The reason “why is solar better for a new house” is because everything in the home is in sync. The roof will age simultaneously with the solar system. The appliances and A/C & Heat are more efficient, conserving power consumption. A more efficient home uses a smaller solar system and it will cost thousands of dollars less. See: How To Prepare for Going Solar to learn how to make any home perform at top efficiency before going solar.
A new home is much more efficient because everything is brand new. