MPPT vs PWM Solar Charge Controller: What's the Difference?

The charge controller regulates the flow of electricity from the solar array to the battery bank. It ensures the batteries are not overcharged during the day and prevents them from being discharged at night.

Some charge controllers come with extra capabilities such as lightning and load control, monitoring, and data logging. However, their primary goal is to maintain the batteries within their safe operating limits.

Charge controllers achieve this goal by regulating the voltage and current going into the batteries. This can be done as simply as a switch that opens and closes to control the amount of current going into the battery.

However, as solar systems have become more popular, charge controllers have had to become more sophisticated. Two of the most common controller types are called MPPT and PWM.

Although both technologies serve the same purpose, they go about it in different ways. In this post, we'll take a look at the key differences between MPPT and PWM charge controllers.

What is a PWM solar charge controller?

A PWM (Pulse Width Modulation) solar charge controller is a linear regulator that uses a series of ON and OFF pulses to control the amount of current flowing into the battery.

The length (or duty cycle) of these pulses is varied according to the battery's voltage. When the battery is low, the duty cycle is high (more current flows into the battery). As the battery charges, the duty cycle decreases until it reaches a minimum when the battery is fully charged.

When charging a 12V battery, the charging voltage can go as much as 14.2V. This is fine since most batteries are designed to accept more than the nominal voltage.

At this point, the PWM controller is going to switch to float mode, where it will pulse at a very low duty cycle to maintain the battery's charge. The battery voltage will fall to around 13.7V which is the ideal float voltage for most 12V lead-acid battery systems.

During the float mode, the charge controller constantly monitors the battery voltage. If it falls below the float voltage threshold, the controller will start charging the battery again.

PWM solar controllers only have two states: all the way ON or all the way OFF.

There is nothing in between. Therefore, the only way they regulate the current is by continuously varying the duty cycle of the pulses.

For example, a 20% modulation would mean that the power is ON 20% of the time and OFF 80% of the time. Similarly, a 50% modulation would mean that the power is ON half the time and OFF half the time.

It is important to note that the pulses that make up the duty cycle are square waves. This means that they have a very high harmonic content, which can cause electrical noise problems in some systems.

That said, some PWM modules have good filtering circuitry that can help reduce the noise levels. However, it is still something to be aware of.

Here is a video that greatly explains PWM solar controllers:

What is an MPPT solar charge controller?

An MPPT (maximum power point tracking) solar charge controller is a digital controller that uses a microprocessor to track the maximum power point of the solar array.

It does this by measuring the current and voltage going into the battery and constantly adjusting the duty cycle to find the point where the solar array is producing the most power.

This maximum power point varies depending on the amount of sunlight, temperature, and other factors. However, the MPPT controller is constantly adjusting to find it.

Once the maximum power point is found, the MPPT controller will regulate the current and voltage going into the battery to keep it at that point.

MPPT solar controllers can convert excess voltage to additional current, which would otherwise be wasted by a PWM controller. We know that most solar panels produce more voltage than their nominal voltage defined by the manufacturer.

For example, a 12V solar panel can actually produce up to 16V – 18V depending on the conditions. However, only a 14.6V is necessary to charge the most 12V batteries.

An MPPT controller can take advantage of this generated extra voltage and convert it to more current, which is then used to charge the battery.

This is why MPPT solar controllers are more efficient than PWM solar controllers since they can make use of all the power that the solar array is producing, instead of just a fraction.

The same rule applies to higher voltage 24V or 48V solar panels as well. Since what these panels can produce is much higher, the MPPT controller can convert even more voltage to current and charge the battery at a faster rate.

MPPT controllers can also help to reduce the amount of power lost due to voltage drop or line losses by handling higher voltage configurations of solar panels.

In other words, you can wire multiple solar panels in series to increase the voltage while keeping the wire size and current the same. This can be particularly helpful when long distances need to be covered without losing too much power due to voltage drop.

As a result, you will be able to run larger panel arrays than with a traditional PWM controller.

If your solar power system is grid-tied, MPPT control is the only type that can handle high voltages in such systems. What that means, the only way to add batteries for backup power with a grid-tied system is to have an MPPT charge controller.

MPPT charge controllers typically offer a 94% – %99 efficiency while PWM charge controllers offer an efficiency of around 70%. This means that an MPPT controller can be as much as 30% more efficient than a PWM controller.

This can result in a significant increase in solar panel output over the course of a day. However, there is a tradeoff that must be considered.

MPPT controllers typically cost 2 to 3 times more than a PWM controller. So, if you have a small solar power system, the increased efficiency of an MPPT controller may not justify the increased cost.

But, if you have a larger solar power system, the increased efficiency can make a big difference and is well worth the additional cost.

