A charge controller sits between the array and the battery. The two types handle the mismatch between panel voltage and battery voltage in fundamentally different ways.
PWM: a switch
A PWM controller connects the array to the battery and rapidly switches to regulate. The array is pulled down to battery voltage.
A 36-cell panel with a maximum power point around 17 V, connected to a 12 V battery at 13 V, is forced to operate at 13 V. The current stays roughly the same, so about a quarter of the available power is simply not collected.
MPPT: a converter
An MPPT controller is a DC-DC converter. It runs the array at its actual maximum power point and converts the surplus voltage into extra current.
That same panel at 17 V and 5.9 A — 100 W — delivers about 7.4 A at 13 V, less conversion losses. The power is captured rather than discarded.
Where the gain is large
MPPT typically recovers 20 to 30% more energy, and the gap widens:
- In cold weather. Panel voltage rises as temperature falls. A cold clear winter day is when the array's maximum power point is furthest above battery voltage, and it is also when you most need the energy.
- On a low battery. The deeper the discharge, the bigger the voltage gap.
- With high-voltage arrays. MPPT lets you wire panels in series at 100 V or more and step down. That means thinner cable for the run from array to controller — often a larger saving than the controller costs.
Where PWM is still fine
- Small systems, under about 200 W
- Where panel Vmp closely matches battery voltage — a true "12 V" panel on a 12 V battery
- Where cost dominates and the array is cheap enough to oversize instead
For a 100 W panel keeping a caravan battery topped up, PWM plus one extra panel can be cheaper than MPPT with one.
Sizing the controller
Rated by output current and maximum input voltage.
Current: array watts ÷ battery voltage, plus margin. A 1,000 W array on a 24 V bank needs about 42 A, so a 50 or 60 A controller.
Voltage: the maximum input rating must exceed the array's cold open-circuit voltage, not its rated Voc. Voc rises as temperature falls, and a string sized at 25 °C can exceed the controller's limit on a cold clear morning — which destroys the controller.
Use the panel's temperature coefficient of Voc and your record low temperature. This is the single most common way charge controllers are killed, and it happens in the first winter.
Both need correct charge profiles
Whichever type, set it for the battery chemistry: bulk, absorption and float voltages for lead-acid; the manufacturer's charge voltage and no float for lithium. A controller on the wrong profile will damage an expensive bank quietly over months.