I appreciate that "alternative energy" is common US usage, compared to the UK say, where "renewable energy" is more common; and that the predominant language of the SE sites is US English.
But I'd like to nominate "renewable-energy" to be the primary tag, with "alternative-energy" as a secondary synonym, for the following reason:
As Wikipedia says (not because I consider Wikipedia a reliable source, but because in this case, I think it's right, and it says it quite well):
Alternative energy is an umbrella term that refers to any source of usable energy intended to replace fuel sources without the undesired consequences of the replaced fuels.
The term "alternative" presupposes a set of undesirable energy technologies against which "alternative energies" are contrasted. As such, the list of energy technologies excluded is an indicator of which problems the alternative technologies are intended to address. Controversies regarding dominant sources of energy and their alternatives have a long history. The nature of what was regarded alternative energy sources has changed considerably over time, and today, because of the variety of energy choices and differing goals of their advocates, defining some energy types as "alternative" is highly controversial.
In contrast, "renewable energy" is reasonably well-defined, constant over time, and its definition is, relative to "alternative energy", less controversial. So, it's clearer, more descriptive, and is less sensitive to the passage of time and changes in context.
I didn't know whether it's good etiquette here just to go ahead and create the tag and then discuss, or raise it here first. I've gone for the latter.