I have to agree, I always have a few CFL's lying around for the winter time. This reminds me of an Erowid article I can't seem to dig up. Basically it depends on what you want from your cacti. If you use a lower light spectrum (2700k) which is more of a red light, your cacti will grow slower but produce more alkaloids. If you use 5600-6400 they will grow faster but have less alkaloid production. That being said, if it's winter time, where most cacti are stressed anyways due to less watering and colder temperatures, they're usually building up alkaloids anyway. (A lot of people swear that the best time to consume one is right after the end of winter, where the cutting is made, placed in a burlap sack for about two weeks and then consumed).
Nonetheless depending on how many San Pedro's you have, I'd say two 100 watt CFL's should be more then adequate for each one (plus they are much cheaper on the electric bill). Set up a timer for 10-12 hours, and it's hands free year round growing. Lastly, try to leave them in a warmer spot as they still require higher temps to grow as if it was summer time. Let me know if you want anymore details or if you want me to go find the Erowid article.