Suppose your motorhome's solar panel is like a tap providing water (electricity) to fill a tank (your leisure battery). The charge controller is the device that manages how water flows into the tank, making sure it fills up properly without overflowing or getting damaged.

There are two main "water flow manager" types: PWM and MPPT.

PWM (Pulse Width Modulation) Charge Controller: “The Simple Tap”
Think of a PWM controller as a simple on/off tap. When your solar panel produces enough electricity, the PWM controller connects the panel directly to your battery.
• How it works: It rapidly switches the connection between the solar panel and the battery on and off. The "width" of the "on" pulses determines how much power goes through. If the battery needs a lot of charge, the "on" pulses are longer. As the battery gets full, the "on" pulses become shorter.
• The Catch: The PWM controller forces the voltage of the solar panel to match the voltage of your battery. This is like forcing a powerful tap to run at the same pressure as a nearly full tank. The solar panel might be capable of producing a higher voltage (and therefore more power), but the PWM controller restricts it to the battery's voltage. This means that some of the potential power from the panel is lost.

Analogy: Imagine your tap can produce water at 20 PSI (Pounds per Square Inch), but your tank is almost full and only needs water at 12 PSI. The PWM controller forces the tap to only run at 12 PSI, even though it could deliver more water at 20 PSI. As a result, you're not getting the full potential flow.

MPPT (Maximum Power Point Tracking) Charge Controller: “The Smart Valve”

Think of an MPPT controller as a much smarter valve that constantly adjusts itself to get the maximum amount of water (electricity) from the tap and efficiently pump it into your tank.
• How it works: An MPPT controller constantly monitors the voltage and current from your solar panel and uses clever electronics to find the "sweet spot" by tracking the maximum power point where the panel produces the most power at any given time. It then converts this higher voltage from the panel into the correct voltage needed to charge your battery, but at a higher current (so more "water flow").
• The Benefit: Unlike the PWM controller, the MPPT controller doesn't force the panel's voltage to match the battery's voltage. It can take the higher voltage from the panel and efficiently "step it down" to the battery's voltage while increasing the current. This means you get more power from your solar panel, especially when the panel's voltage is significantly higher than the battery's.

Analogy: Imagine your tap can produce water at 20 PSI, but your tank needs 12 PSI. The MPPT controller acts like a special pump and valve. It takes the 20 PSI water from the tap, efficiently converts it, and pumps it into your tank at 12 PSI, but with a much stronger flow than if you just reduced the pressure directly. You're now getting much more water into your tank. 

Which controller is better for me?

Weather Conditions (especially in the UK):

o Cloudy or Low-Light Conditions: MPPT controllers tend to perform much better in the often overcast conditions of the UK. Even when the solar panel isn't producing its full power, an MPPT controller can still extract more energy than a PWM controller. This is because the panel's voltage might be significantly higher than the battery's, even in low light, and the MPPT can still capitalise on this difference. A PWM controller would be less efficient as it would still be forcing the lower voltage to match the battery.
o Sunny Conditions: While both will work in sunny conditions, the MPPT controller will generally harvest more energy throughout the day.

Solar Panel Wattage:

o Lower Wattage Systems (e.g., under 150-200W): A PWM controller might be sufficient for very small solar setups and is generally more cost-effective. The power difference might not be significant enough to justify the extra cost of an MPPT controller.
o Higher Wattage Systems (e.g., over 200W): An MPPT controller becomes much more beneficial for larger solar panel arrays. The increased efficiency means you'll get significantly more power to charge your batteries, especially in less-than-ideal conditions. This can significantly affect how long you can stay off-grid in your motorhome.

Other Considerations:

• Cost: PWM controllers are generally cheaper than MPPT controllers.
• Panel Voltage: MPPT controllers work best when your solar panel array's voltage is significantly higher than your battery bank's (e.g., a 36V solar panel system charging a 12V battery). This is often the case with modern solar panels designed for off-grid use.

In Summary:

For most motorhome owners in the UK, especially those with larger solar panel systems or who frequently travel in varied weather conditions, an MPPT charge controller is generally the better choice due to its increased efficiency and ability to harvest more power from the sun. While the initial cost is higher, the extra power gained can significantly improve your off-grid capabilities. However, a PWM controller might still be adequate for small and budget-conscious setups.