Well I’ve had one of these basic plug in types for a couple of years now. Courtesy of Maplin , Kingston. (Right hand image.)  It’s served me well and helped me sample the power consumption of a number of appliances around the home. Mindful of wanting slightly better resolution I’ve recently purchased an Efergy Plug In power meter. Whilst it doesn’t have the power factor readings of the Maplin device it has 0.1W resolution and what appears to be decent accuracy even down at the 1w measurement level. I haven’t checked at the higher end but for me that would be of less relevance any way. So both recommended.  Watch out as sometimes Maplin offers their unit on a sale price basis. The Efergy unit is available for about £12, the Maplin I think around £20.