To get there, use the following formulas; 1 Amp AC = 10 Amps DC. (example, 2AC amps =20DC amp) Add 10% (22 amps) DC amps x 12v = DC watts. (22 x12 =264 watts) 264 would be entered in field # 3. Solar Panel Calculator is an online tool used in electrical engineering to estimate the total power output, solar system output voltage and current when the number of solar panel units connected in series or parallel, panel efficiency, total area and total width. It will display separate results for series and parallel configurations, helping you compare the two setups. . How many watts is a 400W solar panel? The number in the panel's name is its rated wattage. But remember, that's under test conditions. Think of it like a car's fuel rating it shows potential, not. . Estimates the energy production of grid-connected photovoltaic (PV) energy systems throughout the world. It allows homeowners, small building owners, installers and manufacturers to easily develop estimates of the performance of potential PV installations.