add missing area parameter for eta_m arg

[KushalBeniwal]

• Closes pvsyst cell temp docs for eta_m arg missing "area" from definition #1151
• I am familiar with the contributing guidelines
• Pull request is nearly complete and ready for detailed review.

Before:

After:

[mikofski]

Hi @KushalBeniwal, can you check stickler? also what do you think about formatting the math using latex so it looks better? Thx!

[KushalBeniwal]

This is what it looks like now.

[cwhanse]

I don't believe that formula is correct. External module efficiency is based on total module area, including gaps between cells, rather than total cell area.

[mikofski]

@KushalBeniwal Sorry, I think we should remove the underscores. Also what would it look like if we use \frac {} {}? EG: something like this:

or more tersely:

where SUM(P_dc) is the total DC power, POA_global is the global irradiance in the plane of the array, and SUM(A_module) is the total module surface area.

@cwhanse any preference?

[cwhanse]

@cwhanse any preference?

@mikofski you aren't wrong - the DC power and area need to match. But I think it's confusing, and out of synch with the rest of pvlib, to have this function ask for a parameter based on array scale values, rather than single module values. E.g. all the methods in PVSystem and ModelChain calculate DC power for a single module. I think the cell temperature functions should do likewise.

I don't being formal with the equation adds value here. One can't use the output of the DC model as input to this temperature function (it's circular) so I wouldn't use pvlib variable names to avoid suggesting that's what is needed.

I guess I'd prefer the text formula "DC power / (POA irradiance x module area)". OK with me if you'd prefer to render that using LateX, as "P_{DC} / (G_{POA} \times Area}" and then defining each of the three terms.

Sorry @KushalBeniwal that we're having a bit of back and forth on this.

[mikofski]

Agreed. Let's drop the word "total" & keep it at the module (as in PV panel, not a Python "module") level

[KushalBeniwal]

Sorry @KushalBeniwal that we're having a bit of back and forth on this.

Not an issue.
I should have first asked for your opinion before making any changes.

[KushalBeniwal]

I tried both, so, which one looks better or should I try something else?

1. 
2. 
   Is "Area" fine or should I put {A}_module?

[mikofski]

My vote's on 1.

[cwhanse]

My vote's on 1.

Mine also.

[mikofski]

Thanks @KushalBeniwal !