Hi Keith, welcome from another newbie to the ebike world, if i can add my 2 pence worth from building my own ebike and making the batteries myself from Lithium ion 18650 cells, i have found that the Ah rating of the battery is very important. i commute to work on mine which is a heavy 722 ft incline in the space of 2 miles with a total commute of 11.5 miles each way, my mountain bike and setup is heavy but i am only 12.5 stones so that negates a little of the extra strain..
the total commute takes 12Ah out of my battery when i come to recharge on a night for a commute of 23 miles in total. your calculation will depend on what kind of terrain you are on.
i just wanted to give you a real world example for the 15Ah battery as mine currently has a 22Ah battery with an ideal of minimum 60Ah when i get the housings made for them.