Hi All
Rather than Watts vs VOlts it should be relationship of Watts to Volts.
Imagine a Water Tank. We all know that the higher it sits the greater it's Potential to release water. So this is Volts, the 'potential' of the system.
Now current is the flow rate. In a tank the larger the pipe the higher the flow rate can be, just like a copper wire.
Now in the takn if we take the potential (height) and multiple it by Pipe size we get force.
Electrically this is Volts * Current = Watts.
So to obtain a certain Power (watts) output you need to take the motor Voltage and mutilpe it by the motors current.
So simply speaking a 240W motor at 24V has the same power output as a 240W motor at 48V. The DIFFERENCE is in the current, for 48V you need 5A while for 24V you need 10A.
Now what comes in to play here is battery capacity (Ah) and more importantly here, its delivery rate (C).
Battery packs are made up of cells. Say for NiMh it is 1.2V. So a cell may be rated at 1.2V 10Ah. So a 24V pack will be 20 cells giving 10Ah or 240Wh. A 48V pack would be 40 cells at 10Ah or480Wh. But also keep in mind that the 48V pack is twice the size.
Now here is where the delivery rate comes into play. If the 24V pack has a rate of 2C then it can deliver its power at the equivalent of 20Ah for 30 mins (ideal). So would be suited to power a 240W motor easily.
But lets us say, for example, the 48V cells had a deleivery rate of 0.5C. That means they can not delivery at a rate greater than 5Ah and so would just be acceptable.
With eraly battery technology higher than 1C was not readily available and so a higher voltage was need to keep current draw down. Now with higher C rates it is less of an issue and comes down to cost, and more importantly size and weight.
It can be argued now that there is an advantage in using lower volatge systems now. A modern cell typicaly is a minimum of 2C so 24V 10Ah can handle 250W motors easy. Want more distance then get 2 packs. So this presents an option to keep cost and weight down. If you commute short distances then you can use a light weight 24V pack. For weekend trips just add a second pack. This is not an option with 48V. You have the size and weight all the time.
Having said that, once you get up to 500W or more you need to consider either a 24V 20Ah pack or 2* 24 10Ahr packs or a 48V 10Ahr pack. Either choice will be the same weight and size. But again with 2 * 24V 10Ah packs you can put on the bike in two locations to balance the bike, or make less obvious.
So for me I go the smaller option and then option up if needed.
Hope that helped explain Watts and Voltage system advantages.