Check your residuals. A third degree polynomial doesn’t look particularly appropriate here.

As I said below another post you made, don't do a regression if you don't have a model in mind. It may just be hypothetical, but you must have an explanation as to why you chose this regression in particular, other than "it fits pretty well". A 100th degree polynomial function will fit better, a Ngh degree polynomial, with N the number of points, will fit perfectly.

Also, the problem you have here is that you have "positive" outliers but you don't have negative outliers for the lowest values, because energy production can't go below 0. So you have a regression which is higher than the truth. You should find a way to identfy and eliminate these outliers.

And if you can't that's not a problem! We don't need a regression all the time. We see the relationship pretty well, the red line is not needed. It just shows a model which is obviously wrong for many reasons.

Thanks! Like I said, "dumb question" but I had to ask :)