A pal of mine is building up his house, and he’s determined to install both solar heat collectors and photovoltaic panels on the roof. He asked me for advice and, talking about that, he came up with a horrific idea that made me shiver with unease by his tone of conviction “well…I’ll have electricity and hot water from 100% renewable power: I can forget at last about turning off the lights or caring about closing the hot water tap [… ]” (I cut the rest for the sake of politeness).