Errr, assuming that your electricity comes from a combustion source and a simple cycle power plant, at _best_ the work derived from the heat is about 35%. You need to account for that in your 80-90% calculation. (That is, .9 * .35 = .315 using the upper range of your guesstimate.)

In other words, the waste heat has already been lost to the environment at the power generation source in that case.

As always "it depends"... (In this case on the power generation mix in your local grid)

The specific mix depends on the local grid, but nobody in the US gets electricity entirely from combustion, unless you're in like an Alaskan village that only has a generator.