How to size an MPPT controller
An MPPT controller converts the panels' higher voltage down to battery voltage, raising the current. 400 W into a 12 V battery is about 33 A, or 42 A with 25% headroom, so a 40–50 A controller suits it. The same 400 W into 24 V needs only 17 A, so a 20 A controller would do.
Many MPPT makers allow the array to be somewhat bigger than the controller rating ("over-paneling"). The controller simply caps the output on the sunniest days. Check your controller's manual for its limit.
How to size a PWM controller
A PWM controller connects the panels straight to the battery, so the controller must handle the panels' full short-circuit current. The 1.25 factor covers bright conditions and reflections (the same factor used in the US NEC).
MPPT or PWM?
- MPPT harvests 10–30% more energy, works with high-voltage household-style panels, and allows thinner cable from the panels.
- PWM is cheaper and fine for a single small 12 V panel trickle-charging a battery.
Frequently asked questions
What size charge controller for 400W of solar?
On a 12 V battery, about 33 A, so a 40 A MPPT controller (50 A with full 25% headroom). On 24 V, a 20–30 A MPPT controller.
What size charge controller for 200W?
On 12 V, about 17 A (21 A with headroom), so a 20–30 A MPPT. A PWM controller for two 100 W panels with 6 A Isc each would need 15 A.
Can I use a bigger charge controller than I need?
Yes. A larger controller is never a problem except for cost, and it leaves room to add panels later.