I doubt that the type of chair you refer to uses much electricity. I've been trying to Google for the technical specification of a similar chair, but without success. However, my searches did show that stair lifts and powered wheelchairs generally have motors rated at around 300W to 400W. It's likely that your chair will have (at most) the same power consumption. That means that one hour's continuous use would cost you around 4p or 5p.
The chair should have a plate or label on it, declaring its power rating. (You might have to get a more able-bodied person to look for it, as it could be underneath). Electricity tariffs vary but if the label says 1000W (which is probably much higher than what it actually will say), you'd typically be paying around 12p for every hour's use. (Obviously, you only you use it for a minute or two at a time, so you'll have to estimate the total use over a day, week, month or quarter). For other (probably lower) power ratings, simply use a pro rata figure.
Chris