What to Know About Solar Charge Controllers——How to size your MPPT and PWM solar charge controller

There are many ever-changing variables that affect how much power is generated by your panels. The level of sunlight, temperature, and your battery’s state of charge will all have an impact. Charge controllers perform the essential role of monitoring this to ensure the optimal amount gets fed into your battery bank. This ensures your system runs efficiently and safely, as well as prevents battery drainage. in addition to preventing battery drainage. There are a range of different types and sizes of charge controllers. Let’s discuss what the different types are and what size to purchase for your solar installation.

What do solar charge controllers do?

Solar charge controllers prevent the overcharging of batteries by limiting the amount and rate of charge to your batteries. They also prevent battery drainage by shutting down the system if stored power falls below 50 percent capacity and charge the batteries at the correct voltage level. Charge controllers also offer some other important functions, including overload protection, low voltage disconnection, and blockage of reverse currents.You typically need a charge controller for any panel larger than 5 watts.

What are the different types of charge controllers?

There are two main types of charge controllers to consider: Pulse Width Modulation (PWM) controllers and Maximum Power Point Tracking (MPPT) controllers. PWM charge controllers are cheaper and ideal in smaller applications. MPPT controllers are the most efficient, ideal in large applications, and are the most expensive option. Both pulse width modulation and maximum power point tracking charge controllers have a lifespan of about 15 years, although that will vary based on the specific controller.

Pulse Width Modulation Charge Controllers

Wanderer Li 30A PWM Charge Controller

Best for: Those with smaller systems (vans, motorhomes, tiny homes) and those living in warmer climates

Cost: £15-£40

Pulse width modulation controllers are simpler and less expensive than MPPT controllers. PWM controllers regulate the flow of energy to the battery by reducing the current gradually, called "pulse width modulation". When batteries are full, a PWM charge controller will supply a tiny amount of power to keep batteries full. PWM controllers are ideal for small scale applications because the solar panel system and batteries have to have matching voltages.

PWM charge controllers are the cheapest charge controller option, best for warm sunny weather, and performs best when the battery is near the full state of charge

Maximum Power Point Tracking Charge Controller

Best for: Those with larger systems (cabins, homes) and those living in colder climates

Cost: £60-£300

Maximum Power Point Tracking controllers are efficient at using the full power of your solar panels to charge your batteries. MPPT controllers will actively monitor and adjust their input to regulate a solar system’s current, and they will step down the voltage and boost the current. For example, if it becomes cloudy, your MPPT charge controller will decrease the amount of current drawn in order to maintain a desirable voltage at the output of the panel. When it becomes sunny again, the MPPT controller will allow more current again. You can expect efficiency ratings of 90% or higher.

MPPT charge controllers are suitable for situations where the solar array voltage is higher than the battery voltage, highly space and energy efficient, ideal in larger systems where the additional energy production is valuable, best in colder, cloudier environments, and performs best when the battery is in a low state of charge.

What size charge controller do I need?

In order to size your solar system and evaluate your energy needs, we first recommend using our solar sizing calculator. Charge controllers are rated and sized depending on your solar array current and the solar system’s voltage, and you want a charge controller that is large enough to handle the amount of power and current produced by your solar panels.

If your solar system's volts were 12 and your amps were 14, you would need a solar charge controller that had at least 14 amps. You’ll need to factor in an additional 25% due to external factors, such as light reflection. So you’ll need a solar charger controller of 17.5 amps. By looking on the Renogy website, you’ll see your best option is a 12 volt, 20 amp solar charge controller.

Keep in mind you can also use more than one charge controller when a single charge controller is not large enough to handle the output of your solar panel array. However, be sure to use the same type of charge controllers if you are using more than one.

MPPT Charge Controller Sizing: MPPT controllers limit their output. So even if your panels have the potential to produce 80A of current, a 40A MPPT charge controller will only produce 40A of current. An MPPT controller will have an amp reading for it, for example a 40 amp MPPT controller. This represents how many amps the controller will allow, or 40 amps in this case.

Charge controllers will also have a voltage rating, but unlike PWM controllers, the input voltage rating is much higher than the battery banks it will charge. This is due to the special property of the MPPT controller being able to lower the voltage to the battery bank voltage and then increase the current to make up for lost power. You do not have to utilize the high input voltage if you want to avoid series connections in small systems, but it is very beneficial in larger systems.

We can also look at the maximum solar input voltage. For example, if an MPPT Controller can accept 100 volts of input, it will then take up to 100 volts. Let’s say you have 2 x 100 Watt panels in series, each with an open-circuit voltage of 22.5V. Those 2 in series will be 2 x 22.5 V = 45 Volts, which the controller can easily accept.

PWM Charge Controller Sizing: Unlike MPPT charge controllers, PWM controllers are unable to limit their current output. Whatever the output from your panels is, that is the output that will go to the charge controller. This means it is crucial to ensure your charge controller is matched, compatible with, and properly sized for your panels.If your solar array can produce 40 amps of current and the charge controller is only rated to 30 amps, then the controller could be damaged.

Generally you’ll want to look at the amperage and voltage rating. A PWM controller will have an amp reading for it, for example a 30 amp PWM controller. This represents how many amps the controller can handle, or 30 amps in this case.

We can also look at the nominal system voltage. This will tell us what voltage battery banks the controller is compatible with. In this case, the nominal voltage is 12v, which means you can use 12V battery banks.

You’ll also want to check the rated battery current. For the same 30 amp PWM charge controller, it has a rated charge current of 30 Amp rating. We recommended a factor of safety of at least 1.25, meaning you would multiply the current from your panels by 1.25. For example, if you have five 100 Watt panels in parallel, that would be 5.29 x 5 = 26.45 Amps. 26.45 Amps x 1.25 = 33 amps. This means those five panels are too much for the 30 amp controller.

Finally, you can look at the maximum solar input. This tells you how many volts you can have going into the controller. This controller cannot accept more than 25 volts in. Let’s look at having 2 x 100 Watt panels in series for a total of 22.5V (open-circuit voltage) x 2 = 45 volts. In this case, it is not ok to wire these two panels in series.


Charge controllers play a key part of every solar installation. Both MPPT and PWM charge controllers limit the amount and rate of charge to your batteries, provide overload protection, disconnect at low voltages, and block reverse current. Be sure to take the time to do the research to properly select and size your charge controller, and you’ll have an efficient and safe system for years to come.