MPPT vs PWM

Let me begin by saying I am not a highly versed person in technical terms, I tend to speak in laymen terms. Let me also say that neither MPPT or PWM is better, they are just two different ways of doing things with advantages and disadvantages to both.

Lets star with a basic discription of each.

PWM is Pulse width modulation. We call most simple controllers this even though even MPPT controllers use PWM for their charging. The difference is how the power is connected to the battery and controlled. With a PWM controller the battery is directly connected to the panel and its voltage and acceptance rate are what controls the output of the system. The downside is that when the batteries voltage is low, say 12v, that anything the panel produces above the battery voltage is lost until that voltage comes up to what the panel puts out. Your 100w panel will not be worth 100w's until the battery is near fully charged.

MPPT or Maximum power point tracking is the program that controls the output of a MPPT controller. Here the panel is not connected directly to the battery. It comes in to a buck converter that takes the DC current from the panel, turns it in to AC and then back to DC again. The buck converter allows for the full use of the panel no matter what the battery voltage is. It doesn't happen in just bulk mode, the bulk converter is in use in all modes.

So what is the MPPT part then you ask. Since the battery no longer controls the system, something has to decide how the power is applied in bulk mode. The program can somehow tweak the array to get the most out of it as well as decide how to best bring the batteries voltage up. So not only does the buck converter allow the use of higher voltage panels or even panels in series, it gets you the most power when you need it the most early on when the battery is down.

Aside from the additional cost, MPPT controllers have a disadvantage in high heat. Actually the panels will lose voltage in heat and the controller uses that voltage to create current. The lower the voltage, the lower the amps. The loss isn't big, maybe a amp or so and usually only mid day when your battery is likely to be in float anyways. 

So why go one or the other. For me it was equipment related as my panels are 24v grid tied panels and had no choice. They have higher voltage and lower amperage and while I could get a PWM controller that would clip the excess volts, it would give me the amps of a panel half it's size. Quite the waste of roof space.

Part of the decision can be budget. Until recently MPPT controllers cost considerably more the PWM controllers. The common thought was it wasn't worth the cost of buying a MPPT controller for less than 400w of panel. You coujld get a MPPT controller a single 12v panel or run two in series but the advantage you would see at peak wouldn't be enough to justify the expense. Three things this doesn't take into account are early production when the advantage is higher, the fact that less expensive MPPT controllers are available or the fact that 24v grid tied panels can be had for less than a 12v panel half it's size.

An example is I am looking for 140-150w panels to make a hinged portable. They each cost more than a 250w 24v panel does. 

On the other hand say you have one or two 100w panels and want more power. You would be better off adding a third panel to the PWM set up than running what you have on MPPT.  MPPT would get you more out of what you have if you were out of room for more panels.

So don't start the design of your system by choosing one form of charging controller first. Consider it a option as you look at how much room you have for panels, what panels are available to you, what the budget is and even what type of batteries you have. Find the balance that is best for you out of the different options before you start buying.

Last thing is to check on what your batteries require before you pick a controller. Know what voltages, modes and features you want before you buy. A cheap controller of either type may not properly charge your batteries and replacing them is certainly not cheap.