Like Martin says, battery range is variable - so many factors have an influence.Things like ;
Weight of the rider , age and condition of the battery, air temperature (yes really, the temperature affects the battery chemistry) , tyre pressure , wheel bearings , terrain and how much pedalling you want to do.
As an aside, the Battery Vehicle Society have suggested using Lithium Ion power packs. The plus side is far greater range and a huge weight reduction, they weigh about one third of the weight of lead/acid battery's.
The downside is huge cost (£500 for a C5
) and complex battery management systems. Proper charging and maintenance is essential,or they die a horrible death. Bit too fragile for my liking and not cost-effective enough.