TVA's costs to generate electricity vary – wholesale lots both by the time of year and the time of day. During the peak periods when consumers use more dropshipping electricity, TVA has to bring its higher cost power plants into service.
If consumers could lower their electric usage during peak periods, TVA would save the money spent on those wholesale power plants that are needed only for peak dropshipping periods. Customers would be rewarded for those savings in the form of wholesale lower rates over time.