If you can find the perfect end user then $xxx, but if you leave the search to a reseller then $xx to maybe low $xxx.
Solar energy is going to become simply huge in the next decade, so even if you don't get an acceptable offer I think it's value is going to increase nicely. So if you can afford the renewals, then you should keep them until you get a good offer.
I know the urge to flip a good profit, but I don't quite think every domainer appreciates the unique situation we find ourselves in, in 2010-2030. The right domain names are becoming increasingly valuable, and what do you think will happen in 2030? To make an analogy, 300 years ago physical property and land was in abundance, like trying to sell ice to an eskimo. Now even the smallest property in the right place is very valuable.
In short, now and 20 years ago is the time to invest in good domains and let them age like fine wine =P