[Question]: What should Cap_Size be set to for generators?

[NLaws]

Question

Cap_Size is used in many places in the source code, and it appears to default to 1, but the documentation only mentions it in relation to maintenance and fuel. It seems like Cap_Size should be documented in the inputs section. What are its units? When should it be specified? Should it be equal to the Existing_Cap_MW in general?

Additional context

No response

[NLaws]

@lbonaldo or @sambuddhac ☝️ ?

[cfe316]

Hi Nick,

For almost anything* it doesn't matter, you can leave it as 1 or set it to a 'realistic' value like 100 (for 100 MW), if it's a typical power plant or something. Most of the time* it's only used for internal scaling of variables; the result should cancel out. If you want to prove it to yourself, try two different values and see where it makes a difference.

It's supposed to represent the capacity of a single 'plant unit' of the resource under consideration. Where it does matter I will describe below:

1. If you're using integer unit commitment (UCommit=1). Plants can be built/retired (might need to check that part) or turned off/on only in these increments. If you're using linearized unit commitment, (UCommit=2) the value will cancel out and not be relevant to the problem, in a mathematical sense. It will affect the output in a minor way:
   in all cases, Cap_Size is used in files (start.csv, commit.csv etc) that describe how many plants for a given resource turned on / stayed on / etc during each time step. For example, to get the total number of MW that turned on in a step from start.csv you'll multiply the number in that file by Cap_Size.

2. If you're using the PiecewiseFuelUsage module for thermal plants, Cap_Size matters for those resources. I'm not familiar enough with the module to explain it without looking at the details. I would naively guess you are not using that module, though. If you are, let's explore further.

It seems like Cap_Size should be documented in the inputs section.

Fair!

What are its units?

Megawatts.

When should it be specified?

It's required to sensibly use UCommit=1, or if you're using the PiecewiseFuelUsage functionality. Otherwise it essentially just makes the output prettier.

Should it be equal to the Existing_Cap_MW in general?

No.

It can interact with Existing_Cap_MW (and the minimum and maximum capacity fields) if you're using integer unit commitment. For example if you set Min_Cap_MW==Max_Cap_MW then it had better be an integer multiple of Cap_Size or the problem will be infeasible.

[lbonaldo]

It seems like Cap_Size should be documented in the inputs section.

Thanks, @NLaws, and sorry about that. Based on your feedback, we've added Cap_size back to the input sections of the docs in the latest version.