226 ELEMENTS OF ELECTRICAL ENGINEERING. 



Curve K shows the cost of attendance of electrical equipment 

 in addition to attendance of steam plant. 



Curve L shows the cost of oil and supplies for electrical equip- 

 ment. 



Curve M shows total cost per kilowatt-year of electrical out- 

 put ; and 



Curve N shows the total cost per kilowatt-hour of electrical 

 output. 



Cost of electrical power delivered to the consumer. The curves 

 in Fig. 141 give a general idea of the cost of electrical power at 

 the switchboard on the assumption of unity load factor. The cost 

 of electrical power delivered to the consumers is greater than the 

 cost at the switchboard by an amount sufficient to cover the cost 

 of the following items : (a) The power lost in the distributing 

 system ; (b) interest on the cost of the distributing system ; (c) 

 depreciation and repairs on the distributing system ; and ( d) 

 taxes. The sum of these items for a whole year divided by the 

 number of kilowatt-hours delivered per year often amounts to 

 more than the original cost per kilowatt-hour at the switchboard, 

 especially where the consumers are scattered over a large district 

 or where current is distributed through costly underground cables. 



98. The load factor of a station and its influence upon the cost 

 of power. In most electric stations the demand for current 

 fluctuates greatly, so that a station which is designed to deliver 

 the maximum amount of current demanded is operated for a 

 large portion of the time at a fraction of its rated capacity. 



Fig. 142 * shows the daily load curves of an electric light- 

 ing station in Christiania, Norway. The upper curve AA is the 

 load curve for midwinter when the days are extremely short and 

 dark, the lower curve BB is for midsummer when the days are 

 very long, and the dotted curve CC is the mean daily load curve 

 for the whole year. The difference between the winter and sum- 



* Taken from an article by Norberg-Schulz, Electrotechnische Zeitschrift, October 

 5, 1905, pp. 919-921. 



