If you look at your bill closely, it'll tell you what the average cost per unit was of your electricity. Go and IO work well if you can shift some of your usage (dishwasher, washing machine, tumble drier etc) off peak, and reduce peak use. You can then use your bill to compare what you're paying with the tracker rate. So say tracker is 20p for electricity today, if your bill says on average you're paying 25p, moving to tracker would be good, if it says on average you're paying 19p, then moving might not be so good.
In essence they've done the hard work for you.