We've covered Powerline (Homeplug) adaptors previously (see http://ctacs.weebly.com/1/category/powerline/1.html), but things have changed a bit since then. Firstly, they have come down in price and it is now possible to buy a pair of 200 Mbps adaptors for as little as £19.99, or a pair of supposed 500 Mbps ones for around £40. Secondly, new wireless routers (hubs) often just don't seem to be as good as they used to. You'd be surprised at the number of times we hear people saying things like: "My internet used to be okay but since BT upgraded me to Infinity it's been a lot worse!". In fact, what this usually means is that the raw internet speed is indeed better, but the range of the new super-duper BT HomeHub is so poor that they can't enjoy it (a replacement router from Apple or Billion usually fixes this complaint). These two factors have led to a big growth in the market for powerline adaptors, such that far from being the technology of last resort they are now an incredibly useful resource in many situations.
But the issues of speed and performance as quoted by manufacturers remain deliberately opaque. As our original article revealed, the quoted speeds are duplex, meaning they are the total simultaneous speed of both directions. In other words, if an adaptor is rated at 200 Mbps then this really means 100 Mbps in each direction. Inefficiencies in the mains wiring system and the distance between the adaptors means that the rated speed can never be approached, must less reached. Under good circumstances you might achieve up to half of the rated speed, but it could be as little as 10%.
Which brings us back to the original question. Suppose a 500 Mbit adaptor was capable at running at more than 100 Mbit/sec in the real world; the limiting factor now becomes the 100 Mbit Ethernet adaptor inside it! The solution to this is to buy powerline adaptors that have the word Gigabit on them; this doesn't mean that they can run at Gigabit speeds (i.e. 1000 Mbits/sec), rather that they have a Gigabit Ethernet adaptor inside them so it no longer becomes the bottleneck that limits the speed to 100 Mbit/sec.
Does any of this matter? If your main purpose in using powerline is so you can access the internet or connect a smart TV or set top box, normal 200 Mbit adapters will usually be fine. However, if you have a network with a file server or NAS box then you want powerline adaptors that are rated for Gigabit operation.