Why is the actual output power of the solar panel less than the nominal rated power Pmax?Updated a year ago

(1) The nominal rated power Pmax is the test data under Standard Test Conditions (STC), but the sunlight intensity, temperature, dust shielding degree, and installation angle in the actual environment might be different from the testing environment. Thus, the actual output is likely to be different from the nominal value of the rating. At some point, the actual output power will be greater than the nominal value. 

(2) Power diminishing of solar panel

The power diminishing refers to the phenomenon that the output power of solar panels decreases gradually with the increase of illumination duration. The diminishing is a normal phenomenon. It can be divided into two stages, light attenuation, and wear-out attenuation. 

Light attenuation: when the power of the battery decreases due to irradiation of sunlight. It generally occurs in the first year of use, the decrease rate is 3%. 

Wear-out attenuation: a very slow power decline that occurs over time, with panels typically decaying by 0.7% per year over the 25 years usage period. 

(3) The solar panels put in series are not compatible with each other. The output current of the series of solar panels is limited to the lowest one in the series. It is recommended to select solar panels with the same specifications.