Power Generation and Distribution. 471 



provide themselves with steam engines and boilers, or other sources 

 of power, to the total capacity of the motors, would greatly exceed 

 the capital to be spent on one generating station of only 25 per cent, 

 of the total capacity comprising a small number of much larger and 

 more economical generating units. The cost of working a large 

 number of scattered small engines and boilers as compared with a 

 few large ones in one building, would also be much greater. 



3. Station Load Factor. 



In the case just cited, the working hours are the same for the 

 whole factory ; but when supplying a large number of consumers 

 having very different businesses and different working hours, the 

 duration of the average load will be extended. A generating plant, 

 capable of developing 1000 h.p., would, of course, be capable of 

 developing 24,000 h.p. hours per day, but if the aggregate load per 

 day corresponds to only 8,000 h.p. hours, or one-third of the 

 possible maximum — although the maximum power taken at some 

 time during the day was 1000 h.p. — the load factor is said to be 

 :^^ per cent. In other words, the station load factor is a fraction 

 of which the numerator is represented by the units generated in a 

 given time, and the denominator by the product of the observed 

 maximum load in Kilowatts, and the total number of hours in the 

 given period — usually one year to cover the variations of the seasons. 



The longer the machinery can be fully employed, the less, of 

 course, will be the cost per unit generated, owing to the capital 

 charges, management, and other costs, being reduced per unit, 

 pro rata with the output. 



4. Plant Load Factor. 



As a result of the diversity factor, it follows that the average 

 load on a generating station, supplying a number of power-users, 

 must be much nearer the maximum load than in the case of each 

 individual motor, and it is well-known that a steam engine lightly 

 loaded consumes considerably more steam per unit generated than 

 at full load. 



It is the practice in all generating stations to keep only so many 

 of the generating units running as will cope with the total load for 

 the time being. By this means it is frequently found possible, where 

 the station load factor is, say. 25 per cent., to keep the plant load 

 factor as high as 50 per cent., that is to say, the machinery in use is 

 on an average half-loaded. 



By increasing the number of generating units indefinitely, it 

 would, of course, be theoretically possible to approximate closely 

 to a 100 per cent, plant factor, but this would involve smaller and 

 less economical units, greater capital outlay in plant and buildings, 

 and greater attendance and repair charges. 



In practice, about five running sets, and a sixth as a stand-by, 

 is found to give economical results, though generating stations are 



