Factory drains 4 mass/sec, most of construction like factories, pgens drain 4 mass/s per 5 BP (1 engie).
In regards of energy, building factories and pgens is expesive, Air factory drains a lot of e, Land factories drains only 20.
In more Detail:
You need 2 mexes + 1 pgen to run t1 factory. You need 1 mex + 4 pgens to run Air fac. You need 2 mex + 2 pgens to build factory per 5 BP (1 engie), 1.5 mex and 1.5 pgen to build a pgen per 5 BP ( 1 engie)
Everything else is too complex for human brain to calculate in real time during the game and so is done by experience by playing hundreds of games.