It is possible to do it "the hard way". In 2012/2013 I consulted on power consumption measurement of one communications app where separate measuring jigs were b

It is possible to do it "the hard way". In 2012/2013 I consulted on power consumption measurement of one communications app where separate measuring jigs were built for iPhone 5. Little bit of custom electronics and automation made wonders -- IIRC the app shipped with 4x lower power consumption than initially measured. There are really no barriers to doing this the same way today for web apps. On platforms where you have a good access to the battery monitor and high res RTC, you may even get along without extra hardware.