While it may only cost $739 to replace one micro inverter, there are still 29 more that will eventually fail. This inevitability brings the replacement costs to a much more realistic number of 30 x $739 = $22,170, assuming the installer performed a service call to replace each failed inverter.
Lower installation costs are another dubious claim that requires examination for each installation. Balance-of-system (BOS) costs and installation times might favor either topology since there are AC conductors that must be run down off the roof, there must still be an AC service disconnect (AHJ dependent) and optimizer DC home runs must be connected to an inverter, therefore doubling the amount of work required for the same size install as a string inverter.
3. Micro inverters and optimizers generate significantly more power in shaded areas than a string inverter.
This claim requires no further research than a PV Evolution Labs (PVEL) study published in 2012. Using the NREL test protocol for conducting experiments on shaded arrays6, the PVEL concluded that micro inverters do produce more energy than string inverters in light, moderate and heavy shaded conditions by 3.7 percent, 7.8 percent and 12.3 percent, respectively. Since the overwhelming majority of partially shaded arrays in residential installations fall into the light and moderate categories, the heavy shaded array value can be discarded. Most respectable designers would not consider installing a heavy shaded array anyway and it would be very difficult to secure financing for this type of system.
Optimizers make the same kind of claim but use a different number before getting to the core of their extra yield. For example, a leading optimizer company, citing a similar NREL study7, claims “up to 25 percent more energy and 99.5 percent (max) efficiency” for their DC optimizer system. The 25 percent figure is, of course, the heavy shaded result, but they do
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Optimzing PV Systems eFeature | July 2014