Solar System Size Calculator

System Size in kW rises as Monthly Usage rises, holding other inputs fixed, following Array Size = Daily Energy Need ÷ (Peak Sun Hours × Derate Factor). Monthly Usage is divided by 30 to estimate your average daily kWh need, then that need is divided by how much usable sunlight your location gets per day after accounting for System Losses, giving the DC array capacity required to offset it.

System Size falls as Peak Sun Hours rises across this calculator's full 1 to 10 hrs/day range, because more usable sunlight per day means each kW of panels produces more energy, so a smaller array can still meet the same Monthly Usage. This is why an identical household's recommended system is smaller in sun-rich Arizona than in a cloudier northern climate.

No -- by design, Annual Production stays tied to Monthly Usage regardless of Peak Sun Hours or System Losses, because System Size is solved specifically so that the recommended array's production matches your annual usage under whatever sun-hours and loss assumptions you enter. Raising Peak Sun Hours shrinks the recommended System Size by exactly the amount needed to keep Annual Production unchanged.

System Losses bundles inverter conversion loss, wiring loss, soiling and shading, and temperature derating into one percentage, applied as a 1-minus-loss multiplier on production. The 14% default matches NREL's PVWatts v8 default total system loss, a widely used industry reference; real systems commonly range from about 10% in a well-designed cool-climate install to 20%+ in hot, shaded, or older installations.

No -- Panel Wattage does not move System Size at all across its full 200 to 600 W range, since System Size depends only on Monthly Usage, Peak Sun Hours, and System Losses. Panel Wattage only changes how many individual panels are needed to reach that same System Size: higher-wattage panels mean fewer panels for the same total capacity.