It's from Tchibo and costs just £9.99
It gives read outs of voltage, current, watts and cost
AND you can set current limits on it so that it beeps if the limit is exceeded.
VERY useful when on the low current hook-ups abroad so that you don't trip the CB's in their hook-up box. Much cheaper than the fancy Reich Fuse-Control.
I'm going to hard-wire it in, next to the Hobby electrics box in the wardrobe; between the RCCB and the MCB's.
Some have suggested wiring it in between the mains inlet socket and the caravan mains 'consumer unit'; easier done perhaps, but then it's not covered by the 'van's RCCB (if you've got one!!)

