When sizing a PV system, it is necessary to consider the energy demand before considering the supply. Therefore, PV- system sizing, starts at the load side and proceeds backwards to the array. The objective is to first determine the requirements of the system loads and then to determine the size of the inverter and then the array needed to meet those requirements.
Sizing interactive systems require relatively simple calculations and allow the widest variation in component sizing. This is due to the fact that interactive systems operate parallel with the utility service and when additional energy is required it will take it from the back up power accumulated on previous times. Interactive systems begin with the PV module which is rated at the Standard Test Conditions (STC) to calculate the total expected array DC power output per peak sun hour. Then this is derated by the following losses and inefficiencies in the system: guaranteed module output that is less than 100%, array operating temperature, array wiring and mismatch losses, inverter power conversion efficiency. The result is a final AC power output that is substantially lower but realistically accounts for expected real-world conditions.
To determine the expected energy production per day, the final AC power output is multiplied by the insolation or solar energy for the month or year. If the final system power output is not within the desired range, such as above a minimum size requirement for an incentive program, different module and/or inverter choices can be made. Though the size of an interactive system is primary limited to by the space available for an array and the owner’s budget.