228 ELEMENTS OF ELECTRICAL ENGINEERING. 



1,040 kilowatt plant, its load factor during midwinter would be 

 equal to the area under the curve AA divided by the area under 

 the dotted line DD, and its load factor during midsummer would 

 be equal to the area under the curve BB divided by the area 

 under the dotted line DD. 



When a station is operated at light load the item of wages may 

 be somewhat reduced, especially if the station is shut down dur- 

 ing certain hours each day ; also the item of fuel is reduced, but 

 neither of these items is reduced in proportion to the reduction 

 of load. On the other hand the fixed charges, that is, the in- 

 terest on the investment, depreciation, repairs, and taxes are ap- 

 proximately constant. Therefore a reduction in the output of a 

 station does not reduce the operation costs very much, so that the 

 cost per kilowatt-hour increases greatly with decrease of load. 



The influence of load factor upon the cost of power can be 

 estimated in a given case when we know how many hours each 

 day the station is to be shut down, how great a reduction will 

 be effected in the item of wages, and how great the increase 

 of coal consumption per kilowatt-hour will be. The following 

 table,* although based upon rather old data, will serve as an 

 illustration of the decreasing cost per kilowatt-hour with increas- 

 ing load factor. 



Example. Consider a 1 ,000 horse-power electric lighting sta- 

 tion which, with its distributing system, represents a total invest- 

 ment of $150,000, and the equipment of which includes four 250 

 horse-power engines each driving a 160 kilowatt generator. 



The fixed charges (interest, depreciation, repairs, insurance, 

 and taxes), estimated in round numbers as 14 per cent, on the 

 total investment, are $21,000 per year. 



*From the Electrical World, Vol. 24, p. 120, Aug. II, 1894. A very complete 

 table of costs of electric power at the switchboard, showing a wide range of load 

 factors, is given by R. W. Conant in a paper entitled The Cost at the Switchboard of 

 Electric Power for Street Railways, Electrical World, Vol. 32, pp. 313-319, Sep- 

 tember 24, 1898. See also Cost of Electric Railway Power by A. S. Richey, 

 Street Railway Journal, Vol. 25, pp. 126-128, Jan. 21, 1905, and Vol. 18, p. 827, 

 Dec. 7, 1901 ; and Pittsburg Street Railway System, Street Ry. Rev., Vol. 13, pp. 

 185-198, April, 1903. 



