£9bn due to not having built more cheap onshore wind, £5bn due to poorly insulated homes, £5bn due to low solar deployment, £3bn because new homes were built less efficient.
My parents 1741 house with new double glazing is like that - cool in summer, hot in winter without even needing the heating
I’m amazed that we collectively forgot how to build houses but I guess 2ft thick walls make a dent in the size of the living space and you’ve got to build a box with paper thin walls and no garden to optimise that