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!!!CAUTION!!!: Since writing this review last November (2013), the product has been substituted with another manufacturer's brand and model, so some of what I have written is irrelevant, and I don't know how many other reviews on this substituted product are applicable. Amazon is currently (Aug14) investigating what happened in merging one product's reviews with another product. I would suggest you stick with more recent reviews to get the best assessment of this current product. The original product is no longer listed on Amazon. Read this review if you really expect this voltmeter to give you an accurate reading. This Digital Panel Voltmeter (DPM) is a model YB5135D made by YB Electronics Co. There are only two wire leads in this 7.5-20vdc version, which serve as both power to the DPM and measurement of the voltage source in question (e.g., car battery). Therefore polarity is important, and if the voltage source is below +7.5vdc, the DPM will not operate. I measured several batteries with the DPM and compared the readings with a Fluke 87 VOM and a Fluke 114 VOM. The YB DPM was consistently reading low by 9.5%. The error I was getting would cause me to believe that my fully-charged car/boat/RV battery was only 40% charged. To correct the problem with the DPM, I turned the screw CCW 6-1/3 turns on the blue potentiometer that is mounted on the printed circuit board. All three meters then agreed (I measured several batteries (9v, 12v, and 20v) again to confirm that the problem was with the meter gain, and not offset as well). I then put a dab of finger nail polish on the screw/potentiometer, so that the screw wouldn't accidentally rotate from where I set it. Before using one of these YB DPMs, I would strongly suggest checking it against an accurate voltmeter before using it, adjust the potentiometer if needed, and then cement the screw to the potentiometer so that it doesn't get knocked or vibrated out of 'calibration'. The YB DPM I got uses 18-19mA to power the unit. Normally a DPM has high input impedance, so that the length of the measurement wires is not important. In the case of this DPM, if you use small-gauge wire (e.g., 28AWG) and the DPM is 20 feet or more from the voltage source, then your DPM will not give you an accurate reading. Or if the voltage source being measured has a high internal impedance (unlike a lead-acid battery), then the DPM reading will not give you an accurate reading. The opening, for the DPM to be mounted in a 0.70-0.125" thick panel, I measured to be 1.535"H x 2.920"W. The overall dimensions of the DPM are 3.128"Wx1.785"Hx1.074"D. The DPM extends 1.025" into the opening from the front of whatever panel you may mount it in.