How to calculate the difference in KWH values received every 5 min from modbus data and save it as a new values in csv

[Colin]

I don't know whether it has already been suggested to you but if you haven't already done it then work right through the Node Red Essentials videos that are linked from the docs page. A couple of hours spent doing that will save all of us time in the long run.
When you have done that should see how you can use node-red-contrib-delta to give the difference. If not then show us what you have tried.