Having now been on holiday I’ve got some real figures for energy consumption:

The average seemed to be around 290 – 310 Wh/mile. This was for a fully loaded car, two adults, two children, towing a Wanderer dinghy with lots of stuff inside it and a towing cover. Speed was around 55mph(90km/h) on motorways and A-roads.

On the way to the holiday the traffic was very slow and this resulted in much higher energy consumption at times:

I suspect that this was due to the air conditioning running while the car wasn’t moving much resulting in high energy usage per mile. In other words, towing a trailer can have less effect than heating or cooling the inside of the car.

Overall – very happy. Towing the Wanderer does affect the range but not enough to make any practical difference.