If you’re planning to use mathematical optimization, Julia and JuMP are great for this, and there is a wealth of experience and packages to draw on.
Some starters:
Packages:
- https://github.com/NREL-SIIP/PowerSimulations.jl
- GitHub - lanl-ansi/PowerModels.jl: A Julia/JuMP Package for Power Network Optimization
- https://github.com/odow/SDDP.jl
“Energy planning optimization” is a bit broad, so if you had more specifics on what you were planning to do, others might have more specific advice.