The second available heater, Coal Burner outputs significantly more heat then Solar Cell does and will definitely require more then one generator to handle the heat when it`s introduced.
Given the lack of available infrastructure at the time when it is obtained, it is used in much the same way as Solar Cell. It`s up to player to decide if switching from Solar Cells to Coal Burners or even using a mix of both is called for, but generally it is an improvement in heat output and therefore, money flow. Just be sure your generators can handle it.
Grand total of heat Coal Burner furnishes without upgrades is 380*400=152000, which with the subtraction of price leaves player with 52 K of profit - if all of the heat is used constructively.
Researching Coal Burner will allow player to access Small Office.