For instance, most fossil combustion heaters operate at around 85%, some better(newer) some worse(older). Where as electric resitance radiant heat is 100% efficient-it delivers all the watts in heat to the air(no conduit, no loss with combustion).

Then heat pumps operate at way above 100% in efficiency as they produce heat more efficiently(varies with temps) than resistance heaters.

But another efficiency is how the energy converted to heat is distributed. That's another efficiency that runs from dismal(a long cold hot air duct or hot water pipe). A rattling combustion inefficient woodstove has a 100% distribution efficiency, if you're sitting next to it. And while you're neighbor may be choking on your smoke, you're 'carbon neutral' as long as the trees keep growing as fast as we burn them.

And none of this much to do with the various costs of the energy(Westie pays 1/3 that I do for power).

From a green standpoint, they all vary. The greenest is electric heat(or solar), supplied by your PV panels. Pretty hard to do to unless you have a super insulated, very small house, with sufficient panels to make what you burn. The up front cost is large but another subject.

Those same KW's supplied by coal fired electric generation plants are the opposite. So we have an ongoing diversity of sources to generate power, renewable-non-renewable(let's not get into that here ).

It's simply not, simple, anymore.