For most of my life we have had a solar hot water system. Now this is great for the environment and most of the time works fine - no problems with hot water. Except for overcast days. Like today. Now we also have offpeak hotwater electricity for days like this - so you can flick a switch on those days but you will only get power to heat up the water during off peak hours i.e. in the evening.
So this evening I started pouring myself a bath but no and behold no hot water and the switch hadn't been flicked. So now I have to wait for half an hour or so. And I love really really hot baths.
Silly hot water system
picture from http://www.emc.cmich.edu