I don't think that's true for steam engines - they produce only what is demanded and scale their steam (and hence fuel) consumption accordingly. This is what makes steam an effective backup & load smoothing source, you can load up a bunch of coal in the boilers for when your laser turrets start going, but it won't get consumed as long as your nuclear/solar power is running. One of my strategies when I run out of coal is to cut the power to everything but the coal mining patch (by deleting power poles), and then let it mine enough to fill up the boiler belt.

It's nuclear that continues to burn regardless of demand. The extra energy will heat your heat pipes up to 1000C but any heat beyond that is wasted.

Also, if you play Space Age you're well accustomed to using batteries (accumulators) to smooth out power spikes. Fulgora power generation through lightning rods is incredibly bursty, and if you don't drop about 20-30 accumulators per lightning rod you will waste power and not make it through the day.

Or S-R latched heat tower with accumulator, though it's more of a hassle than it's worth.

100% efficiency is impossible.