They came with a compass. Which way’s north? Ah, thought so. Do you want it on the front, or the side? I don’t mind… front’s better — I’ll be setting a good example. Behold: a green geek.

The ladder went up. The collector went up and got connected. The old tank got ripped out and taken away. Some rummaging inside the roof. New tank in the old one’s place. It even looks more stylish. (Evidently 90s tanks were square and boxy; this one’s smooth and round.)

The way it works is pretty simple. An electric pump sends the water into the roof, so it goes through the collector and heats up as it goes, before being stored in the tank. When you turn on the tap, the system either gives the water additional heat from the gas if it’s not warm enough, or mixes it with cold water if it’s too hot (which apparently is common on sunny days).

The installation wasn’t entirely without drama. After the two guys had gone, the electrician turned up to install the new power point for the pump. He asked me if I had a shifter.

A shifter? I, and I realise I’m showing my mechanical ineptitude here, didn’t know that he was talking about, and said no. He kept asking, with increasing disbelief that I wouldn’t have such a device.

It eventually became apparent that he spotted a dripping water connection, and what he was asking for was an adjustable spanner. Apparently they’re really called shifters. How did I live on this earth for 37 years without knowing that? He did some fiddling, and all was good.

So now I have (almost) free hot water, at least until the sun fades away into a white dwarf, a few billion years from now.