Grid-Tied PV System Performance Factors: Page 3 of 4

Intermediate

Inside this Article

Cleaning solar modules
Grid-Tied PV System Performance Factors
A remote meter
A remote meter can help you keep track of how well your array is performing.
Utility meter
Taking readings from the PV system production meter can help you identify system problems, but only if you have previous records to compare to.
A temperature gun
A temperature gun checks infrared radiation to give quick readings of cell temperature.
Measuring cell temperature
Placed against the back of a module, a sensor (attached to a meter) can be used to measure cell temperature. Be sure to take readings on several spots and average the results.
Pyranometer sensor
To accurately determine the irradiance striking your PV array, be sure the pyranometer sensor is on the same plane as the modules.
PV module label
While this label does not show the PV module’s power tolerance, it does give the minimum figure, which computes to be 91% of the rated power (154.7 ÷ 170.0).
Dirty PV module
Dirty PV modules can significantly reduce output. Clean ’em!
Dusty PV module
Dirty PV modules can significantly reduce output. Clean ’em!
Partial shading of a PV module
Shading can negatively affect your PV system’s output. For testing purposes, be sure your array is in full sun.
Misshapen junction box
Warped, misshapen boxes or insulation indicate electromechanical problems with one or more PV modules.
Burn marks
Burn marks indicate electromechanical problems with one or more PV modules.
Monitoring display
Finding individual underperforming PV modules is possible with module-level tracking technologies, such as microinverters or DC optimizers.
Measuring the array’s operating voltage
Measuring the array’s operating voltage at the positive and negative terminals, with the array connected to the inverter.
A clamp-on meter
A clamp-on meter is used to measure the current while the array is connected to the inverter.
Cleaning solar modules
A remote meter
Utility meter
A temperature gun
Measuring cell temperature
Pyranometer sensor
PV module label
Dirty PV module
Dusty PV module
Partial shading of a PV module
Misshapen junction box
Burn marks
Monitoring display
Measuring the array’s operating voltage
A clamp-on meter

“Other Losses” Factor

Depending on the PV system, other factors may affect PV system output. Here are some common ones used in NREL’s PVWatts calculator:

PV module nameplate DC rating: Testing has shown that module nameplate power ratings tend to be a bit optimistic, but by how much? A good place to start is the manufacturer’s power tolerance specification. For example, you might see “Pmax = 250 W, -5%/+10%” on the module data sheet, which means this module is warranted to produce between 237.5 and 275 W at STC. It’s generally best to use the lowest rating. In this example, we will use the -5% tolerance, for a nameplate rating factor of 0.95.

Inverter: We also need to account for the power it takes to convert the DC electricity from the PV array to AC, and we can do so by factoring in the inverter efficiency. While inverter specification sheets will list “maximum efficiency,” a more useful value is the “weighted efficiency,” which accounts for the percentage of time the inverter commonly spends at various power levels. This gives a better indication of the inverter’s real-world efficiency. Most grid-direct inverters have weighted efficiencies greater than 90%. You can find inverter weighted efficiency ratings on the Go Solar California website: (bit.ly/CAeligInv). In this example, we use 0.96 for our inverter efficiency factor.

Module mismatch: Include this factor unless the PV system has maximum power point tracking (MPPT) capability at the module level, using microinverters or DC optimizers. Due to slight differences in modules’ IV curves, or power output profiles, the one MPP used for an entire array or series string will not be a perfect fit for each individual module. A small power loss will result. If there are different makes and models of modules in the array, the power loss will be even greater. If individual series strings of modules have their own MPPT, the loss will be less. Most PV arrays with one kind of module and a single MPPT for the entire array will experience a 2% loss due to module mismatch, so we will use a 0.98 mismatch factor.

Wiring losses: Wire connections and the DC wire runs introduce losses due to resistance. Since we are measuring inverter output at the inverter, we should not include AC wire losses. Most well-designed systems will have about a 0.5% loss due to connections, and about a 2% loss in the DC wiring. If system designers did not account for voltage drop on a long wire run, or there are poor wire connections, these losses might be greater. We’ll assume our system fits the numbers stated above, for a total DC wiring loss of 2.5%, for a 0.975 wiring loss factor.

Soiling: Modules get dirty. In places with lots of dust, few significant rain storms, and buildup of other small debris, this factor can be significant. Modules set at a lower tilt tend to have more soiling. Let’s assume we’re in an area with “normal” dust, and our array is tilted to about 40° (steep enough to shed most gunk). The modules don’t look dirty from the ground, but it hasn’t rained in a while. We will use 0.97 for our soiling factor.

Age: On average, crystalline silicon modules lose about 0.5% of their output power capacity per year. A new installation would have no age-related losses. The array in our example was installed six years ago, so we’ll assume a 0.97 age factor. (Note that degradation rates may vary due to climate effects and may not be linear over time.)

Shading: Quantifying the effects of shading on annual production is fairly easy with site analysis tools. But accurately determining the effects of partial shade on an array at any one time is very difficult. Therefore, you need to choose a time when the array is not shaded to make your measurements.

Now that we have determined all of the other losses, we find the total by multiplying them together. This number will be our final factor:

Other losses factor = 0.95 × 0.96 × 0.98 × 0.975 × 0.97 × 0.97 = 0.82

Comments (3)

Justine Sanchez's picture

Hi Gary,
Thanks for posting! Yes a production meter is an extremely valuable asset to any grid-tied PV system. Separate production meters (and faceplate inverter output meters) can give us a good idea if the PV system is meeting energy (kWh) production expectations overtime. But if your kWhs aren't stacking up to what is expected, this article provides the next step in troubleshooting, and describes a method of spot checking the wattage of the system, so that we can determine if there is actually a problem with the system itself, rather than something other such as a cloudier than usual spring or perhaps partial morning or afternoon array shading due to tree growth over time, etc..And yes the method described here does require some specialty tools (ex/irradiance meter and temp gun) which will be common to the installer, but not necessarily to the average homeowner.
Cheers,
Justine Sanchez
Home Power Magazine

gary beckwith's picture

this is an interesting article with some good information for the techies and tinkerers. but for practical purposes, the easiest way to check on your system is to install a separate meter for production if there isn't one already. personally I think it's a good idea to put a production meter on your system even if it isn't required, just for this purpose. then go to the PVWatts website and put in your location, tilt angle, temperature etc, and see what you should be getting every month and per year. the PVWatts calculator already factors in all the derate issues mentioned in this article. So if you're consistently below what PVWatts says, then you should call your installer and have them troubleshoot your system.

Robert Sczech_2's picture

A 3KW system will generate roughly 3 MWH of electricity per year - a market value of roughly $300. If one is really to perform all the maintenance tasks and checks on the system as described in the article and if one values ones own life time at only $10 per hour, then the value of ones human lifetime invested into these activities could easily eat up a significant part of the electricity created.

A personal experience: The state of NJ has mandated the installation of so called revenue grade meters for all solar installation in order to accurately measure the amount of electricity generated. In my case, the process of installing that meter started last July and is still not complete. During the past year many emails and phone calls needed to be made to get the contractor, the permits and the final inspections. The inspection process alone takes many months because of the paperwork requirements and repeated reinspections. If all the human labor in getting these meters installed is all added up, then the gain from renewable does not appear to be that significant. Lots of that gain is lost in the inefficiencies of the full installation process.

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