AE 868
Commercial Solar Electric Systems

System Sizing Overview


In previous lessons, we learned about PV modules, batteries, and inverters in terms of performance characteristics and main parameters. At this point in the class, we will introduce the concept of sizing PV systems. System sizing involves the detailed calculations of the energy produced by the PV system and matches it with the desired energy demand. We will put the knowledge we learned together, in addition to introducing some sizing tools that can be used to determine the exact number of modules needed to form a PV array. The main parameter that PV designers name as a goal when designing a PV array is the load energy demand or energy usage in (kWh).

When describing a PV system in terms of components, it is logical to use the energy flow path from the array side to the load side. However, when sizing a PV system, it is necessary to consider the energy demand before considering the PV supply side. For that reason, PV system sizing starts at the load side and proceeds backward to the PV array side. For example, PV designers need to know the energy usage (kWh) before choosing the PV array size.

Limitations to sizing

Not all clients/sites are ready to accommodate PV arrays. That can be due to one or more of the following factors:

  • Factor 1: Space availability for the PV array (roof square footage/meter)
  • Factor 2: Owner’s budget
  • Factor 3: Annual energy consumption (client's energy bill in kWh)

PV systems are generally sized to maximize the solar utility for a client within these aforementioned limits. For the purpose of this class, we will focus only on the last factors that influence the sizing of any PV system. Before diving into the energy demand, let's elaborate more on these factors.

Factor 1: Space availability for the PV array (roof square footage/meter)

Before installing a PV system, it is important to make sure that the site is suitable for installation by considering the following factors:

  • Shade-free (Sites are best optimized when there is no shading on the PV array.)
  • South-facing (In the Northern Hemisphere, the sun path is generally leaning towards the south. The opposite applies to the Southern Hemisphere where the sun appears to be towards the north of an observer.)
  • No obstruction (Sites need to have minimum obstructions before installing PV arrays. Obstructions may include chimneys, gables, HVAC units on rooftops, and so on.)

Factor 2: Owner’s budget

Budget is an essential factor that plays a huge role in the design process. It varies with available rebates and incentives at each location. As we mentioned previously, this class will not consider the finances of PV systems, as it is covered in other RESS classes.

Factor 3: Annual energy consumption (client's energy bill)

When sizing a PV system, It is important to consider the following criteria:

  • PV system size to meet 100% of client's annual energy usage kWh
  • PV system size to be permissible by utility interconnection rules

In the next topic, we will discuss how annual energy demand is estimated and how to go about understanding the client’s energy bill.