Solar Panel on the Trunk of my Z-20

A quick, rough answer is that, assuming 2100 watt-hours of energy in the Z-20 battery pack, and your 20 watt panel producing 16 watts after the conversion efficiencies, it would require 131 hours of sunlight, or 11 average sunny days to charge the battery pack from a 100% drained state. So, the contribution of this small a panel is not worth the trouble.

However, There is a well-known (on the internet anyway) person who built larger panels on his scooter in the form of folding "wings" which are unfurled when the scooter is parked. These are capable of charging his scooter a significant amount.

Look here: http://www.treehugger.com/files/2005/04/don_dunkees_diy.php

Or google "solar electric scooter"

Hi DonCristobal,

I can't speak in terms of the Z-20 and I certainly am not able to produce any charts for you but I do have an eBike and am somewhat familiar with charging and drawing amps from battery banks.
I very seriously doubt that you could put a large enough solar panel to make the slightest difference in your range.
What you could do with a solar panel is charge over a period of time, like while you are at work etc.
I don't doubt that you could actually charge while operating but I don't think it would make any significant difference.
Using an inverter and a charger will both consume energy and your charging output would be minimal at best given the constraints of sq/ft of solar panel needed to produce enough energy to make a significant difference.

In my experience, a solar panel would best be used in conjunction with a wind generator or other source of Power Generation at a charging station but not as an on-board compliment.
With an eBike, I often pedal assist to gain range, and this works quite well since it helps the motor draw less power (amps) from the batteries. I think you get a lot more range from reducing the draw rather than supplementing with charge input.

Adding to useful Amp Hours is quite feasable, however. By useful Amp Hours, I mean like Lithium batteries vs. SLA type and having more AH reserve (more batteries).

This is based only on my experience and on information that I've learned on this site and other sites.
Others may disagree, I just don't think solar would be a worthwhile investment in gaining range.
Conservative use (going a bit slower) or tweaking friction (wind resistance, tire pressure etc) will likely help to a much larger degree. Perhaps more AH, conservative driving and less friction together would help the most.

Dave
MB-1-E

DonCristobal,
Welcome to the V is for Voltage Community, you might want to also check out How to convert an EVT 4000E electric scooter to a solar scooter.