When/where is it feasible to use a solar cooker to boil water?

1. Can a parabolic reflector-type solar cooker of reliably boil water? If so, what major factors determine

• how much/how intense sunlight is required to do this
• how much water can be boiled
• how long the process takes
• what should be kept in mind to obtain good results?

2. Do any other "passive" solar devices exist that can boil water effectively?

3. Is there any good way to safely boil water using a (small) solar hot water system?

I’m mostly interested in systems of a size that is normally marketed in rich countries as appropriate to households of 2-4 individuals (in the case of solar hot water systems, to systems on the very small end of those available). I’m interested in a range of conditions, but the "base case" I have in mind is a moderately sunny winter day in the United States, around 40 degrees north latitude.

To anyone reading this, I should emphasize that that boiling water does not necessarily make it safe to drink.

Thanks!