What is the wattage of my iPhone?
Anandtech has some nice charts detailing the wattage of the iPhone 4S running various apps, which could be a good starting point for you. 

You may also want to check out existing kits/projects, such as MintyBoost - they seem to work reasonably well with iPhones. There's a specific MintyBoost page detailing Apple device charging that may also yield some useful info.