An MPPT (Maximum Power Point Tracking) charge controller is more efficient than a PWM (Pulse Width Modulation) controller, typically extracting 15-30% more energy from the same solar panels. MPPT is the right choice for most systems, especially in the UK where variable light conditions make efficient energy harvesting critical. PWM controllers are cheaper and suit very small, simple setups only.
A solar charge controller sits between your solar panels and your battery bank, managing the charging process to protect the batteries and maximise the energy you harvest. Choosing between MPPT and PWM is one of the first decisions you will face when designing any solar system.
What Does a Charge Controller Actually Do?
Solar panels produce voltage that varies with sunlight intensity and temperature. Batteries require a specific voltage and charge profile to charge safely and efficiently. Without a charge controller, panels would overcharge and damage batteries. The controller regulates this process and prevents reverse current flow at night.
PWM Controllers: How They Work
A PWM controller works by rapidly switching the connection between the solar panel and battery on and off. As the battery approaches full charge, the pulses become shorter, reducing the average current. The panel voltage is effectively dragged down to match the battery voltage.
Pros of PWM
- Lower cost
- Simple and reliable with few components
- Suitable for small 12V systems where panel voltage closely matches battery voltage
Cons of PWM
- Significant power waste: if your panel's maximum power point is at 18V but your battery is at 12V, all that voltage difference is wasted as heat
- Poor performance in partial shade or variable light
- Cannot handle higher voltage panel arrays
MPPT Controllers: How They Work
An MPPT controller uses a DC-DC converter to track the panel's maximum power point voltage continuously and convert the excess voltage into additional current. Rather than wasting the voltage difference, it converts it into useful charge current.
For example: a panel producing 20V at 5A (100W) into a 12V battery. A PWM controller would deliver roughly 12V x 5A = 60W. An MPPT controller converts the 100W efficiently, delivering closer to 100 / 12 = 8A into the battery, a 60% improvement in this scenario.
Pros of MPPT
- Significantly higher efficiency, typically 93-99%
- Works well with higher voltage panel strings
- Better performance in low light, partial shade, and cold weather (relevant for UK use)
- Allows 24V, 36V or 48V panel arrays to charge 12V or 24V battery banks
Cons of MPPT
- Higher upfront cost
- More complex electronics
When to Use Each Type
Choose PWM if
- Your system is very small (under 200W) and simple
- Your panel voltage closely matches your battery voltage (e.g. a 12V panel charging a 12V battery)
- Budget is the absolute primary concern
Choose MPPT if
- Your system is 200W or larger
- You are using modern monocrystalline panels with higher open-circuit voltages
- You are in the UK and want to maximise harvest during overcast conditions
- You are connecting multiple panels in series to create a higher voltage string
Key Specifications to Check
When choosing an MPPT controller, check:
- Maximum input voltage (Voc): must be higher than the open-circuit voltage of your panel array. Never exceed this rating.
- Maximum charge current (A): determines how fast the battery charges. Size this to your panel array wattage divided by battery voltage.
- Battery voltage compatibility: most controllers support 12V/24V auto-select; better units also handle 48V.
- Communication and monitoring: Bluetooth or RS485 connectivity for data logging is a useful feature in larger systems.
Portable Power Stations
If you are using a portable power station such as the EcoFlow DELTA 2, a dedicated MPPT charge controller is built into the unit. You do not need to buy one separately. Simply connect compatible solar panels to the station's solar input port. The internal MPPT controller handles everything.
