Optimizing PV Systems July 2014.pdf

Whileitmayonlycost$739toreplaceonemicro
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
installerperformedaservicecalltoreplaceeach
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
concludedthatmicroinvertersdoproducemore
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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