Often owners of photovoltaic systems ask about their power production. “If I bought a 100kW system, and it is sunny, why does it only say 65kW at the inverter? What is wrong?”
PV systems have what is called a DC rating. That is the capacity in DC, direct current, for output based on a standardized testing procedure, STC. These numbers reference the amount of energy that the system is capable of producing. If we install (100) 240watt panels, that is a 24kW (DC) system. So is that what it puts out? Well, no. many factors in real world environments impact the production of solar.
Let’s move away from solar for a second and look at your automobile. It may have a top speed rating of 120mph however have you ever been able to get it to that speed? The answer is probably “no,” that is a speed which is given by the auto manufacture to give an example of what the auto is capable of and does not fully represent real world conditions such us traffic, road construction, law enforcement, etc.
The same is true of solar arrays, they have their own “speed bumps” that prevent them from fully reaching their potential. In an ideal install, factors such as temperature and angle of the sun, even on a wide open roof can impact performance. In less than ideal conditions, other variables that can add to a difference in production are soiling (dirt), shading, and inefficiencies with the wiring.
“PV systems deal in power versus energy production so it is important to understand the differences”
This is a simple explanation, and one your renewable energy provider should be able to go into more depth with you about during your site evaluation, system design, and proposal phase. PV systems deal in power versus energy production so it is important to understand the differences which will allow for greater appreciation of how PV systems are designed, how they work, and what to expect from them.
Power is a term that is used to evaluate energy production instantaneously, which is why it is a popular term to evaluate PV systems. It makes more sense for PV systems to be rated based on the energy that they are capable of producing so that they may be compared more easily to similar systems. One solar panel is rated at 240w, so that is what it puts out in standardized test conditions in the lab. In the field, again, with higher or lower temperatures, dirt and different sun angles this panel puts out much different readings.
Energy is that instantaneous power which is accumulated over time. Citing the aforementioned panel, (10) of them together would be a 2.4kW system. If it is partly cloudy and they are running at a lower output, they may make only one kilowatt hour over the course of an hour. Is that low? Not necessarily. If they only had so much light, it should be right in line if the system was installed properly. This is where commissioning comes in.
This is OK! PV installers take these losses into account and size your system accordingly. For example if you were to purchase a system that had a DC rating of 10.000kW your installer would acknowledge that this system would be producing between 7-8.000 kW/hour and that a 8.000kW inverter would be more than adequate to fully maximize your system’s potential. The energy that your system produces is the figure which should be paid the most attention, ultimately any kind of long term benefit from the system will be seen in the form of performance based incentives.
Written by Sean Price, a Contract Manager at Baker Renewable Energy
If you have any questions regarding this post, or would like to know more on the subject please feel free to contact us here or email us at info@bakerrenewable.com
