Can I add a net power used field to the grid?

I have all the usual stuff populated, but ideally I would like a field that I can tell at a glance if my houses power needs are being taken up by the solar or not. I.E, a "Net Used’ column that is simply a calculation of (Power Used - Power). Mostly in the day it would be negative, and in the night it would be positive.

I have tried to create an extended v7 as a calculation rule of v7 = v4 - v2 but i dont understand what I am doing wrong there as the values returned makes little sense.

Just use the “Net View” to show net power and energy.