Modbus Configuration for SMA Sunny Island

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You're right on that one thank you. I've found a new piece of code and I'm just gonna do the same thing where I talk through my thoughts.

Ok so the code i've found is from https://www.industrialshields.com/blog/arduino-industrial-1/modbus-tcp-master-with-industrial-arduino-esp32-plcs-103:

/* 
   Copyright (c) 2018 Boot&Work Corp., S.L. All rights reserved

   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU Lesser General Public License as published by
   the Free Software Foundation, either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#include <Ethernet.h>            // This is the client;
#include <ModbusTCPMaster.h>     // This is the master;

// Ethernet configuration values
uint8_t mac[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };
uint8_t ip[] = { 10, 10, 10, 3 };
uint8_t slaveIp[] = { 10, 10, 10, 4 };
uint16_t slavePort = 502;

// Define the ModbusTCPMaster object
ModbusTCPMaster master;

// Ethernet client object used to connect to the slave
EthernetClient slave;

uint32_t lastSentTime = 0UL;

////////////////////////////////////////////////////////////////////////////////////////////////////
void setup() {
  Serial.begin(9600UL);

  // Begin Ethernet
  Ethernet.begin(mac, ip);
  Serial.println(Ethernet.localIP());

  // NOTE: it is not necessary to start the modbus master object
}

////////////////////////////////////////////////////////////////////////////////////////////////////
void loop() {
  // Connect to slave if not connected
  // The ethernet connection is managed by the application, not by the library
  // In this case the connection is opened once
  if (!slave.connected()) {
    slave.stop();

    slave.connect(slaveIp, slavePort);
  }

  // Send a request every 1000ms if connected to slave
  if (slave.connected()) {
    if (millis() - lastSentTime > 1000) {
      // Send a Read Input Registers request to the slave with address 31
      // It requests for 6 registers starting at address 0
      // IMPORTANT: all read and write functions start a Modbus transmission, but they are not
      // blocking, so you can continue the program while the Modbus functions work. To check for
      // available responses, call master.available() function often.
      if (!master.readInputRegisters(slave, 31, 0, 6)) {
        // Failure treatment
      }

      lastSentTime = millis();
    }

    // Check available responses often
    if (master.isWaitingResponse()) {
      ModbusResponse response = master.available();
      if (response) {
        if (response.hasError()) {
          // Response failure treatment. You can use response.getErrorCode()
          // to get the error code.
        } else {
          // Get the input registers values from the response
          Serial.print("Input registers values: ");
          for (int i = 0; i < 6; ++i) {
            Serial.print(response.getRegister(i));
            Serial.print(',');
          }
          Serial.println();
        }
      }
    }
  }
}

Ok, few questions about the initialisation: Can't find the documentation for "ModbusTCPMaster.h" does anyone know where to find that?
I am also a bit unsure as to what the name "mac" means (I've seen this in other examples and it isn't clear to me), more for my own understanding this one, also a bit unclear what is being referenced in this and what these values mean. I understand the rest of the initialisations.

I think I mostly understand the loop also, but line 64 which is:

if (!master.readInputRegisters(slave, 31, 0, 6))

I am a bit confused on this one, I understand slave as it's the device being requested, but then 31 I am unsure on, is this the master or slaves address or something else entirely? Then 0 for me I am assuming is where I set 30845 which will be the data I want, and then 6 I think I need 2?

I think I understand the rest, does anyone see why this would not work for my intended purpose? Reading in a value on one of the slaves registers.