A single supply may be for a combination of household, business and non-business uses. Although, in principle, each portion of that supply is taxable in its own right, it is acknowledged that, in many cases, it may not be possible to determine the quantity of the supply for each use. Provision is made for an alternative procedure for determining the units to be taxed at each rate.
Most mixed supplies involve household use; this means use in a dwelling other than for the purposes of any business activity in such dwelling. It may be assumed that, on average, one unit per month of a household or business supply is for household use. It may also be assumed that, in the case of a mixed supply for business and (taxable) non-business uses, one unit per month is supplied for business use and is taxable at the lower rate.