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. That's the wattage; we have 100W, 200W, 300W solar panels, and so on. How much solar energy do you get in your area? That is determined by average peak solar hours. Operated by the Alliance for Sustainable. This solar panel wattage calculator allows you to calculate the recommended solar panel wattage according to the energy consumption of your household appliances.