I'm hesitant to use optimization mode at this stage for performance reasons.

UPDATE: It weirdly seems to function in the table at the bottom in analysis view, but then in results view it gives me zero as a result when I try to use the expression Demand:Final Energy Demand[MWh, Fuel=Electricity]/8760/0.3

The most basic way to do this would be to use the PercentShare dispatch rule. Then you can simply specify that solar gets 50% in the Process Share variable. You should not need to write any complex expressions to achieve this. That variable only appears when some processes are set to use the PercentShare dispatch rule. Both these variables are specified at Transformation processes. You can actually specify different dispatch rules for different processes but I don't recommend that.

You can separately specify whether a Transformation module will include capacity data. Do that by editing the Transformation module properties screen shown here. If you do include capacity data, then you can also specify a Planning Reserve Margin (at the module branch, and then use either the Exogenous Capacity or Endogenous Capacity variables to specify capacity additions and retirements. LEAP will automatically add new plants to try and maintain the specified reserve margin based on the processes you list in the endogenous capacity screen. Note that you can also specify a variable called Capacity Credit, which determines how much each process counts toward the planning reserve margin. Baseload plants typically have a capacity credit of 100% but variable renewable processes may be much less.

Hope this helps!

Charlie