Why do the WattNode integer energy registers show huge, sudden jumps in energy?

This happens when your Master is misinterpreting the least significant 16-bit register of the 32-bit energy register as the most significant one. You need to reconfigure your Master to interpret the first 16-bit register of the energy register pair as the least significant one. This is usually done with a selection called “Swapped Long”, “Swapped 32-bit Integer” or “Reversed Long”. If that doesn’t work then try handling it the other way by selecting just “Long” or “32-bit Integer”. Also see: 32-bit Energy Register Details for the Modbus WattNode.