Does it pay the average coal mine to purchase central station power?

In the application of central station power to coal mines the successful operator of today is sometimes at a loss to know just what saving will be effected, for the following reasons: first, he believes that he is producing power at a fairly low cost, due to cheap fuel, simple apparatus and low cost of buildings; second, hot knowing approximately his costs in detail, he is at first unable to see his saving, if any, as compared with a definite rate per kw-hr. for central station power; third, the use of central station energy often requires the purchasing of new apparatus, and the selling of the present generating apparatus under unfavorable conditions.