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67% of electricity comes from fossil fuel.

Efficiency of coal plant is ~ 33%

Natural gas is about ~40%.

So in reality you are getting 90% of ~35% or 31.5%.

There are combined cycle natural gas plants that can go up to 68% eff

That is not what it is currently for a majority of plants. Maybe there is few plants here or there, but I posted the data directly from eia.