The best way to inspect and determine the value of a PV system is onsite and by looking under the solar modules as one would look under the hood of an automobile.
In addition to wiring methods, when evaluating a rooftop solar energy system, the roof under the array should be qualified and evaluated. For a ground-mounted system, the soil and type under the solar array should be evaluated including soil amendments.
A. CODE COMPLIANCE AND THE SOLAR INDUSTRY
How fast can you drive a car or an Electric Vehicle (EV) on the highway? In the solar energy industry, how many solar modules can you connect in series based on the open circuit voltage? For speeding this may be found in the California Vehicle Code 22349. "(a) Except as provided in Section 22356, no person may drive a vehicle upon a highway at a speed greater than 65 miles per hour."
Likewise, in the solar industry, the applicable code would be the California Electrical Code (CEC) / Nationally Electrical Code (NEC) by the National Fire Protection Association (NFPA) Articles 690, 691, 705 and other articles. For example, CEC/NEC 690.7, "In one and two-family dwellings, PV source circuits and PV output circuits that do not include lampholders, fixtures, or receptacles shall be permihed to have a maximum PV system voltage up to 600 volts."
For micro-inverters and dc optimizer or other Module Level Power Electronics (MLPE) solutions there are other techniques in determining the number of modules in panel including manufacturing instructions, module compatibility, and maximum module to inverter ratios,
Like rush hour traffic, it may be acceptable for a PV system to be impacted by shade in the early morning and late in the evening. However, a solar energy system typically generates the greatest power around solar noon. Shading should be minimized.
By the way, 746 watts equals 1 horsepower. If shaded, then the power measured on the inverter output circuit can drop to near zero.
[Sample Inspection Photograph: There are issues in compliance to code, industry standards and best practices. Both the installer and customer were confused on why the system was not working correctly.]
B. INDUSTRY STANDARD AND BEST PRACTICES
One way to maintain a solar energy system would be to inspect to code compliance. For instance, CEC/NEC 110.12, 338.10, ... In other words, physically look under the hood or the modules with periodic inspections.
Simply, a solar energy system not designed, installed or supported using best practices or industry standards may result in premature failures, corrosion or other issues.