Question

An electric kettle rated accurately at 2.5kw is used to heat 3kg of water from 15% to boiling point it takes 9.5 minute. Then the amount of heat that has been lost is

Answer 1

power=heat/time=3kg*4.18kJ/kg*(100-15)/9.5*60

so power into water then is 3*4.18*85/9.5*60=1.87 kw

so the power lost is 2.5-1.87 kw
and heat lost=powerlost*9.5*60 kiloJoules

Answer 2

The temperature must be in deg C if using water's specific heat capacity of 4.18 kJ/(kg*C).

Heat absorbed by water
= 3kg * 4.18 kJ/(kg*C) * (212 C - 15 C)
= 2470.38 kJ

Heat created by 2.5 kW kettle in 9.5 minutes
= 2.5 kJ/s * (9.5 min * 60 s/min)
= 1425 kJ

This question doesn't make sense then, because the kettle can't even produce enough heat for what the water requires to boil it.