Know how light mixes with the trees around your property. You may think you've got the perfect spot for a solar panel, but watch it throughout the day. The sun coming in from different angles may lead to surprising shadows from trees and foliage. This will affect your solar output.
Your solar panels' density will impact their efficiency. Getting high density solar panels may cost you quite a bit, but after a while they'll pay for themselves because they produce more power. Be sure and compare densities before choosing a solar panel.