Derating 24v Solar Panels?

True, I've only done a few charges, but I have a pretty good idea of what the current set-up can produce, having compared pointed to flat several times, under varying sky conditions. I do know 280w/pointed will not suffice, spring and fall, and that 500w/flat will be too much, most of the time in summer. But I need to go big to cover the worse case scenarios.

The 140w/12v panels I bought are no longer available, so adding a third is no longer an option, either.

I'm seriously considering a 24v system, in the future, so the 24v panels will be necessary for that, as well.