I also have the same meter. The right side of the meter is the load side (battery) and the left side is the Source (panel) The switch is needed in battery side so as to Disconnect the battery power (Red) for two seconds to reset for data readout. I've read the two reds are tied together in the meter. Hmm lot's of fun
I have my Watt Meter just handing below the MPPT controller in the shop connected to my 24v/120w Cynergy Panel and two 12v batts in series.
My Watt Meter was mfg for SUN-BORN Energy by Turnigy It's on EBay at $42.95 plus shipping.
O$S
Floyd