15 U.S.C. § 391

No State, or political subdivision thereof, may impose or assess a tax on or with respect to the generation or transmission of electricity which discriminates against out-of-State manufacturers, producers, wholesalers, retailers, or consumers of that electricity. For purposes of this section a tax is discriminatory if it results, either directly or indirectly, in a greater tax burden on electricity which is generated and transmitted in interstate commerce than on electricity which is generated and transmitted in intrastate commerce.