Hi thgoebel
It was not necessary to test in another scenario, I left the device on overnight and I already got a power reading, these days it has been a good solar index, the water has been heating up only with the power of the sun (which is good) but at night, when the water temperature drops, the backup resistor is automatically turned on, and that must have been the case.
I was a little anxious to see the power reading working, and as I didn't get reading I was worried, but now everything is ok
Thank you very much
See you in next thread.