Choice of Size and Number of Generating Units: 1. The size/rating and number of generating units in such way that they approximately match with the load curve as closely as possible. 2. In order to calculate the size of the units, the station auxiliary load should be taken in to account. 3. Also the transmission line losses should be considered. It can be approximately taken as 20 % of the consumer load. 4. The future demand and expansion should also be considered as the load on the station always increases. 5. The plant must have some reverse capacity at least 15-20 % more than M.D. under abnormal conditions. 6. Select size/rating of generating units in such way that reliability to maintain supply will be more. 7. Select size/rating of generating units in such way that the plant capacity factor, load factor diversity factor, plant use factor will be more. 8. Select size/rating of generating units in such way that unit almost run at full load or at load which gives maximum efficiency. 9. Select size/rating of generating units in such way that power generation will be economical. 10. Initial and operating cost also to be taken in to account 11. Space required also to be considered. 12. The minimum number of units should be two. 13. As far as possible, the units of equal capacities are 14. While selecting the size/rating and number of generating units there are two options i) To select single generating unit of large capacity ii) To select more numbers of small capacity generating unit either of same ratings or different ratings. Both options have its own advantages and disadvantages. 15. In summary:- Load on the power system is variable where reliability of supply is important so it is neither practicable nor economical to use a single unit of large capacity. But, if power plant is connected to grid system then generating unit of higher capacity can be installed.