This is a good question and one that even the experts cannot agree on!
Solar setups are so efficient these days that some homeowners become self-reliant and actually have surplus electricity to sell back to the grid. In these cases it doesn't seem unrealistic that this will add considerable value to a property - some estimate it can add up to 14%. At the other end of the scale you may have a single, old style panel that is producing negligible returns. It is not a surprise that in this instance it may not add any value. One thing they do seem to agree on though is that it will not affect saleability.