By Justine Sanchez, HomePower Magazine
Aug/Sep 2010 (#138) pp. 28
One of the beauties of a solar-electric system is that there are no
moving parts. The PV modules silently sit in the sun, doing their job of
producing electricity. But, do you know how well they are doing their job?
With access to grid electricity (unless there is a utility outage),
owners of grid-tied PV systems often are not as engaged in monitoring their
systems compared to their offgrid counterparts, who critically rely on their
systems for every bit of their energy. For many, the only indication of a
problem comes in the form of a higher electric bills a month later and even
that might go unnoticed for some time. It is important to spot-check the
system regularly to make sure it is performing as expected.
Checking Watts
Choose a clear, sunny day around noon to check system power output (watts).
Find this value on your inverter's faceplate meter or remote monitor, and
compare it to your array size, adjusted with a derate value to account for
system inefficiencies. System efficiency ranges from 70 to 80%, accounting
for power losses from module heating, dust, inverter inefficiency, wiring
voltage drop, and module production tolerance and mismatch.
For example, if you have a PV array rated at 3,000 W, you can expect the
output to be 70 to 80% of that value (2,100 to 2,400 W) on a sunny day. If
you read a value that is significantly lower than expected, and there is no
obvious reason (like shade, a day without full sun, or extremely dirty
modules), a call to your installer is a good idea. A common culprit for
lowerthan- expected performance is a blown fuse in a combiner box (or in the
inverter, if it has integrated series string fusing).
Checking Watt-Hours
While checking watts is a good spot-check on power, also periodically check
the kWh (energy) production total. Using this value requires that you know
when your system was installed, its shading factor, and array orientation
and tilt.
You can use NREL's PVWatts program (http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/)
to find the expected system output (monthly and annual) and then compare
them to the actual kWh reading. (See
Sizing Batteryless Grid-Tied Arrays in this
issue for more information.)
For example, let's say a 3 kW system was installed on October 1, 2010, on
a home in Billings, Montana. Eighteen months later, the inverter or
production meter shows total energy produced is 5,050 kWh. You can plug in
system variables into PVWatts to compare the predicted value with the kWh
total. In this example, assume a 0.90 shade factor and a south-facing array
set at a tilt angle equal to latitude. PVWatts reports expected annual kWh
production at 3,597 kWh. But this system has been running for 18 months, so
we need to add in the monthly totals from the additional six months
(October & March adds 1,541 kWh). Using this, we get 5,138 kWh as a predicted
value are within 2% of the actual value. Note that while inverter faceplate
meters (and production meters) generally only keep a running kWh total, and
not monthly totals, users can track this by recording their kWh total at the
beginning of each month. Then, by subtracting the preceding month's total
kWh from the current month's kWh, they can use these values to compare to
the PVWatts monthly predicted values. However, keep in mind that variable
weather patterns can impact energy production. For example, if a particular
month is much cloudier than usual, the system's energy production will
likely be lower than the PVWatts predicted value.
If there is a wide discrepancy (i.e, more than 5% and lower than
expected) that cannot be explained by unusual weather, examine the solar
window for increased shading (from growing trees, etc.), and call your
installer to look for problems such as poor inverter maximum power-point
tracking or failed module bypass diodes.
Milholland Electric is a licensed electrical contractor in Arizona
and California with an experienced team in solar panel design and solar
panel installation and maintenance. Contact us for a
free estimate today!
