Controller managing monthly energy production (kWh) using power (kW) as the set point

Hi, I am currently looking to develop a controller whose purpose is to limit the energy production of a plant over a period of 12 months, which is calculated at the end of each month. This task has historically been managed manually, however I would like to try and automate this process.

While instinctively I initially considered the use of a PI controller, with:
PV = live energy meter data (kWh) read from the revenue meter
SP = monthly energy data (kWh) taken from the database, plus the required 12 month energy limit for the site

Digging a little deeper into the concept, I realised that as:
- the solar plant has a known issue where it must have a minimum power set point to solar plant, and
- the process variable is automatically reducing over time, by virtue of the solar plant producing power during sunlight hours

I am unsure how to conceptualise a controller, whether it is PI controller, or otherwise.

Could anyone prompt me with some ideas as to how I can progress this further.

Thank you all in advance.