How accurate are these? Just bought one - it's one with a magnetic clip which goes around the live load cable coming out of the meter. It seems to read about 20-30% under at low usage (500W) compared to the meter, but more accurate at higher usage (2-9kW). I've tried another one which gives the same results. Could I have a faulty meter? Or do these things usually read under at low usage?
TIA
-- Andy