Ok, let's do it that way then. Here after a few theory, then you'll see if it still fits your time frame and possible efforts.
Since you're a hardware guy, you certainly have an oscilloscope available, it may be necessary later on.
I also consider you're familiar with UART and asynchronous serial communications, which apply to RS485, from a logical point of view.
In your project, that's not only RS485, it is Modbus over RS485, or Modbus RTU for the correct naming. This is a Master-Slave communication model, where the meter is the slave and the master must be unique (in your case and most of the time, but it's possible to build multi-master ;-)).
So, only one master on the line, in the context of your application. And here we hit a first problem/question: If you have a meter installed, probably it is already used, thus there is already a master on the line... You cannot interfere with it, especially by adding your own master!
No worries, since you intent to act on low level, then you still can spy silently what is going on the line without acting yourself as a master. Just spy, recognize the frame that is of interest for you, and you're (nearly) done!
Well, recognizing the frame is just detecting the start/end according to the correct framing, check the CRC checksum, recompose the bytes in the correct order to finally get the power expressed as a multi-bytes floating point or integer value. (No worries, all this will look simpler once you'll see the simple structure of frames.)
But if the meter is there just because you had to buy it and is not used, for example you have no monitoring of the power flow using it, no export limit or the like that uses the RS485 line, then you have to play the role of master from your microcontroller. It means you have to formulate the requests to the slave (meter), so that the slave can reply to you, then you process the reply. There is probably some Arduino library available to do a part of the Modbus job. (By opposition to spying, where you'll need to go low level yourself, since no library can be relevant there.)
The request structure is very simple too, the most difficult is just the CRC calculation, but there again you'll certainly find some Arduino library, it is a common CRC calculation.
That's all for the theoretical/simple principle, now some of the many subtleties, per level...
RS485 lines are, in theory, quite simple. This is just a twisted pair, differential, and you may/should/have to add or not add some resistors (pull-up, pull-down, terminator) to both balance the line and ensure that relative levels are in the range of the RS485 specification. Seems ok, right? Well, indeed when you reach that stable state where everything if fine, RS485 can bring your signal on long distances... Not your interest here, but just to mention that it CAN be strong, if properly tuned.
In practice, especially for DIY stuff, it's often a nightmare to setup, depending on the line, devices, resistors, environment and, most important than all others, a common valid reference. It must be common to both ends, or you necessarily get out of the valid RS485 ranges at some point, possibly randomly. => This is where the oscilloscope may help you to check if the signal is clean or not.
Modbus is not really a concern once RS485 is stable. It is just a "standard" way of communicating on top of RS485 (or another media). That way of communicating is based on "registers" that are "mapped" and can be either read-only or read-write. Typically, in your project you want to know which register you have to read to get the active power. This is documented in the "manual" document you shared. There, you'll find the map of registers, that is what information you can find in which register address. Also, the WattNode implements both its own register map and also another one, standardized as "SunSpec". Typically you'll find the "active power" is spread on 1 or 2 registers, depending which format you want.
If your are the master, just send a request for that registers to the meter, it will then send you the content of those registers. You'll just have to order the bytes appropriately to get the active power value.
Ok, how to formulate the request? If you use a library, look at "read holding register" (function 0x03) in its documentation. If you write your own code to build and send that request, than the typical format is:
<slave address>[1 byte] <function code>[1 byte] <first register address>[2 bytes] <count of registers>[1 byte] <CRC>[2 bytes]
NOTE: The master as "no address", because it doesn't need one, since it is the only one allowed to request something. The slaves reply with their address, for convenience and redundant check.
So, with this you have a global view. Now the deal will be different according to what library you can find to accomplish a part of the job. And if you do everything on your own, well it may take time but yes it is "funny".
I hope to publish my C++/linux software tomorrow evening (CET timezone), I'll share the link here when available. You'll find some reference code that may help you do the same or similar with an Arduino.
