There are two kinds of solar panels. One is to heat water and the other is to generate electricity. There are even two kinds to heat water. One kind is good for swimming pools for which you need a small increase in temperature for a large amount of water; the other (for house hot water) requires a large increase in temperature for a relatively small amount of water.
Since the cost of solar panels is not a function of the price of a house, it's hard to forecast a % increase in value. Two years ago when I checked electricity generating solar for my home, after all the rebates and discounts, it was still a $10,000 cost, and to recover that if I sold today would require a 2.5% increase in price. The same $10,000 cost (if indeed that's the current cost) on a $200,000 home would require a 5% increase in price.
My brief take is that if the home has a pool, a solar heater is a must. For heating hot water, people are more interested in an on-demand system. For generating electricity, the excitement is hard to attach a dollar sign and harder to attache a % gain in value