One device I have been playing with is the TURNIGY 130A Watt Meter and Power Analyzer for about $30 shipped from Hobby King online. dnix71 was nice enough to provide a defective one to me which I fixed. It had a defective 2937H 3.3V regulator (little chip in upper RH corner) that was supplying over 10V to the processor. Don't know how it survived or if it will continue to work. Anyway this was left in a box at home that never packed on the trailer along with a bunch of electronics. I really wanted to use it this year on the system so went looking on ebay for another. One vendor is blowing these look alikes out for under $23 shipped, bought two. The G.T.POWER device is identical and rated up to 60V DC and 130A peak. The two line lighted LCD displays the voltage and amperage to two decimal places and wattage to one decimal place. A sequencing fourth section indicates watt hours, V max and min, peak amps, amp hours. The boards are folded open with the repaired unit and will likely never fold back since the would likely break the traces. The two blue pots calibrate V and I. Current sensing is in the negative leg with a better than expected current shunt and only senses current flow in one direction.
On message boards there are numerous reports of these solder connections melting with high continuous currents. I wouldn't use it at over 25A full time, one vendor says 50A. The wires are very flexible #12 with a silicone jacket. Passing current through the red positive lead should be avoided since this will only cause additional IR loss and bring more heat into the case. The two pots allow calibration of the volt and amp display. That makes it possible to divert some current to an external shunt for higher currents. The picture shows it connected to my fridge inverter with a 100A 50mv shunt. The multiplier for this setup is 5.71 to multiply A and W readings adding a little length of wire between the shunt and meter would easily make it a nice number like 10. Just calculate out what length of wire will give you about 40mv to make your own shunt. My fridge tested out at 122A peak current, 13.5A running and 191W.
The three pin connector on the side allows external power to keep the electronics alive and external reset. Diodes isolate external and line power. The instructions give no indication how this connector hooks up. Top to bottom, common, power, reset to ground.
As said before, the maximum voltage is 60V, the rating of the regulator. I recommend that this device be disconnected from internal power and always be powered externally if measuring higher voltages. There is no spike protection for the regulator. On the back side of the board there is a single perpendicular trace that can be cut to isolate power from the red sense wire. That may also allow higher voltages to be monitored safely and protect the regulator. dding an external shunt makes this low cost device is pretty handy for solar panel and battery monitoring.
My eyes aren't what they used to be and the blue is a little hard to focus on. This is a standard C1602A two line display easily googled. Once the warranty is up I will be adding a pot to adjust the display contrast for better viewing. The hookup circuit shows how I connect in my system. Placing a 150 ohm resistor in series with the 12V 400 ohm relay turns on the display at first light and off at night and protect relay from overheating. This will save you having to turn it on each morning for accurate daily totals.
With a variable load this would be a cheap solar panel tester. Get one before they are gone. I've seen the same thing selling for up to $129.
http://www.ebay.com/itm/High-Precision-G-T-Power-RC-Watt-Meter-Tester-and-Power-Analyzer-130-Amps-/380644150825?pt=LH_DefaultDomain_0&hash=item58a028ce29#ht_1999wt_1059
