It's been a few years since I went through the process, but I think the OP is not talking about estimated production. I believe when you initially start the process and pick a system size they estimate the effect on your energy bill to help guide you on what size system to get. This is before the system is designed. Given this and the vagaries of your rate plan, provider, NEM status, and time of day you use your energy I don't think that estimate will be very accurate especially now with NEM 3.0.