There is no sane universe in which we lie about how fast we can do something in order to account for differences in how long we can do it. If you have two models identical save for one having twice the battery we don't account for this by pretending the lesser endowed unit is actually only half as fast.

While these things are interrelated you actually want both pieces of data and you don't want them complected by using them to forge some number made entirely useless and incomparable by incorporating both facets in one useless whole.