This is a feature of OpenEVSE. You just need to get your solar output in watts to MQTT. OpenEVSE will subscribe and follow along.
@openevse, will it actually use up all the solar generation? Or will it wait untill other users for eg battery and household appliances are finished charging/being used?
It depends on the value that your energy monitoring system publishes.
OpenEVSE just tracks to the number of watts published. If you say 2000w is available, then OpenEVSE uses 2000 watts.
If your energy monitoring system just measures solar and publishes, then OpenEVSE will use all the solar regardless of household load.
If your system measures both solar and load and sends solar - load = available, then OpenEVSE will use available.
Here is some example code: