Hi Guys, Long time no post!

I have started a new project to build a DIY stereo I can take camping that will run off battery.

I have drawn up a quick schematic of off the shelf parts I have collected but my electrical expertise is not up there with most of you so I am hoping someone can put some eyes over this and advise on if this will work or if any changes should be made.

The components:
12V SLA Batteries
16V Mains Transformer
Volt/Amp Meter - This has wires for power source, voltage reading and load wire to measure amps
PicoUPS - Connections for Battery, V-In, V-Out (This basically switches between battery and mains 16V source and charges that batteries at the same time)
Power Regulator - Has V-In (6-36V) and V-Out (12V) This is to regulate the voltage output of the PicoUPS as it has no voltage regulator built in
12V-USB Transformer - Takes 12v input and outputs to female USB connectors to charge USB devices (has own transformer)
12V Amplifier - To power speakers - Has required inputs volume control etc.

Here is my drawn circuit diagram:

I am wanting the Volt/Amp meter to measure the total current of the circuit and voltage of the battery but realised after drawing this, the way it is currently wired will only measure the current when running off battery and will read 0 when running of Mains Transformer - My solution would be to wire the Load wire to the output of the PicoUPS unit but then I will have no way to tell how much current that device is drawing.

This is the type of meter in question: http://diyprojects.eu/how-to-wire-digital-dual-display-volt-and-ammeter/