ilepisch, it is because the heat/fuel production is exponential, meaning rather then 2 cells meaning 2x production, 2 cells is to the power of 2 production, and four is to the power of four, which mean 4 x 4 x 4 x 4, not 4 x 4.
Here are some tips if you are just starting out:
DO NOT USE YOUR TIME TRAVEL TICKET. this is a mistake you will often make. it will run 3 real life day instantly. if you use it when your income is $25 a tick you will not make nearly as much money if you instead use it when your income is $1000 per tick.
Do not be afraid to start selling steel even if you don't have a high enough income to prevent your money from going down at first. the steel will more than make up for it because of how much it sells for.
For anyone new to this game the dev is no longer updating the game. i played this game on a different account but i decided to start over. i fell like progression for the game is really slow especially in the further stages of the game. i get that it's an idle game but i shouldn't have to have the game running for weeks on end with another 5 percent of progress towards the next component. but like i said the dev is no longer on kongregate and hasn't updated the game for almost 4 years.
 "Reactor Redux"){kind=icon}
 "Factory idle"){kind=icon}