People say that it is better to buy or install things as per the requirement as this will save you money, but the parameters are opposite in the case of your solar roof. Actually, installing over-sized solar power system at your rooftop could prove to be a better investment choice for you.
Before starting with the concept of the over-sizing, you should first know the sizing & the feasibility of your solar power system. You can go through to my article "How to design the solar power system by yourself?" for better and the deep understanding and look at my solar feasibility spreadsheet to know know the complete techno-economic viability of your residential solar roof.
Let us say that I have the following facts and the figures:
(The intensity/insolation of the sun is one of the very important factors in sizing your solar roof)
Why the concept of over-sizing?
When my requirement are getting fulfilled then why should I buy extra panel or oversize my solar power system, which will also result in over-sizing of all the components attached like solar charge controller & solar inverter?
How much extra do I need to add to my present requirement?
I think we should at least consider and foresee our needs at least for next 5 years.
By looking at the benefits, durability of the solar power systems, incentives by the government, increasing electricity demand and the rising grid prices, installing a oversized solar power system at your rooftop is obviously a smart move.