Another big factor to consider if you are buying a house with an off-grid PV system is that it can be in perfect functional order, working well for the previous owner—but may not work well for you or your family. Off-grid system performance is dependent on the system’s electrical loads—both the power draw of the appliances and the user’s usage patterns. For example, the system may have been sized for only a few loads and miserly power consumption, with the previous owner judiciously using only a few lights at night and timing laundry loads to coincide with the sunniest hours of the day.
If this is your first time living off-grid, it’s always best to make sure a working backup generator is part of the system, as you will probably use more energy than you expect. Is there already a generator with the system? If so, what quality is it? If you are expecting to use the generator frequently (such as in areas with very little winter sun), you’ll want to see a heavy-duty water-cooled generator, usually propane- or diesel-fueled, designed to run for several hours a day—for years. An air-cooled backup generator may be suitable for sunnier areas, in which it’s only used a few times a year. Portable generators are generally only good for the smallest cabin systems. If in doubt, consult with an RE installer about what sort of generator is appropriate for a particular system and climate.
And make sure you get a tutorial on operating the system. Smaller off-grid systems can be quirky, and being privy to the system’s idiosyncrasies—such as “never start the table saw unless you turn the well pump off first, or it’ll overpower the inverter, tripping it offline”—is important. If the system was professionally installed, consider contacting the installer to explain the operation of the system, discover what issues it has had over the years, and what upgrades they recommend. You might learn more about the system than you will from the previous owner, plus it is valuable to develop a relationship with the company that can service it. You might also find out that the previous owner bought the highest-quality components available and oversized the system, or the worst possibility—that they cut corners wherever they could, and did not properly care for the batteries. If you are planning a much different load profile than the system was originally designed for, this also gives you a chance to discuss what upgrades will be required so the system can meet its new requirements.
If you are buying a used PV system, inquire about its warranty. If something does go awry, it might be possible to get it fixed—without having to pay full price. There are usually several separate warranties: a manufacturer warranty on the equipment—this can range from two years (some charge controllers and off-grid inverters) to 10 years (grid-tied inverters to 25 years (PV modules); and an installation warranty, which will vary in length and terms. Many grid-tied systems require at least a five-year installation warranty to qualify for rebates.
If the system still has the installation warranty in effect, make sure to give your name and information to the installer to take advantage of remainder of the warranty. During this period, they may service system problems for you without charge. However, once their warranty has expired, they may charge a fee for labor to service a manufacturer warranty. (Note that, in some circumstances, manufacturers may cover the installer’s service calls.)
If the original installer is no longer available, you may have to find a new installer to service the system. Unfortunately, it sometimes happens that installers leave the business or stop working in some geographical areas. This may mean that you have no installation warranty at all, or you may need to contact an out-of-state office of the installation company.
Examine the PV system for evidence of chewed insulation on cables and for debris that may clog the inverter’s cooling fins. Insulation-hungry squirrels, for example, can turn a PV array into junk in a matter of weeks, and critter damage is a serious threat to PV array longevity. Rats, raccoons, pigeons, and wasps also can damage PV arrays. Ideally, a rooftop array, mounted parallel to the roof, should be completely screened around the edges with wire mesh. If an array has been disabled due to chewed wires, the inverter will most likely indicate a ground fault. This damage can usually be repaired, but usually at a great expense to remove and replace the array, and to repair the wires. Flush-mounted roof arrays are the most prone to harboring critters, because they create an inviting hiding place for squirrels and birds. Tilt-up or ground-mounted arrays are less inviting, but can still be damaged by animals. Bird or wasp nests blocking airflow on inverter cooling fins are more easily removed.
Zeke Yewdall is the chief PV engineer for Mile Hi Solar in Loveland, Colorado, and has had the opportunity to inspect and upgrade many of the first systems installed during Colorado’s rebate program, which began in 2005. He has also upgraded many older off-grid systems. He teaches PV design classes for Solar Energy International.
The owner/manager of Bozeman, Montana-based Onsite Energy, Orion Thornton has focused his recent efforts on contracted inspection work for first-generation grid-tied PV systems installed prior to 2005. He also works as a PV instructor for Solar Energy International.