Grid-Tied PV System Performance Factors

The number of PV systems is increasing all over the United States. By the end of 2012, PV capacity was more than 7,700 megawatts (MW), with more than 3,300 MW of PV installed during 2012 alone. But are all of these systems performing at the highest possible level, even after five, 10, or more years of operation? Here are some of the common signs and symptoms of underperforming PV arrays—and their remedies.

Assessing Your System

Figuring out if your PV system is working properly can often be difficult, especially for owners of smaller residential PV systems who have never been given proper instruction on system monitoring. Many folks keep tabs on their grid-tied PV systems just by examining their monthly electric bills. They know roughly what the bills have been since their PV system was installed, and assume everything is fine if their payments stay about the same.

In areas with production-based incentives, a separate meter records PV system generation. This can make monthly comparisons of PV performance more straightforward from year to year. Any significant drops in production for a particular month from one year to the next should raise a red flag—while it could mean only that there was less sun than the previous year, it is a good idea to check the system for problems.

Net-metered PV systems can be difficult to pin down since the utility’s meter only shows excess PV energy produced after all of the home’s electrical usage. If the loads change from year to year, then the net production (if there is any) will change as well, even if the PV system is performing as in the past. Most grid-tied inverters display instantaneous power and energy production totals, but someone needs to check them regularly, record the readings, and compare them from year to year to really know if the system is performing to specifications. And what happens if the system was not installed properly in the first place and has never worked properly? We need a way to know if a system is doing what it is supposed to do.

System Spot-Check: Power Output

Let’s use a 3 kW net-metered PV system as an example. The system owners notice that their monthly electric bills are a lot higher than the previous year. Nothing looks amiss—the modules are still on the roof, and the inverter has a little green LED lit up when the sun shines. The inverter’s screen shows the array is producing 925 watts at about 3 p.m. Should this 3 kW array be producing more?

With a little math and a couple of measurements, we can get a pretty good idea of what an array should be producing any time there is full sun. Here are the parameters you can use to calculate PV system power output:

STC ratings. PV modules are factory-tested to determine their power output. When we talk about the size of a PV system or module, we are using the “STC ratings”—the numbers detailed on the back of the module and on its data sheet. STC, or “standard test conditions,” is a solar irradiance of 1,000 W per square meter and a module cell temperature of 25°C (77°F). Although both of these values impact a PV module’s output, its rating at STC rarely reflects real-world conditions. The result is that a PV module rated at 250 W STC will only produce 250 W under those specific conditions. In full sun and up on a rooftop, the actual conditions are usually much different. For instance, during the winter in an area with no snow and a slight haze, the irradiance will be lower (perhaps 700 W per square meter) and module cell temperature might be close to 25°C ( 77°F). In the summer, irradiance might be closer to 1,000 W per square meter, but module cell temperatures may be 60°C (140°F).

When calculating the power output, we’ll start with the STC rating of the PV array—in this case, 3,000 watts. Then we will include several derate factors, which will lead us to the array’s expected power output.