I assume this thread was prompted by the discussion on this thread:
Why such low amps from 150w solar panel?
From that, it's clear that many people do not understand the finer points, so maybe this thread can help sort that out. I don't want to be in the position of telling people what is or isn't worth it for them, but up front I will say, that if PWM controllers and MPPT controllers cost exactly the same price, that few PWM controllers would be sold.
For the purposes of this discussion, I will assume we are talking about relatively small camping systems like might be useful on offroad trips. RV discussions with huge battery banks, multiple panels, etc are a different discussion.
I really like the Morningstar PWM controllers. They seem very well made and robust, the built in firmware has a charging profile I like, and they are physically small, which helps with mounting options. If they have a down side, it's that they are not programmable. So if you want a different charge profile or a longer or higher voltage absorb stage, you are out of luck. This is a minor issue though, and it's mostly a plug in and forget situation. Mine has run off road on trips and for months at a time in my driveway.
I have the Ecoworthy MPPT controller as well. It's about $100, so double what the Morningstar PWM controller costs. But for that $50 you get a fully programmable charge controller, slightly better efficiency and a built in display. I changed my float voltage from 13.5 to 13.8 and raised my absorb voltage to 14.8. I recently used it on a 10 day trip to death valley and mounted the controller inside the truck, and left the 160 watt panel connected full time-ie even when underway, driving etc. So it was fully exposed to miles of washboard, alternator power spikes (if any) any and all loads. Not a single problem.
More generically, if you have a "12 volt" panel isn't really 12 volts. Your average '12 volt' panel will have an open circuit voltage (VOC) around 22volts, but will make it's maximum power (this is the point of maximum efficiency) around 17.5-18volts. A PWM controller works by intermittently connecting the panel directly to the battery. It does this very fast but the important concept is the direct connection. During the "pulse" (connected to the battery) the panel will be at battery voltage, and so is producing power lower on the power/voltage curve, and so is giving up a bit of panel's maximum power potential. With a "12 volt panel" this isn't a big difference (maybe 10-20%) and so a PWM controller works perfectly well, especially if you use less power overall, than your panel is capable of producing.
Take home point: with a 12 volt panel, PWM or MPPT controllers will both work fine, with the cost nod going to PWM, and the efficiency nod going to MPPT.
The real difference comes when not using 12 volt panels, or when using two 12 volt panels wired in series. Then the VOC (Voltage, open circuit) will be close to 40 volts. A PWM controller will still force the system to operate at 12 volts on the panel side. But the panel's maximum power point is more like 32-35 volts. An MPPT controller operates the panel side of the circuit at the maximum power point, and still allows the battery side to work in the 12 volt range. So in effect the panel can operate at the voltage where it is most efficient. Pretty cool, and in this case the entire system would be significantly more efficient.
An MPPT controller allows one other cool trick that will decrease line losses from the panel to the controller. Since the panel side can operate at a significantly higher voltage, the current to supply the same battery power will be less. Lower operating current (amps) means lower resistance losses in the wiring from the panel to the controller. So in effect, you could have a longer wire out to your controller or a smaller wire. It's the same concept that explains why long distance power lines operate at 500,000 Volts and your house operates at 115 volts.
Take home point: Use an MPPT controller if you use 24 or 36 volt panels, otherwise you're throwing 1/2 or more of your power out the window.
So where to go. From a practical perspective, especially if amps, volts and watts sound like gibberish, and you hated college physics, get a 12 volt panel and a PWM conttroller. But get a good PWM controller like a Morningstar. Monitor your battery with a volt meter, even the Harbor Freight $5 meter will do.
But if your power needs are greater than about 160 watts or you are right at the limit of your existing panel-ie your 60 watt panel barely keeps up with your fridge, an MPPT controller will be slightly more efficient. Slightly. You should probably just buy a bigger panel, but that's up to you.
Now if you just have to squeeze every possible watt out of your system, or you run 24 or 36 volt panels, you will want to use an MPPT controller.
But watch out if shopping on
Ebay. The MPPT controllers that seem too cheap, are really PWM controllers with an MPPT sticker.
