This has been discussed before, but prob hard thread to find. I've taken 1 bike on a standard Halfords carrier, rests on bumper and tailgate glass with canvas straps fed to inside of tailgate with a plastic drum thing which gets trapped and held there. Important to pad the bumper v well as the paint isn't super strong. Also a bit inconvenient to use as have to load boot first, and remove bike if wanting access to boot. Fine for my use, carting son +kit to Uni, but wouldn't want to use it daily.
@Squasher Not sure what to say really, the kit fits, and it's secure. Going from a Passat estate, and I've used a few for shifting loads, to an Ampera will be a bit of a shock. The Ampera boot is certainly on the small size. Also, if your kids are small then the rear leg room is fine, but...
In short, I use roof bars plus a tow bar mounted bike carrier on the back. Others have found the Thule Raceway works. I was quite tempted by this too, but it seems more precarious compared to a proper bike carrier like my Thule 9503, which I had anyway. However, that or the Thule Clip-On were my potential options for that kind of carrier. I've attached some pics of the Raceway fitted to an Amp/Volt. (I've actually got loads of photos of roof/bike racks I've found for reference's sake.)