Glossary

PWM

What is PWM?

Pulse-width modulation: a simple control method in which the solar controller switches the panel on in pulses. In doing so, it pulls the panel voltage down to battery level.

What matters about PWM in practice?

The difference between panel and battery voltage is lost in the process. With a panel built for 12-volt systems this matters little; with larger panels at high voltage it costs considerable yield.

PWM is cheaper and more robust. It makes sense for small systems with matching panels — as soon as the panel power matters, MPPT quickly pays off.

When is a PWM controller enough?

When the panel voltage suits the battery well, for example a 36-cell panel on a 12 V battery, and the system is small. Then the extra yield of an MPPT controller is small, and the PWM controller scores with a low price and simple technology.

How much yield does PWM cost?

The PWM controller pulls the panel down to battery voltage. If the panel’s voltage at its maximum power point is 18 V and the battery is at 13 V, it uses only about three quarters of the possible power. With 60- or 72-cell panels the loss is much greater; an MPPT controller belongs there.

What to watch when choosing one

  • Rated current above the panels’ short-circuit current.
  • Adjustable charging profile for the battery type, essential for lithium.
  • Temperature sensor on the battery for lead.

The solar calculator estimates how large the yield will be.

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