My point of view :
An operation is said to be "profitable" when the gains are greater than the costs. This is the basic definition, used by manufacturers to define whether a product is profitable, or in other words profitable.
For an individual, the notion of profitable has another meaning: in general, it is the fact that the chosen operation (purchase of a car) generates less expenditure (compared to a reference value).
For example, an investment is often said to be profitable when it exceeds the reference that is for example booklet A.
It should be noted that in the context of the purchase of a car, the fact of having to buy the car is acquired. The gain (mobility) is considered necessary, the question of investment no longer arises.
Therefore, only the question of "profitability" arises, namely which purchase will minimize the expenses compared to a typical car.
This question is often not asked because the gain (mobility) amply justifies the investment.
As part of buying solar panels, the only thing that can push the investment is pure financial profitability. The gain (autonomy and renewable energy production) is very rarely preconceived as a sufficient reason to justify a solar panel project.
The project is therefore for him only the financial profitability.
So the whole problem revolves around what value is associated with the gains and the costs. As long as the safeguarding of the planet will not be seen as an objective alone justifying investments (we will come to it perhaps), the only gain estimated for the majority will be counted only in € ....
In short I probably do not teach you much but it's always interesting to formalize this kind of concepts