A 40W solar panel, under optimal conditions, can yield a current output around 2. 5 to 3 amps, generating an estimated 200Wh of energy when adequately harnessed. Estimate daily, monthly, and yearly solar energy output (kWh) based on panel wattage, quantity, sunlight hours, and efficiency factors. Losses come from inverter efficiency, wiring, temperature, and dirt. Increasing panel count or choosing higher wattage. Most common solar panel sizes include 100-watt, 300-watt, and 400-watt solar panels, for example. How Much Sun Do You Get (Peak Sun Hours).