206. Save in so far as any Federal law may otherwise provide, no law of a unit shall impose, or authorise the imposition of, a tar on the consumption or sale of electricity (whether produced by a Government or other person) which is–

(a) consumed by the Federal Government, or sold to the Federal Government for consumption by that Government; or

(b) consumed in the construction, maintenance or operation of a Federal railway by the Federal Government or a railway company operating that railway, sold to that Government or any such railway company for consumption in the construction, maintenance or operation of a Federal railway;

and any such law imposing, or authorising the imposition of, a tax on the sale of electricity shall secure that the price of electricity sold to the Federal Government for consumption by that Government, or to the Federal Government or any such railway company as aforesaid for consumption in the construction, maintenance, or operation of a Federal railway, shall be less by the amount of the tax than the price charged to other consumers of a substantial quantity of electricity.