12Standard method of charge
(1)Subject to sections 13 and 14 below, a public gas supplier shall charge for gas supplied by him according to the number of therms supplied, that number being calculated in the prescribed manner on the basis of the declared calorific value of the gas.
(2)In this Part—
" calorific value ", in relation to any gas, means the number of megajoules (gross) which would be produced by the combustion of one cubic metre of the gas measured at a temperature of 15 °C and a pressure of 1013.25 millibars and, if the Secretary of State so determines, containing such an amount of water vapour as is specified in the determination;
" declared calorific value ", in relation to any gas supplied by a public gas supplier, means calorific value declared by the supplier in accordance with regulations under subsection (3) below.
(3)Regulations shall make provision—
(a)as to the time when, and the manner in which, the calorific value of gas supplied by a public gas supplier is to be declared, and is to be brought to the notice of consumers;
(b)as to the time when any such declaration is to take effect; and
(c)for the adjustment of charges for gas in cases where an alteration of declared calorific value occurs in the course of a period for which such charges are made.