I like the idea of using a microwave for decarboxylation:
http://cannabisculture.com/articles/3037.html

Some of the cannabinoids vaporize at temperatures lower than THC's boiling point, and it's not just THC but the other compounds that are part of the experience. I've heard that the cannabinoids can also evaporate a little bit during the extraction process.

I think the most efficient method would be to use the microwave for decarboxylation, then use the solar method for extraction. No losses in either step.