backchannel: 300w panel making 20w

from this reddit post:

I pointed to the RVwiki Is My Solar Working? article, but let’s flesh this  one out a bit.

> I have a 300 watt solar panel. I have it wired into a MPPT charge controller. On a super bright sunny day I get about 40V coming in, but I'll be lucky to get more than 0.5 amps. That's only 20 watts. Much less than I should be getting. Anyone have any idea why this is happening?

The most likely scenario is the bank is fully charged and it requires 20w to maintan Vfloat (or late Absorption).

A 300w panel is likely nominal 24v, with Vmp around 36v and Voc a little over 40v.  OP’s “40v and 20w” exactly describes what it looks like when MPPT is limiting current to keep battery voltage at a given setpoint.

Note:  OP elsewhere describes the bank as 200Ah AGM,   Trailing amps at the end of Absorption would be C/200-C/100 (1-2A, likely 15w-30w).    Look familiar?  :-)

Consider the diagram below.  The blue line is the power curve of a fictional panel.  Moving along the curve (ie, MPPT choosing different panel voltage) gives different power outputs.  The maximum power point (MPP) yields about 225w when the panel is running at the voltage that gives max power  (Vmp).

This fictional panel makes max power (Pmax) at about 32.5v (blue line).   Notice what happens after the MPP;

power drops off sharply.  MPPT controllers exploit this behavior to control power outputs.  Need max power?  Run at Vmp.  Need less power?  Run the panel away from Vmp (usually higher).  Yes, you could also run the panel to the left of the peak (lower voltages) but you could end up at a voltage too low to feed the battery.  Simpler to move to the right (higher voltages).

comparison with PWM

This  Vpanel shift toward Voc to control current is one of the ways MPPT differs fundamentally from PWM.

PWM runs the panel at Vbatt, and turns the panel off/on very fast to control current.  Running the panel at Vbatt can be done on very simple/inexpensive hardware but Vbatt rarely overlaps with Vmp (where the panel makes the most power).  Hence the yield  advantage of mppt, and also increased cost.