PWM

PWM stands for Pulse Width Modulation, a simpler and less expensive solar charge controller technology that regulates charging by rapidly switching the connection between the solar panel and battery on and off. By adjusting the ratio of on-time to off-time (the duty cycle), a PWM controller limits the charging current to prevent battery overcharge.

Unlike MPPT controllers, PWM controllers do not perform voltage conversion. They essentially clamp the panel's output to the battery voltage. This means if you connect a panel with a Vmp of 36V to a 12V battery through a PWM controller, the panel is forced to operate at 12V — far below its optimal power point. The voltage difference is lost as heat, not converted to usable current.

This voltage mismatch is why PWM controllers harvest significantly less energy than MPPT — typically 15-30% less. The only scenario where PWM approaches MPPT efficiency is when the panel voltage closely matches the battery voltage, as with small 36-cell panels specifically designed for 12V PWM charging.

PWM controllers remain popular for very small systems — a single panel keeping a marine battery or RV house battery topped off — where the cost savings over MPPT matter more than peak efficiency. They are also simpler, more robust, and generate less electromagnetic interference.

For any system larger than about 200 watts, the energy recovery advantage of MPPT controllers typically justifies their higher price within the first year of operation.