Sadly I see a lot of off-grid solar systems fail in the fall. They fail for all kinds of reasons, from shading to improper wiring, but the most common is that the solar array can simply not produce enough energy to support the load. In other other words, the solar system was incorrectly sized. There are several reasons the system could be sized incorrectly, from incorrect solar data to incorrect load estimates, but one that should NOT happen is that the system was sized for an MPPT charge controller.
If you refer to my blog regarding industry standards for sizing off-grid PV systems, standard IEEE 1562 sizes solar arrays using the "sun-hour" and amp hours, and specifically does NOT use MPPT charge controllers.
Unfortunately in an effort to sell more expensive products, some controller manufacturers overinflate the energy production from MPPT charge controllers, sometimes by as much as 30%. Now these numbers can be achieved, however they are achieved in a lab environment where they can control the variables, and NOT in the real world. A good example of this are the STC ratings of solar modules, it got to be such an issue that California required the manufacturers to state the power output under real world conditions and NOCT ratings were born. Unfortunately, this is not the case yet for MPPT charge controllers, however a general rule for off-grid solar systems is that you can get an additional 5-10% energy production from a MPPT charge controller than you would from a PWM charge controller. With 5% being in warmer climates and 10% in cooler climates. Quite a bit less than the 30% stated, and enough for your system to fail in winter months.
The main reason for the reduced output is due to the voltage temperature coefficient on the solar module. In other words the solar module has less voltage and therefore less power when it's warm out. I'll spare you all the math details, but you are welcome to do the calculations yourself, or even just do it based on the NOCT ratings of the solar module and you'll quickly realize that actually output is only 5-10% better.
So then why bother using an MPPT charge controller? Well for 24VDC and 48VDC systems it's mostly a waste of money, the small gain in energy production does not offset the additional cost of the MPPT, so I typically don't use them. However for 12VDC systems it might make economical sense, as a lower cost solar module could be used, or smaller wire to offset voltage drop.
So if you find your solar system failing in the fall and you have an MPPT charge controller, you might want to double check your calculations and see if you have fallen victim to the MPPT hype.
For questions on this or any of our other blogs, for help on your solar system, or other ways to make your system more reliable and cost effective, please feel free to contact us.