New testing conducted at France's oldest PV system have shown that its solar modules can still provide performance values in line with what the manufacturers promised.
Sounds like you should install double what you think you need. Reason? The panels will start losing efficiency over time and your electricity usage over time will do nothing but grow. That's very common.
I didn't say they lost half but they lost 20% so you need to have at least 120% today and then you need to account for the fact that you will get more electric devices in the future. How are you going to charge your electric car if you don't have the electric to do it? So therefore you should probably double it.
Average US residential power usage hasn’t really changed much over the last 20 years. This is generally because things become more energy efficient with time - e.g. typical light bulbs have gone from 60W to 8W.