It's the fundamental problem that needs solving in order to achieve what this announcement claims is the ultimate end goal of all this - a "net-zero carbon economy" for the country as a whole - and it has been the fundamental obstacle to achieving this for decades now. So long as this problem remains unsolved, no amount of supposedly zero energy homes will ever get the country as a whole to that goal because they're still reliant on selling excess energy to the remaining "non zero energy" parts of the country in the summer and buying energy back from fossil fuel plants in the winter.