Part of the solar installation process is determining the size of the system you need. In order to estimate the size of the system you need, start by gathering your daily kWh energy requirement. Once you have this, divide it by your peak sun-hours. This will give you the kW output required. Next divide the kW output by your panel's efficiency. The answer will equal the total number of solar panels your system requires. Typically, the larger the system size, the cheaper the average cost per watt is. For example, a 4kW system costs $3.72 on average per watt while a 8kW system costs $3.21 on average per watt.