[idanoeman]

Hold on, coal plants are already only ~35% efficient at converting heat into electricity. Subtract 30% and you're left with 5%, not 70%.

[Sacho]

Err... that's not how an efficiency rate works.

If it takes you 30% of the initial energy to get to the heat->electricity phase, then the heat->electricity phase receives 70% of the initial energy. From that point, a 35% efficient transfer means 24.5% efficiency from the initial energy.