The steam power system is a little odd.
Talking specifically about boilers and steam engines/turbines in this mod. You should upgrade steam engines before boilers, or both at the same time, because it is based on steam temperature.
If you put lower temperature steam in a steam engine than what it expects, it will produce less power, proportional to the difference, minus starting temperature. So T1 steam in a T2 steam engine will produce half the listed power, T1 steam in T3 steam engine will produce 1/3 power. or T2 steam in a T3 steam engine will produce 2/3 power, etc.
If you put higher temperature steam in a steam engine than what it expects, it will still only produce the listed value, but consume all the steam.
So T2 steam in a T1 steam engine will give you the listed value on the steam engine. Do not do this, you're wasting the energy it took to produce that steam.
When you take other steam powered things into account, like the steam powered assembling machine in Bob's Assembling machines mod, or the steam powered miner in Bob's Mining mod, they assume T5 steam, so if you use T1 steam, you only get 1/5 of the listed speed.
