watt-hour load, the home solar panel estimator works that way and reports the panel count and system size in kilowatts.

Fewer large panels (400W+) make sense when roof or ground space is limited, you want fewer mounting points, and your charge controller supports higher-voltage strings. A single 400W panel produces as much as two 200W panels but takes up less total area (panels have dead space around the frame) and needs half the wiring connections. Installers prefer large panels because they reduce labour time.

More small panels (100-200W) make sense for irregular mounting surfaces (curved RV roofs, boat decks, odd-shaped ground areas), portable setups where you fold and move panels, and systems where partial shading is unavoidable. Smaller panels with individual MPPT inputs or microinverters handle shade better because one shaded panel does not drag down the others.

Efficiency also favours larger panels. Modern 400W residential panels achieve 20-22% efficiency, while budget 100W panels sit around 17-19%. Higher efficiency means more watts per square foot, which matters when space is your constraint.

A typical van conversion uses 1,500Wh per day (12V fridge, lights, fan, phone/laptop charging). The van travels the US Southwest with 5.5 average peak sun hours. With 20% system losses, the 96% inverter efficiency and the 302 / 320 cell-temperature factor this calculator applies: 1,500 / (5.5 x 0.80 x 0.96 x 0.944) = 376W minimum.

Option A: Two 200W panels (400W total). Cost (early 2026): ~$300-360. Fits on a standard Sprinter roof with room for a vent fan. Provides a 6% buffer above minimum. Each panel is light enough for one person to lift onto the roof.

Option B: One 400W panel. Cost (early 2026): ~$250-350. Uses less roof space but is heavier and needs a larger charge controller input. A single panel failure means zero solar until replaced.

Option C: Four 100W flexible panels. Cost (early 2026): ~$320-400. Follows the van roof curve without a rack. Lighter total weight but lower efficiency and shorter lifespan (flexible panels degrade faster). Partial shading on one panel does not kill the array if each panel has its own MPPT channel.

For most van builds, Option A wins: two rigid 200W panels balance cost, reliability, and roof space.