Free online solar panel output calculator — estimate daily, monthly, and yearly kWh energy production based on panel wattage, number of panels, sun hours, and system efficiency. Losses come from inverter efficiency, wiring, temperature, and dirt. Increasing panel count or choosing higher wattage. Now, the amount of electricity in terms of kWh any solar panel will produce depends on only these two factors: Solar Panel Size (Wattage). Most common solar panel sizes include 100-watt, 300-watt, and 400-watt solar panels, for example. The calculator gives that value. It says solar circuits must not exceed 120% of the busbar rating of your.