Should I use a PWM or MPPT Solar Controller?

Enerdrive offers a range of both PWM and MPPT solar controllers. We have compiled a small amount of information to help you better understand the difference between these two methods of solar charging and their role in your electrical system.

What is a Solar Charge Controller for?

A solar charge controller is necessary in virtually any solar power system that utilises batteries. The role of the solar charge controller is to regulate the power going from the solar panels to the batteries. They prevent overcharging, which significantly reduces battery life and can damage batteries to the point that they become unusable. A solar controller also prevents your panels from draining your batteries at night when their voltage drops below that of your battery.

The most basic charge controller simply monitors the battery voltage and opens the circuit, stopping the charging, when the battery voltage rises to a certain level. Older charge controllers used a mechanical relay to open or close the circuit, stopping or starting power going to the batteries. Modern charge controllers are entirely electronic and do not have any mechanical components to break. Modern solar controllers fall use one of the two following technologies

Pulse Width Modulation (PWM)

PWM is the older and more popular of the two modern solar charge controller types. PWM solar controllers work by pulsing a switch on and off at a high frequency to control the amount of power that is fed into the connected batteries as they approach full charge. This reduces the stress on batteries and extends battery life. PWM controllers can also pulse their output to hold batteries in their fully charged (or float) state indefinitely. This style of charging draws current out of the panel at voltage just above the battery voltage and works best when the nominal voltage of the solar panel and battery match.

Maximum Power Point Tracking (MPPT)

MPPT is the more recent and superior type of solar charge controller. MPPT controllers can draw current out of a solar panel at the maximum voltage regardless of what the battery voltage is. This gives MPPT controllers a significant advantage over PWM controllers because solar panels produce voltage far in excess of what is required to charge batteries. This difference between the panel output voltage and battery charge voltage is wasted by a PWM controller, but an MPPT controller can convert the excess voltage into current. Under ideal conditions, this means that an MPPT controller could produce up to 30% more power than a PWM equivalent.

In addition to getting the most out of each panel, the ability of an MPPT solar controller to efficiently step down panel voltages allows you to use higher voltage panels or connect panels in series. For example, a nominal 24V or a grid connect solar panel could be used with a MPPT solar controller to charge a 12V battery.

Some MPPT controllers, including all ENERDRIVE | DOMETIC MPPT controllers, allow you to install oversized solar arrays for better power availability in poor solar conditions. This type of setup will remain limited by the controller's maximum output, but extra panels will allow the system deliver maximum output for a larger portion of the day. For more information on "overdriving" an MPPT controller, click here.

Despite the benefits of using an MPPT controller, there are a few downsides as well. MPPT controllers can be more than twice the price of PWM solar controllers. If you have the space for more panels, you could take the savings you had from buying a less expensive PWM controller and use that money to purchase an extra solar panel. This would most likely make up for the inefficiency of a PWM controller and leave you with some money left over.

The bonus efficiency of an MPPT controller can also be somewhat overstated depending on the environment they are used in. Solar panel voltages fall as the temperature rises, which means that the advantage of MPPT over PWM falls as well. The warm climate in most of Australia means that an MPPT controller will almost never see a charge efficiency 30% higher than PWM.

Finally, although there are benefits to be gained from connecting panels in series for higher voltages, ENERDRIVE | DOMETIC does not recommend connecting panels this way in most instances. This is because shading a has a more negative impact on series connected panels than parallel connected panels. This means that the benefit of running higher voltage panel strings is not applicable in most setups. For more information on connecting panels in series or parallel, click here.

Quick Comparison

There are a lot of factors to consider when choosing which type of solar controller is right for your setup. A quick summary of the key points can be found in the table below: