I've got a setup with four 125w solar panels that have been in use for about 20 years, connected to a 40a PWM charge controller. Over time, I've noticed that the maximum output has only been around 19a at ~14v, resulting in a total of about 275w from what should be 500w of panels. Recently, I switched to an MPPT controller hoping for an increase in efficiency, but it only peaks at 20a output while showing the panels are putting out 15v. I double-checked this voltage at a junction between the panels and the controller.
According to ChatGPT, older panels lose amperage rather than voltage, which raises some questions. I initially predicted about 20% degradation (1% per year) after all this time, but could it actually be more significant? My wiring consists of 2 strings of 250w each via 16ft of 6awg, which then merge into a single 1/0 for another 16ft. I'm considering upgrading to 800w in the same footprint with new panels, but this low voltage makes me second-guess the wiring. Could these panels still be functional, or is the resistance in connections and wiring causing the issue?
1 Answer
I’d really need more specs on your panels to give a solid answer. However, even my new 100W panels don’t exceed 20V VOC, so a 15V output under load sounds normal for panels of your size, like the 125w ones. Lack of detailed model numbers makes it tricky to assess further, but it seems like your setup is performing as expected. By the way, keep in mind that my own 400W panels rarely pump out over 300W, so don’t always rely on the rated outputs!
I can relate! I have three strings of newer 4 x 100w arrays that managed 27a at 14v today (380w). Comparatively, your 500w array output seems low. My newer panels are rated for 18.6v, but they're at about 16.75v now, so that 15v isn’t unexpected!