Purchasing a PV-Powered Home

What You Need to Know Before You Buy
Intermediate

Inside this Article

Shaded modules may mask other problems.
Dealing with the climbing foliage is an easy fix, but be sure the shade on the modules is not masking any other production problems.
The inverter is still functioning fine.
This paint on this 2001 model inverter has oxidized from being in the weather, but the inverter is still functioning fine.
Reading the inverter’s power and daily energy production
Reading the inverter’s power and daily energy production can tell you a lot about how a system is performing.
The glass face on this center module is shattered
The glass face on this center module is shattered and may allow moisture to penetrate, leading to corrosion of the internal electrical connections. Also note the non-flashed feet, which should be inspected to make sure the sealant around them has not deteriorated.
Module failures are relatively rare
Module failures are relatively rare, but they do happen. Although there are “snail trails”—perhaps due to a reaction with the encapsulant—this module still functions. Test modules that are different in appearance to assess their performance.
This PV module’s electrical traces were burned from a lightning strike
This PV module’s electrical traces were burned from a lightning strike, but testing showed that the module still functioned.
Trace Engineering began making these off-grid inverters in the 1980s
Trace Engineering began making these off-grid inverters in the 1980s, and many are still providing electricity to homes today.
Replacing the roofing material necessitates removing, and then reinstalling, the entire array
The cupped and deteriorated shingles underneath this array are a cause for concern, as replacing the roofing material necessitates removing, and then reinstalling, the entire array—a pricey proposition.
Older arrays may not have flashed roofed attachments
Older arrays may not have flashed roofed attachments, and rely only on sealant to keep water from penetrating the roof.
These batteries are an inappropriate type for use with residential RE systems
Besides being old and poorly maintained, these batteries are an inappropriate type for use with residential RE systems and housed in a non-Code-compliant box.
The rubber boot around this flashed conduit has disintegrated
The rubber boot around this flashed conduit has disintegrated, creating a route for water to easily enter the home. This is an old solar thermal installation, but the same rubber boots are often used on PV installations.
Dangling cables
Dangling cables can be easily damaged through abrasion or from chewing critters, and can be one indication of a poorly installed system. This system also has grounding daisy-chained from module to module, which does not meet code requirements.
Off-grid inverters are still functioning fine.
Although they have more than a decade of service behind them, these off-grid inverters are still functioning fine.
A loose MC connector between two modules.
Inspection of an underproducing array revealed a loose MC connector between two modules.
This wire was pinched between a module and the rack
This wire was pinched between a module and the rack, and eventually caused a ground fault. This can be a common problem with sloppy installations.
Squirrels have severely damaged this wiring
Squirrels have severely damaged this wiring, putting it at high risk for a ground fault. (The taut wire is due to lifting the module for inspection.)
Shaded modules may mask other problems.
The inverter is still functioning fine.
Reading the inverter’s power and daily energy production
The glass face on this center module is shattered
Module failures are relatively rare
This PV module’s electrical traces were burned from a lightning strike
Trace Engineering began making these off-grid inverters in the 1980s
Replacing the roofing material necessitates removing, and then reinstalling, the entire array
Older arrays may not have flashed roofed attachments
These batteries are an inappropriate type for use with residential RE systems
The rubber boot around this flashed conduit has disintegrated
Dangling cables
Off-grid inverters are still functioning fine.
A loose MC connector between two modules.
This wire was pinched between a module and the rack
Squirrels have severely damaged this wiring

More and more, it is common for houses on the market to include grid-tied PV systems. Depending on the design and condition of the PV system, it may or may not be a selling point. Here’s how to evaluate an existing PV system and how to make sure it adds value instead of liability.

Is It Working?

Because the utility also provides energy to the home, it may not be evident when a PV system stops working. Problems may show up on your energy bill, but if the system only provides a small portion of the home’s total electricity usage, or if the occupants (and load) of the house have recently changed, there may be no clear indication of a problem on the latest energy bill. How do you know if the system is operating properly and producing its full potential?

To check if it is producing power, look at the inverter during the daytime—at the very least, it should have a green light (or something equivalent) indicating that it is “online.” Most will show how many watts are being produced on the AC side. Red lights that say “ground fault” or “off-line” indicate the system is not working—no power from the system is feeding the home (or the grid).

Even if the system is operational, is it producing as much power and energy as it should? Many older systems may be operational, but producing low power for various reasons—fuses blown, foliage grown in front of the array, or perhaps a poor design that’s been stifling system production from the beginning. If the inverter has a cumulative kWh meter, you can compare its overall energy production with PVWatts estimates based on module specs. (This may require disconnecting and removing a module from the rack to see its label.) You will also need to know when the inverter was commissioned to make an accurate comparison to PVWatts data. This comparison can tell you if the system has produced expected energy production since commissioning, but doesn’t tell you if the array is producing expected power now.

An inspection by a qualified PV installer who has the proper tools and know-how to analyze the system’s electricity output is a good idea. This requires special equipment that can measure several parameters as well as accounting for ambient temperature, irradiance, and system efficiencies. With the right tools and some math, a qualified PV technician can tell if the power output is too low, indicating a potential problem. (See “Grid-tied PV System Performance Factors” in HP156 for more information.)

If system data collection includes historical data, it should be a lot easier to determine if the system is performing as it should. However, monitoring failures can be more common than PV system failures. Sometimes the monitor just fails (monitors are often much lower quality compared to other PV components) or a vital component (like the monitor’s network gateway box) is unplugged. Make sure to get any Web-based monitoring addresses (and passwords) from the former homeowner(s) so you can access the data.

How much of the house’s energy use is the system expected to provide? It would be great if every PV system resulted in a net-zero energy home. But most PV systems are designed to provide only a portion of a home’s energy usage. For example, a 3 kW array on an all-electric, 3,000-square-foot house is unlikely to make a major dent in the electricity bill —production problems will be less noticeable, so it does make the need for monitoring slightly more important.

If it’s an older array that’s been installed for at least a year, the previous owners should be able to tell you whether or not the system has ever met the home’s monthly electricity usage. If it is a new system, find out how much electricity the array was designed to offset. It’s not uncommon to ask for historical energy bills when purchasing a home, so don’t be shy about requesting information about the PV array and its subsequent energy production. A small array may just be serving a battery backup system in the event of a power outage, keeping critical loads running—its main value is in the backup electricity rather than in utility bill reductions.

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