PV Ratings Jim Cooley As one becomes involved in the design of a PV power system, be it large or small, a critical factor is the number of modules needed to supply the power. This is, in theory, a simple case of arithmetic, somewhat equivalent to balancing a check book. In reality, you need to put back what is taken out, plus the percentage lost to inefficiency. Basically, you figure your loads, and then use the module rating to estimate haw many are necessary to replace what is used. A PV module in southern Arizona in July will produce different curves then an exact module in Montana in July. One sees that the PV module rating is, and probably never will be, an accurate indicator of its actual output under ever changing real world conditions. As in most things today, we are applying a certain amount of science to the output specs. This is where an understanding of how the ratings are achieved is very important to any design. If you purchase a 50 watt (manufacturer rated) PV module, it's a sure bet that when it's out in the sun, you won't always get 50 watts. Among the many things affecting that spec are; A) actual surface temperature of the cell B) actual light intensity at the cell's surface C) wiring resistance from module to application D) angle of cell to sun E) age and condition of the battery bank (when charging batteries) F) quality and number of connections between the module and the load G) age and condition of the module itself H) accuracy of the instrument used for the measurement It is fairly safe and somewhat optimistic to say that under the controlled conditions at which the module is initially rated, an accurate output would likely fall within reasonably close range of its factory spec. But the range of realities under which most PVs are utilized, leaves the end user with somewhat less than they paid for. In the 10 odd years I have been involved with this science, I have been witness to both ends of the truth. In many cases a lack of spec'd output can honestly attributed to the conditions of the atmosphere at the time, or one or more of the factors A)-H). And, in a few cases, I have actually seen the modules outperform the specs', under less than perfect atmospheric conditions (this can also prove to be a problem when the design thresholds are passed). But in other applications, I have been at somewhat of a loss to explain the reasons for low output and would draw conclusions of miss-ratings on the part of the manufacturer, be they intentional or not. But before we all run out to instigate litigation against the PV manufacturer's, we should consider the many variables involved, the need for some type of logical base rating, and that when we consider all the pieces of the system (regulators, batteries, inverters, etc.) the fractions lost from errant PV ratings are, in most cases, the easiest to overcome. As it applies, simply add some additional input. The case of overkill is no stranger when choosing a generator, a battery bank or an inverter, (definitely when choosing an inverter). In the real world, PV overkill follows common sense. At best, ratings and specs give us a base to start with, while actual experience under specific operating conditions is the only true critic. Access Jim Cooley, Energy Transition Technologies, Inc., 540 14th St., Douglas, AZ 85607 ù 602-364-7121.