I don't know if it is in the 1.4.7 versions, but King Lemming said you only pay the 5% thermal expansion penalty once in the network. I think he said you pay it at the engine -- so if you use his engine's you don't pay 5% at all.
What I've read (not tested, so keep that in mind) is that you pay 5% at every source of MJ. So every engine runs at a 5% penalty, basically. My reasoning is that the energy cell is a source of MJ, therefore you pay 5% again. Is that wrong? Beats me, I'm guessing.
Also, what does "so if you use his engine's you don't pay 5% at all" mean? It was my understanding that the redstone conduit applies the penalty - why would the type of engine matter? And if it does, are you saying that engines specifically from a mod King Lemming has done are exempt from this? I'm skeptical, does anything back this up?
To automate on demand engines buildcraft gates can be effective. I usually use the 'full energy' setting and stop engines when a redstone energy cell is full. Doesn't work reliable on just conduit, so you will need to buff the input/output through energy cells to read them with a gate. So if you have the boiler always running the energy cell and it keeps it at 100% no extra engines will start. If the energy cell dips the engines can be triggered to start off the gate signal.
So you've done this and it works pretty well? I'm asking based on
what I read about how gates interact with the cells. I'm not sure I'm smart enough with the tools to build the workaround mentioned and even then it sounds like a poor way to automate things.
You need to pair engines that are good at start/stop to fill this on demand roll -- don't use hobbyist steam
I was planning on setting up huge biofuel tanks and 20 combustion engines in 5 banks that can be fired up sequentially as the need grows. If that seems to work, it shouldn't be hard to expand it with more banks of engines if it seems like my need has grown.