The basic formula to estimate solar output is: Daily Energy (kWh/day) = Panel Wattage × Number of Panels × Sun Hours × Efficiency ÷ 1000 This calculator automates that process and gives you daily, monthly, and yearly energy estimates. Quick answer: A modern 400W solar panel produces about 1. residential median of 5 peak sun hours. 70. 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. solar installer uses is kWh/day = kW × PSH × derate, where the derate factor is 0. 83 by NREL PVWatts v8 default — or 0. When you look at a solar panel label showing “400 W,” it's natural to wonder: how many kilowatt-hours (kWh) will that actually produce in a day? The Solar Panel Wattage to Daily kWh Output Converter.