There have been a lot of innovations in the solar
market. Affordable PWM controllers were a nice step forward, and is what I have on my Scamp
. Only recently, however, have MPPT controllers come down in price to where they're not more expensive than the solar
panels they support.
MPTT controllers are like many tools. They are very good at what they do. The question, as always, is, "Are they the right tool for the job?"
The answer to that can be yes or no, depending on things like money, the size of your solar panels and battery, and how you use the power your batteries store and panels make. What follows here is a long-ish, boring-ish discussion you can skip (just head down to the "=========" line below the boring stuff) explaining what scenarios an MPPT controller makes a lot of sense in.
First off, a good MPPT controller will charge your batteries at least as well as a good PWM unit. It may not do it better, but it will do at least the same job. Whether you get more out of an MPPT controller has a lot to do with the size of your solar panels, the size of your batteries, and the amount of electricity you use from your solar system and batteries.
MPPT controllers make sense when your solar panel
output is just adequite to getting your battery fully charged. So, let's say you have a 100w panel and get the equivalent of four hours of full sunlight. After doing the math, that means your solar system can optimally provide about 16 Amp-Hours (AH) of battery power each day. If you have a fully-charged 50 AH battery that you discharge down to 12 volts or so by the next morning (30AH used) you will never use the features of your MPPT charge controller
, and your battery will go dead over time.
Double the size of your battery, to 100 amps and use the same amount of power, and you'll get some small benefit from an MPPT controller on the first day or two, but your battery will still go dead, and in slightly less than double the time compared to using a 50 AH battery.
If, on the other hand, you discharge that same 50 AH battery to 12.4 volts by the next morning (20AH used) you will use the MPPT features for the first several days while it trys to keep your battery charged.
Your battery will still eventually go dead, but it'll take longer, so you do get some value out of your MPPT controller. Bump your battery size up to 100AH, and it'll still discharge in slightly less than half the time, but will again use the MPPT features a lot during those first several days.
Let's say your daily usage is right at 16 AH, the amount of power your solar panels can produce. Because more energy is lost during the absorbtion and float phases of battery charging, your solar system won't quite keep up for the first several days running on solar, but then you'll get back from a day in the great outdoors and discover your battery voltage is about 12.5 volts (80% charged), same as the day before, and same as the day before that. This is the break-even point where an MPPT charger makes the most sense, when your solar panel
power production and daily consumption are just about in-balance, regardless of the size of your battery.
What, you might ask, gives you the most bang for the buck if your solar panels make more electric power each day than you use? Say your 100w panel makes 16AH each day but you use just 12AH. In that case, it doesn't matter whether you have an MPPT controller or a PWM controller. Both will bring your battery up to full charge each day.
In the end, MPPT charge controllers make a lot of sense if your solar panel
generating capacity more or less matches your average daily consumption, say plus or minus 25%.
If you have an abundance of solar compared to your daily demand, your solar system is already beefier than you need, so why spend more on an MPPT controller? If your solar panel capacity is less than 75% of your daily demand, you won't get much out of those fancy MPPT features, and perhaps your money would be better spent buying a larger solar panel, a second panel, or buying a larger battery.
Yes, it gets complicated. And, rather than working all the details out I can really see the logic of saying "Screw it. I'm spending the extra $50 and getting an MPPT controller."
Which is, in the end, what I decided to do for our Surfside