Given that a solar power system for your home is a big investment, it's only logical that you might consider whether used solar panels are worth the potential savings. There are a few things to consider when shopping around for used solar panels:
* the age of the panels
* the state of the panels (are they damaged?)
* and whether you are using one centralized inverter or a number of microinverters
Due to technological advances, the efficiency of solar panels has improved over the past few years. New solar panels pack more punch for the same surface area than older panels. In other words, the panel size hasn't changed much but you get more capacity today than you did in the past. If space is limited, this is even more relevant. Manufacturers today are able to cram higher efficiency modules into the same size panel, meaning that today you only need twenty new 250 W panels to make up an average-sized system of 5kW. Whereas with used panels, you may need fifty similarly-sized 100W panels to have a 5kW system, something that is just not feasible for the average sized rooftop. Moreover, panels are subject to a certain amount of natural degradation over time, on the order of about 1% per year, meaning that an older panel will have a lower capacity factor than a newer panel. It's important to do the math, because while you may be saving on your initial outlay of money, you may be dramatically reducing the amount of electricity you can produce, and consequently the amount of money you can save on your electricity bill, or if you're one of the lucky, the amount you can earn selling that electricity back to the grid using a feed-in tariff.
Another very important and more obvious consideration is the state of the panels you are considering to buy. Are they damaged in any way? Do you see evidence of moisture or burn marks? If you cannot see the panels in person, than ask for as may photos as possible. If the vendor is unwilling to provide them, it may be time to walk away. Damage to solar panels is a real concern if you are looking to set up a system for your home. It may not be as relevant if you are looking for small panels used in battery maintenance or small appliance charging. Damage to a second-hand solar panel, even to only a small section of the panel, reduces the efficiency and output of the whole panel because the modules within the panel work in serial. Think of the old strings of Christmas tree bulbs, where when one bulb went out, the whole string was affected. With solar panels, where a number of strings of modules make up a panel, the whole panel might not be rendered useless, but the output will be reduced by more than just that from the damaged section - the whole string connected to the damaged part will be affected.
The old adage that your team is only as strong as its weakest member could not be truer when it comes to solar panel systems that rely on a centralized inverter. Just like the individual modules within a panel, panels that are connected to make up a system, are often connected in serial with one centralized inverter. If one panel is damaged or less efficient due to age, it will bring down the output of the other newer or undamaged panels. The problem is essentially compounded: the small decrease in efficiency due to age, or the small area of damage, not only brings down the output of the individual panel, but that of the entire system. So if you have this kind of a system for your home and you are thinking of adding to it or replacing a panel, this is not generally the time to consider a used panel, unless you are very fortunate to find one that is relatively new and in pristine condition. However, some systems do not rely on one centralized inverter, but on multiple microinverters. In this set-up, each panel has its own inverter and therefore, a panel that is not performing as well as the other panels, for whatever reason, will not bring down the potential output of the whole system. In this case, you may decide to add to your system or replace a panel, with the knowledge that the panel itself may not be as efficient, but that it won't affect the output of the rest of the system.
One last point to consider with regard to used solar panels. A certification process exists for new solar panels, offering you some assurances about the efficiency of your panel. Such a process does not exist for used panels, meaning that you have no warranty that your panel will deliver as expected. When all is said and done, you want to get what you paid for and be happy with your investment. It may be worth looking at used panels, but just know what you're getting into. And good luck!
Image credit: http://groundswell.azgs.az.gov/2011/02/