Guides / Panels & Charge Controllers
PWM vs MPPT Charge Controllers
What a charge controller does, how PWM and MPPT controllers differ, and which one suits your panels and battery.
A charge controller sits between your solar panels and your battery. It stops the battery being overcharged and charges it in the right stages for its chemistry. There are two main types: PWM and MPPT.
1. How a PWM controller works
A PWM (pulse width modulation) controller is basically a fast switch. It connects the panel straight to the battery and switches it on and off to control charging. That means the panel is pulled down to the battery's voltage.
A typical "12V" panel works best at around 17 to 18V. Pulled down to a battery at about 13V, it can't deliver all of its rated power. The extra voltage is simply wasted.
- Good: cheap, simple and reliable.
- Good: fine for small systems where the panel's voltage closely suits the battery.
- Not so good: wastes some panel power, and can't use panels wired in series for a higher voltage.
2. How an MPPT controller works
An MPPT (maximum power point tracking) controller keeps finding the voltage where the panel gives its most power, then converts it down to suit the battery. The extra voltage becomes extra charging current instead of being wasted.
- Good: harvests more energy from the same panels, especially on cold days and when panel voltage is well above the battery.
- Good: lets you wire panels in series, so you can use thinner cable for long runs from the panels.
- Good: lets you use larger home-style panels on a 12V or 24V battery.
- Not so good: costs more than PWM.
The gain from MPPT is often quoted as about 10 to 30 percent, but it depends on your panels, the weather and the battery voltage.
3. Which one should you pick?
- PWM: one or two small "12V" panels on a 12V battery, on a tight budget.
- MPPT: almost everything else, including any workshop or home system, any system with panels in series, and any system using large home-style panels.
4. Sizing a controller
- Current: the controller's rated amps is its output to the battery. Divide the total panel watts by the battery voltage, then add a margin. For example, 800W of panels on a 24V battery is about 33A, so a 40A controller suits.
- Voltage: the panels' total open-circuit voltage must stay below the controller's maximum input voltage, even on a cold morning, when panel voltage rises.
- Battery type: make sure the controller has a charging profile for your battery chemistry, such as lithium (LiFePO4) or AGM.
Not sure which battery chemistry to choose? Revival Academy 101 compares them in LiFePO4 vs AGM.
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