I was using an E.on supplied clip on usage meter, and never could get the supply down to zero. I even worried that the supply to our ex shop (attached to the house) was coming through the house meter before reaching the shop meter. That was proved false.
You know where this is going. We have a flat roofed ground floor carbuncle of a bathroom added by a previous owner 40 years ago, and never used by us. It still contains tea chests and stuff I stacked in there after we moved in 15 years ago. Had occasion to go in there earlier today, and happened to touch a heated towel rail, which was hot. Oh bugger, bugger. Yes, electric, and no, I have no idea how long it has been on. 15 years? Possible, but unlikely (I hope!). The master switch is outside the bathroom, next to a light switch (we rarely use), and I suppose the towel rail switch was once switched on instead of the light switch. Bugger. Probably around 500 watts? Dare not work out how many units per annum.