Welcome, Daniel, to the forum and to OEM.
Phew, what a question! It’s a simple question, the answer is not nearly so simple. In short, there are no simple startup tips, you need to know some electronics, some electrical engineering, some programming, and knowing about energy comes into it too!
“Emon” is an abbreviation for Energy Monitor. The first of these was the emonTx - energy monitor with a radio transmitter. Then came the emonPi, an energy monitor with a Raspberry Pi. That leads me to the second question - on the face of it, if you are using DIN-rail meters, you don’t want an emonPi - because you will have the ‘emon’ part in the meters themselves. Just a Raspberry Pi and an adapter to connect to your meters will serve your needs.
Now comes the tricky bit, I’m not sure where we stand with Modbus. I know there’s software to read a single Modbus meter, but although you can have many slave devices on a Modbus, I don’t know the state of our software to get the data into the RPi.
If you have our RPi software, it (“EmonCMS”) and the RPi’s operating system comes on an SD card. EmonCMS will run on anything from a Model 2B upwards, but drawing big graphs on one of those is slow. You can buy an SD card from our shop, or download the software and flash the SD card yourself. I think you’ll need the specific add-on software to read the Modbus meters. (I’ve recently been very busy with other aspects of the system, so I haven’t kept up with this side - but I know there have been some developments. Somebody else will need to fill in this area for you.)
Modbus is a protocol. Usually, the wiring between the meters will be RS-485, and you’ll need an RS-485 to USB adapter plugged into your RPi to make the connection. Like all bus systems, the wires will start at the controller (which will be the USB adapter), and connect to the first meter, from the first to the second, and from the second to the third. The way the protocol (Modbus) works is the controller asks each meter in turn for its data, and that meter responds. The meters never initiate any action, they only ever answer. The controller of course gets its instructions from the software on the RPi - which meter to ask, what to ask for, and how often, etc.
As far as your PC is concerned, the RPi is a server on your LAN and emonCMS on the RPi looks just like another website, so once you find your way around, that should be reasonably familiar. There are really two parts to emonCMS. There’s emonCMS itself, which handles the input data and the operations you can do on it (like totalling all 3 meters, storing the data in its database, drawing graphs and charts on web pages); and emonHub, which is the interface between the hardware and emonCMS. It’s emonHub that will issue the instructions to drive the Modbus.
If you want to search the forum, the words I’d suggest you start with are Modbus, Mbus (not the same, but does roughly the same job) & RS485.
I also suggest you look through - don’t under any circumstances try to take it all in, just get an idea where the various types of information are to begin with - the ‘Learn’ and ‘Resources’ sections. ‘Learn’ is the basic principles and theory, ‘Resources’ gives detailed technical data principally for the ‘emon’ hardware. I’m not sure that much of this will be relevant to you, but you should know roughly what’s there. Finally, you’ll probably need to look at the ‘Guide’ but much of this concerns setting up hardware that you don’t have - only the descriptions of emonCMS will concern you initially.