Why can't I bank a solar generated summer kWh to use in the winter?

Yeah, it seems like one would have to either store it physically (for which the battery technology doesn't exist. Or sell it and buy it again in winter, where you lose some money as the energy is worth more in the winter.

It's an interesting blog though, I switched to Octopus Energy a few years ago and it's been the best electricity company I've ever had, far better than the others here in Spain.