Just bought two 75 panels to run a Norcold RV refrigerator at my camp. Of course, I'll be wanting to monitor how long the refrigerator runs each day to determine system requirements. A quartz clock and a reed switch met this need perfectly. The glass rod of the reed switch was wound with 22 turns of #18 enameled magnet wire. Two wires in parallel were used for lower resistance and because I didn't have any thicker wire. This gives me a switch closure at a little over 2A and a drop out at about 1A. The switch contacts were wired in series with the clock battery. I soldered one contact directly to the battery and the other to the battery clip. You have to be artful in doing this so you don't melt the case. A thin piece of cardboard then insulates the battery from the clip. A piece of double sided copper clad circuit board would be a better way to go.
My refrigerator draws 5.5A so multiplying that by the time on the clock would give you the amp hours. Just start out each measuring period by setting the clock to 12:00. Best guess is that this wil use about 20AH a day. This could also help in monitoring available sunlight. From what I have seen, 2A out of a 4.5A rated panel is basically full sun. Any little wisp of a cloud drops it down to nothing.
Reed switches may be a little hard to find and the number of turns will vary greatly. Still, this is a very simple and cheap way to monitor. I'm going to design one using a LM339 op amp that will be adjustable have a series resistance of about 0.05 ohm to replace the reed switch. Only have one reed switch left in the junk box!
