Solar Panel Voc-MPPT?

BFL13 wrote:
The practical value of Voc and Isc is that you can measure them with the panel disconnected and that should tell you how things will go once you connect up. With PWM, whatever the amps you got as Isc, you should now get that connected. It is Voc that doesn't matter in that case.

The OP question is how can you do a similar thing with MPPT? If you measure Voc and it is lower than rated, does than mean the controller will not be able to find Vmp but only some lower number even if you got rated Isc ok?

I want a way with MPPT to tell I am getting the best amps possible at the time, same as I can now with PWM.

With MPPT the amps you are seeing are not related to Isc but are from the controller. You can't compare 'with controller' vs 'no controller' (panel -direct) amps unless it is a 12v panel. With a 24v panel your panel- direct amps could be the same as Isc but you could get way more than that with the controller, even if the controller were failing to provide what you should be getting. (Such as if you had a wiring or connection issue that needs fixing.)

How do you tell what you should be getting so you can see if you are getting it?

BFL. If you have a reasonably accurate clamp on DC current probe, you could measure the panel output current on one conductor going to the MPPT controller and then with a voltmeter MPPT input voltage or at the panel the panel Vout.

Presently I have my second Voltage input for my trimetric wired to the inverter input to monitor inverter voltage, but I'm tempted to either wire it to the MPPT controller input or put a SPDT switch near the controller and inverter (they are side by side) that will let me either look at MPPT input voltage or inverter input voltage on the trimetric.

This would let you then calculate power input to the controller, and with the trimetric by looking at battery voltage and battery current MPPT output power and measure controller efficiency as well as compare MPPT input power to what the panel theoretical power should be.

With a MPPT controller, you want to shoot for getting Imp not Isc out of the panel to the controller. and Vmp not Voc. Isc will be at a much lower panel voltage so panel output power will be below max. And Voc is with no load.

And Voc is the maximum output voltage over the operating temperature range of the panel. Voc would normally occur in max solar conditions at a panel temperature well below ambient, like 0F or lower. A main reason for the Voc spec is to give a panel max output voltage that you can compare to controller max input voltage and make sure you will be within the spec'd operating range for the controller and not damage it. For example, my panels have a Voc of 22V. even in bright sun and normal 75F on initial turn on, Voc. is in the 20 to 20.5V range.