Electrical current can be transported in two different ways: direct and alternating current. There are advantages and disadvantages for both types of current, but the main reason we use alternating current (AC) is that it can be easily transported and transformed. The whole electrical grid is designed to transport AC. That is why the power you get out of the wall outlet is AC, and therefore the equipments and appliances in your home are also designed to work with AC.
However, the electricity produced by the solar system is DC. You will need to convert (or rather, invert) your power to AC before being able to feed it into the grid. For this purpose you use inverters. They come in a variety of sizes and designs. The inverter power will determine the “nameplate capacity” of the system.
Here we should distinguish between DC and AC system sizes:
DC system size = no. of panels x panel power output
AC system size (nameplate capacity) = inverter power
In Ontario, for optimum system output, the system is designed in a way that the DC system size is 20% greater than the AC side (called oversizing). You may not be able to use all the area available for PV system installations because of this technical constraints.
Let’s do an example to help you understand all these.
I want to use 250W panels for a solar system on my rooftop. I realize there’s available room on my rooftop for installing 120 panels. Therefore the DC system size is:
DC system size: 120 x 250 =30,000 W or 30 kW
Since I have to oversize my system on the DC side, I have consider 20% less power output for the AC output. This will determine the inverter power required. Therefore:
AC size = 30 / 1.2 = 25 kW
I check some inverter manufacturers only to realize that they have wide increments of power; 20 kW and 35 kW are the closest to my system. If I choose the 35 kW inverter, my system will be “undersized” on the DC side, and will never be able to produce maximum power (this will also increase my initial investment). If I choose the 20 kW, my system will strongly oversized on the DC side, and I will produce power only to waste it. Therefore I will have to adjust my system according to the available inverter power. For this purpose, I consider the lower end of inverter powers available. Thus, my nameplate capacity is 20 kW, and adding 20% to it will determine my DC system size:
DC system size = 20 x 1.2 = 24 kW or 24,000 W
no. of panels requiered = 24,000 / 250 = 96 panels
As you realized, I had to compensate on the available area for PV installation, in order to reach an optimum point both for system production and cost.
When we speak of the power output or the system cost, we consider the “DC side” of the system. For instance we express the system cost as $5/W DC; or annual system production as 1,100 kWh/kW DC.