Yes, usually. However, depending on how cloudy it is, longer times in the sun might be necessary.
In rainy conditions, it is better (according to the SODIS site) to collect the rainwater.
It should be added that this approach needs to be tweaked (by painting the bottles black on one side, among other measures) to be effective against giardia and certain other organisms.
The simple approaches and rules of thumb given here http://www.sodis.ch/methode/anwendung/index_EN are designed to help people in third world countries by significantly improving their water supplies. However -- and I have to criticize the SODIS site for this -- they do not make it clear that these approaches are not 100% (or even close to 100%) effective against giardia and certain other organisms.
In order to use this technique properly, so that it is closer to 100% effective, you have to do additional research and take additional steps, beyond the simple rules of thumb.
A useful scientific paper that gives additional information and links: