Philadelphia electric company´s system

Maximum system economy is attained when the sum of operating costs plus fixed charges on investment is a minimum. Considerations of reliability usually lead to increased capital investment for sturdier facilities and for spare equipment, and consequently add to the fixed charge burden. Fortunately, however, operating costs are to some extent inversely proportional to fixed charges, and analyses of system performance often will reveal operating savings which can result from judicious use of investment in spare facilities.