Ah. Didn't think of that. This could get tricky.
You're going to need some way of counting how many Solar Panel outfits you've bought.
Here's a thought:
The OnBuy field of the original Solar Panel sets (say) b1.
Then we have a series of cröns that are used to count up in binary style. It's late at night, so I'm possibly not spotting some glaring problem, but I'm sure someone will point it out to me. Maybe there's even a simpler way. Either that, or someone's already thought of this.
1. Activates on b1 & !b2, and has b2 !b1 OnEnd
2. Acivates on b1 & b2 & !b3 and has b3 !b1 !b2 OnEnd
3. Activates on b1 & b2 & b3 & !b4 and has b4 !b1 !b2 !b3 OnEnd
4. Activates on b1 & b2 & b3 & b4 & !b5 and has b5 !b1 !b2 !b3 !b4 OnEnd
Then your original Solar Panel outfit has !b5 in its Availability field. The procedure for upgrading to the advanced panel is the same as described in GutlessWonder's post above. The trick here is that the procedure only works if the player can only buy the normal Solar Panels one at a time.
So, to explain what I mean, when you buy your first panel, crön 1 activates, setting your counter to 0001. When you buy the second, crön 2 activates, since the least significant bit is set, which sets the second bit and clears the first, setting your counter to 0010. When you buy your third, crön 1 activates again, since the first bit is clear, but since only the second bit is set, none of the other cröns activate - this sets the counter to 0011. When you buy the fourth, crön 3 activates, since the lowest two bits are set. This sets the third bit, making your counter 0100. The fifth, sixth and seventh solar panels repeat the above procedure, setting the counter to 0101, 0110, and 0111 respectively. On the eighth solar panel, crön 4 fires up, setting the most significant bit, and blocking any further purchases of solar panels.
... Does that make any sense, or am I just spouting gibberish?
The main thing I can't work out is how to restrict the player to only buying one at a time, other than making it outfit limit 1 (which would prevent you from buying any more until you upgrade the one you have).