Yep, by definition solar depends on sun, though ambient air temperature apparently has little effect. This means solar effectiveness varies by season and climate. You wouldn't want solar in a winter BOB in a cloudy climate, but if that climate has a sunny summer ....

But I know I already change gear, like my sleeping system, in my BOB and car kit for winter even here in California. Ditto for the rest my camping and hiking gear. A great piece of gear with seasonal limitations is nothing new.

“Clear weather is essential. On partially cloudy days, solar cooking takes longer. On extremely cloudy days, we suggest that you rely on the old energy-consumer standbys like house ovens or barbecues.

Outside temperature has little effect on solar cooking; if the sky is clear, you can cook on a snowbank high in the mountains.”

http://www.azsolarcenter.com/technology/solcook-5.html