I tried to connect the power plant to different liquids, but none of the fuels gives maximum energy output. Which fuel is most efficient?

you sure? i use all 4 and available power is alaways max.

It's very strange, either I was looking somewhere wrong, or it worked on the second try as you describe :)

The fluid with the most fuel_value is petroleum-gas. The power window shows just how much thte machines are contributing to the overall demand. If you want specific infos about a machine just hover over it. There is a green bar showing its current output.

This seems to be a pretty frequent question and i think i will change the output priority so that it will run even if there are full accumulators.
Just use a pump when you plan to use them as a backup.