But when the energy source is not expendible, cost of production is mostly in the construction and maintenance of equipment. To even mention cost per unit without accounting for the only real expenditures is meaningless. And there's no point in discussing the cost of the facility unless you're going to amortize it — in other words, calculate its value inside of a projected lifespan.

To say that a given method of solar harvest "costs X cents per killowatt-hour" is not logical, because the definition makes no account of the cost of the equipment divided by the probable duration of useful service.

Without using a time-frame, the discussion of cost cannot be valid.