January 9, 2019

My ARCO M75 solar panel

This is a 47 watt panel. In the best of conditions it should produce about 3.5 amps and 14.2 volts. It measures 13 by 48 inches. I expect to weld up a frame to bolt it into that attaches to my house and will allow it to stay put during severe thunderstorms. All due consideration for lightning issues will also be pondered.

It is almost 30 years old, but has been indoors most of its life and seems to work fine. It was given to me, and I have always wanted to fool around with solar energy, so why not?

A Charge controller

As luck would have it, I was also given a pair of 12 volt sealed batteries. They were removed on some schedule from a large very critical UPS, so have done little more than sit around most of their lives.

Now what I need is a charge controller (and a sturdy mounting for the panel) and I should be in business.

The main drawback with the $15 unit is that it doesn't display amps being charged, which I think is an important and interesting measurement, so I am passing on it, despite its attractive (maybe too attractive?) price.

I ended up ordering the HQST unit. They have a 10A unit for $16, but the main reason I bought the bigger unit is that it displays the amps while the 10A unit does not. I vetoed the PowerMr units -- the deciding factor was no PDF manuals available online. Renogy provides nice PDF manuals (as does HQST).

I discovered the HQST unit, because the 10A HQST unit is offered by Renogy as part of their 50W starter package. The Renogy Wanderer for $30 (only $10 more) might have been an even better choice.

MPPT versus PWM

We all know what PWM is, but what the heck is MPPT? Apparently you can get either "flavor" of charge controller. See the link above. For a small system (100 watt or less, PWM is fine and perhaps half the cost).
An MPPT controller is advantageous with a big system where panels are in series to produce a higher voltage. An MPPT controller will be more efficient as it "works" the panel at the voltage and current that makes it yield the most power (the MPPT point). On the other hand, PWM is simple and reliable and cheap.


Panels are rated at 25 C (77 degrees F), but their voltage drops significantly as their temperature goes up. The drop is something like 0.12 to 0.15 volt per degree C.
Have any comments? Questions? Drop me a line!

Tom's notes / tom@mmto.org