Your solar panel doesn’t produce a steady voltage or current, so it’s not a good idea to hook it straight to your devices or battery. Our panels, for instance, can produce 38 volts or more. That would fry any regular 12v equipment.
Luckily, solar controllers are designed to take the varying voltage and current and turn it into a steady output. There are two main types of controller – MPPT (Maximum Power Point Tracking) and PWM (Pulse Width Modulating). Victron has a nice technical document explaining the differences between MPPT and PWM (PDF). It boils down to this: MPPT technology will get more power out of your panels, and is useful with more types of panel, but costs a little bit more up front.
MPPT controllers take a high voltage from the panels, and turn it into the correct voltage for your system. By lowering the voltage, they increase the current. Because they can be flexible in the voltage they accept, they can find the “maximum power point” that delivers the most efficiency from your panels. The result is, they use more of that energy.
PWM controllers want panels that are at or near the voltage that your system will use. They pass the power on to the system, and if the voltage gets too high (or your battery gets fully charged), they start to disconnect and reconnect the solar panel many times a second in order to reduce the overall amount of power going in to the system. This very fast connecting/disconnecting is called pulse width modulation. Because they can only control how much power the battery and the rest of the system gets by switching on and off (even if it’s very fast switching), they aren’t using all the energy from the solar panel. This is less efficient than MPPT controllers.
We also chose an overkill solar controller/charger. It’s Victron Energy’s MPPT 150/70. Victron make nice equipment, and the controller will “talk” to our other Victron components. For smaller solar arrays, Victron also make smaller controllers.
Whatever controller you choose, make sure it’s compatible with your batteries as well as with your panels. If, like us, you chose to use lithium batteries, read the battery manufacturer specifications and make sure that the controller is capable of providing what your battery wants. Don’t just believe that your battery will be fine if the charger has a “lithium charging profile” – different lithium battery types want different charging profiles. The same is true of lead-acid batteries – different types such as AGM, Gel, and flooded cell lead-acid batteries like different charge voltages and different charge times.
Finally, a word on the wires you use. There’s no point spending lots of money on solar panels and a fancy MPPT controller if you don’t let the energy from the panels get to the controller. If your connecting cables are too thin, they will create too much resistance. The energy running through them will be lost as heat. Make sure that you use a proper sizing calculator to work out what size cables you need. One advantage of MPPT controllers is that you can use solar panels that produce a higher voltage (38v) and a lower current. Because it’s (mostly) the current that’s a limiting factor in wire sizing, this lets you get away with running thinner cables than if you had to start with 12v panels.