Then you take your array size and divide that by the watt rating of a panel like a 455W panel to find out how many solar panels you''ll need. EG: 900kWh average monthly usage / 30 days = 30kWh daily usage 30kWh / 5.5 average maximum production hours = 5454.54kWh array size needed 5454.54kWh / 455W solar panel rating = 11.988 …