A little late in posting but here is a chart we developed for our customers. http://california-ebike.com/how-to-select/ it's an empirical chart and is based on you answering 3 questions. That being said, your 20" wheels will even give you more torque than reflected in this chart. I personally would buy a geared hub 500+ watts and at least a 48 volt-10Ah battery with at least a 5c rating. If you are able to charge at work 10Ah should be plenty of power. Now the icing on the cake. Get the Lyen 6FET controller (made in San Francisco) and the Cycle Analyst to monitor your performance and power levels. This kind of package may sound expensive but just remember, you don't live in Kansas and those beautiful hill of SF are a real challenge for the best of eBikes.
Good luck