Another thing, MPPT controllers are much larger than PWM controllers. That is worth knowing if the space in your solar power system is limited.

If you have an off-grid solar PV system in a cold climate with lots of cloud cover, you might want to consider an MPPT controller. Since they can extract every last bit of power from your solar array, they could make the difference between having enough power to get through the day or not.

One of the key disadvantages of MPPT is that it does not perform well in low light conditions.

Because in low light the controller may have a hard time finding the maximum power point, which can result in lower than expected solar panel output.

Fortunately, this is not as big of an issue since the controller makes up for it by working better in bright sunlight.

When selecting an MPPT controller, be wary of cheap products that appear to be too good to be true.

If you buy an MPPT for around $100, it is most likely a knockoff that will not last as long or be as efficient as the original brand product. Thus, I would always recommend investing in an MPPT controller manufactured by a reputable company.

Remember your solar PV system is only as good as the solar charge controller you use. So, if you want the best possible performance, invest in a quality MPPT controller.

Advantages of PMW Charge Controllers

Inexpensive

PWM charge controllers are the most common type on the market. They have been around for a long time and are proven to be reliable.

PWM charge controllers are also much less expensive than MPPT charge controllers. In fact, they cost 2 – 3 times less than MPPT controllers. So, if you are on a tight budget, a PWM controller is the way to go.

Small and Compact

PWM controllers are small and compact, making them easy to install in tight spaces. MPPT controllers are much larger and can be difficult to install in small solar systems.

Good for Low Light Conditions

PWM controllers work well in low light conditions, whereas MPPT controllers do not. So, if you live in an area with a constant cloud cover, a PWM controller can still be an acceptable option.

Good for Small Solar Systems

PWM controllers are also a good choice if you have a small solar system. The increased efficiency of an MPPT controller may not justify the cost for a small system. Thus, if you only have a few solar panels, a PWM controller is the way to go.

Doesn't create heat

PWM controllers do not create heat, whereas MPPT controllers can get quite hot. This is because PWM controllers don't create resistive circuitry regulating the current. They are either ON or OFF, which doesn't create any resistance and thus no heat.

Disadvantages of PMW Charge Controllers

Lower Efficiency than MPPT

PWM charge controllers are not as efficient as MPPT charge controllers. As we have mentioned earlier PWM can be as much as 30% less efficient than MPPT, which can make a big difference over the course of a day.

Can't Handle High Voltage Inputs

PWM charge controllers can't handle high voltage inputs, which means they are not a good choice if you have a large solar array. PWM can be okay for small solar systems, but if you have a lot of panels, you will need an MPPT controller.

Nominal Voltage must match the Battery Voltage

If we are going to use PWM, the nominal voltage of the solar input must equal the nominal voltage of the battery bank.

No Controller can generate more than 60 amps of DC

There is currently no single controller capable of handling more than 60 amps DC.

System Growth Isn't Possible

PWM controllers offer very limited growth potential. When system requirements outpace the controller's ability to meet them, a new controller must be installed.

Cannot be utilized with 60A panels effectively.

PWM controllers cannot be effectively utilized with 60A solar panels.

Advantages of MPPT Charge Controllers

Increased Efficiency

MPPT controllers are over 90% efficient, whereas PWM controllers are only about 70% efficient. So, if you want to get the most out of your solar array, an MPPT controller is the way to go.

Can Handle High Voltage Inputs

MPPT controllers can handle high voltage inputs, which means they are a good choice if you have a large solar array.

Allows oversizing array

MPPT controllers also provide the option of having an array with a higher input voltage than the battery bank.

Adjusts for differences in the I-V characteristics

MPPT controllers have the ability to adjust for differences in the I-V characteristics of the solar array. This means that they can extract more power from the solar array than a PWM controller.

Can reduce system complexities

MPPT controllers can reduce system complexities by eliminating the need for a micro-inverter on every solar panel.

Disadvantages of MPPT Charge Controllers

Higher Cost

As we have mentioned earlier, MPPT charge controllers are more expensive than PWM charge controllers. In fact, they can cost 2 – 3 times more than PWM controllers.

Large and Bulky

MPPT controllers are also large and bulky, which can make difference for certain installations.

Creates Heat

MPPT controllers create heat, which should be taken into account when deciding where to install the controller.

Conclusion

So, there you have it. The difference between MPPT and PWM charge controllers. As you can see, there are pros and cons to both types of controllers. Ultimately, the decision comes down to your specific needs and budget.

If you have a small solar system and are on a tight budget, a PWM controller is a good choice. However, if you have a large solar system or want the most efficient system possible, an MPPT controller is the way to go.