Sure, hopefully this is what you are asking for:
In EmonCMS, go to Setup → Inputs and click the little spanner to add the circled part to your solar production (assuming your solar generation reads power in W or kW, not energy Wh or kWh) to get that Feed.
In EmonCMS, I have 3 power inputs: solar production, grid power (this one goes negative for export) and a house fuse-box (or consumer unit) consumption. Not directly measured is outdoor EV charging, but it can be easily calculated.
When you add a process, the blue description text box below tells you the output from that process, the output is then fed into the next process, this way you can do maths or logic. Very elegant solution to provide huge amount of flexibility, well done whoever designed it.