I want my steam engine to turn on in just two cases:
1) complete blackout due to full discharge
2) network surge/overload
This one does just that, without any energy bleed.
The inserters for my main steam engine are connected to separate network, powered by single "controller" engine.
Controler engine is out of fuel, thus inserters for my main engine are not powered, keeping it idle too.
There is a box with fuel (I use wood, because coal has too much energy for this purpose) and two inserters, powered from the main network. First inserter puts a piece of fuel on the belt, second one puts it back in the box.
In case of any problems with the main network second inserter starts skipping pieces which are already on the belt.
Controller engine springs to life and powers inserters for my main power plant, bringing it to full power in the matter of seconds.
I use two radars as a ballast to turn off my main power plant faster. Without any load single piece of wood can keep it powered for days.
Click to open 2880x1800

Update: Check this thread for clean build: My take on using steam as a backup. Works perfectly for me.