(6) Auxiliary equipment. Solar energy property does not include equipment (auxiliary equipment), such as furnaces and hot water heaters, that use a source of power other than solar or wind energy to provide usable energy. Solar energy property does include equipment, such as ducts and hot water tanks, which is utilized by both auxiliary equipment and solar energy equipment (dual use equipment). Such equipment is solar energy property (i) only if its use of energy from sources other than solar energy does not exceed 25 percent of its total energy input in an annual measuring period and (ii) only to the extent of its basis of cost allocable to its use of solar or wind energy during an annual measuring period. An annual measuring period for an item of dual use equipment is the 365 day period beginning with the day it is placed in service or a 365 day period beginning the day after the last day of the immediately preceding annual measuring period. The allocation of energy use required for purposes of paragraphs (d)(6) (i) and (ii) of this section may be made by comparing, on a Btu basis, energy input to dual use equipment from solar energy with energy input from other sources. However, the Commissioner may accept any other method that, in his opinon, accurately establishes the relative annual use by dual use equipment of solar energy and energy derived from other sources.