George (dicegeorge) expressed precisely :
I've read here that clamp meters are about 5% accurate,
so if I bought one like this for £20 (+vat+postage)
http://www.tlc-direct.co.uk/Manufact...Test_Meters_7/

0.1amp = 24 watts
so could I have one before the Consumer Unit
and use it to check if anyone's left lights on,
or got a electric fire on etc?


http://www.tlc-direct.co.uk/Manufact...Test_Meters_7/
Digital Clamp Test Meter


Data hold and peak hold functions

It actually says +/- 2% on the current range, which is more than
accurate enough to decide whether a light(s) have been left on. There
are though better ways - look on ebay for a gadget which provides a
wattage read out remotely. You clamp it around a cable, then a remote
wireless display displays the wattage being used constantly.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk