While larger conductors lose less energy because of their lower electrical resistance, they cost more than smaller conductors. An optimization rule called "Kelvin's Law" states that the optimum size of conductor for a line is found when the cost of the energy wasted in a smaller conductor is equal to the annual interest paid on that additional cost of the line construction for a larger conductor. The optimization problem is made more complex by additional factors such as varying annual load, varying cost of installation, and the discrete sizes of cable that are commonly made.