Based on e bike having a 250w motor & being able to harvest 40w that gives you about 18% of your total power. That should boost your range quite nicely.
Don't take this the wrong way I am trying to save you a lot of heart ache and money, the short answer is it won't work well enough to justify in any way.
A 250w ebike motor can actually draw upto 800w, plus a 40 watt panel would be 40w under ideal condition and aimed correctly for maximum sunlight, whilst riding for an hour ideal orientation would be minutes of that time.
Next issue is a flexible solar panel is less efficient than a rigid panel and a 40w rigid panel is roughly 530 x 650mm.
A designer previously tried promoting a jacket as described, it charged a USB bank that could charge upto about 6 ah at 5v with no statement of how many hours or days it would take to charge the power bank, now that pack would add 1 amp hour at 36v without any losses of which there would be some ie. stepping up the solar panel voltage, charging circuit etc.
The tech sights made fun of how useless it would be for any charging function, the most damning thing was that the designer never released charging specs.
http://gizmodo.com/no-one-needs-a-solar-powered-jacket-1667736727
The only kind of successful usage of this idea I have seen or can conceive of working is mounted as a roof on recumbent trikes to give you the required area and good exposure to sunlight, plus they can charge whilst parked.