Useful energy (kWh or MJ)
The final amount of heat or work for which fuels were used; the actual demand for energy and the purpose for its use. Useful energy is the product of delivered energy by the efficiency of the appliance used for the task.

Useful (solar or internal) gains (kWh or MJ)
The amount of useful heat contributed by solar or internal heat gains to help meet a building's heating load.

U-value (or thermal transmittance) (W/(m²K))
A thermal conductance that includes the thermal exchanges at surfaces and in cavities; the reciprocal of the sum of the thermal resistances of constituent layers of a composite element plus the surface resistances at the external and internal surfaces. The U-value decreases with thermal insulation.