When we do hit whatever +1.5C or +2C, Q will be 0. The amount of energy input to the earth system isn't changing - that's defined by the temperature of the sun,

When we do hit whatever +1.5C or +2C, Q will be 0. The amount of energy input to the earth system isn't changing - that's defined by the temperature of the sun, it's the output that is decreasing. What your energy look describes is dT/dt, how quickly we will reach the new equilibrium. At the new equilibrium, the temperature is likely still to be the best descriptor of the new normal - we describe most processes and states in terms of temperatures they happen at, because it's and intensive property rather than an extensive one. You don't have to temper this 300 TW number by the mass or surface area of the earth to get a sense of what's happening when you use temperature What's the energy number at which a glacier will melt? What's the temperature? ∆T is one of the most important numbers in thermodynamics for good reason