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.
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.