The cost of supplying electricity in a grid is minimized in a nonlinear optimization framework The formulation of the problem gives the incentive-compatible trading prices of electricity among many independently operating state utilities. The problem is solved both for a central planner (central grid operator) and for the case of a slate planner (independent state utility). It is proved that there exist trading prices that are compatible with both. Finally, as an example, the theorem is applied to the case of the Indian National Grid.