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. For 10kW per day, you would need about a 3kW solar system. 75 / 1000. Caution: Photovoltaic system performance predictions calculated by PVWatts ® include many inherent assumptions and uncertainties and do not reflect variations between PV technologies nor site-specific characteristics except as represented by PVWatts ® inputs. The mode changes what you provide (e., daily vs monthly load, or target kW vs usage-based sizing). Here's a practical example: Imagine you have a 100-watt lightbulb turned on for 10 hours. Whether you're designing a residential solar installation.