How to figure out how much power for your vehicle

In order to size a motor and batteries in a vehicle it is necessary to figure out how much much power it takes to go the speed and range you need.

I use this online bicycle calculator, it can be used for any kind of vehicle as long as you know the parameters:

http://www.machinehead-software.co.uk/bike/power/bicycle_power_calculator.html

For the VW bug, I looked up the CD (coefficient of drag) which was about 0.485. The frontal area is 1.8 meters squared, the weight is around 1140 kg. The transmission efficiency for a car is usually around 85%.

You can then enter a velocity and it reports back how much power at the motor is required to drive that speed.
For my bug I have a maximum of 27hp, so my top speed will be around 69mph, 19.6kw.
Once you have sized the motor you can figure out how far you can go on what sized battery pack. For example at 69mph if you want to travel 69 miles IE one hours worth of travel you would need 19.6kwh (kilo watt hours) for your battery pack.
My pack will be 144 volts because that is the operating voltage of the Honda IMA motor, so divide 19.6kwh by 144 gives 0.136ka (kilo amp hours) or 136 amp hours. I'll be using lead acid batteries which are nominally around 12 volts, so I'd need 12x 136 amp hour batteries.

In actual fact I would need more than that. Batteries have a much shorter life if you use their full capacity, if you only use for example 25% capacity of a lead acid battery you'll get over 2000 charges from it before it starts to age.
You'll only get a few hundred if you run them down real low.
So for the above example I'd actually need 12x 544 amp hour batteries which would weigh too much for the bug.

I'm shooting for a range of 30 miles (at a discharge of 25%) at a low speed of 35miles an hour average. This is fine for driving to work and back and general runs to the store etc.