So do the energy rebates exceed depreciation? That would make solar panels a more reliable investment than expensive artwork.
Well, a solar panel eventually depreciates to some rather small worth, but the thing is, depreciation is basically a myth. In the real world, depreciation exists because used items are considered less attractive than new items, as they have additional wear and tear, damage, etc. In the Sims world, depreciation is just an arbitrary number: The item itself is as functional and looks exactly the same as it did in mint condition, and a depreciated item would thus be MORE attractive due to its lower price, which is why wanded houses are so much nicer: The same stuff for lower price, which wouldn't exist if used items really were cruddier.
I forget what exactly the daily depreciation is, but eventually the solar panel hits the minimum "wanded" price and ceases to decrease in value, while continually providing revenue until the item is destroyed in a game bug.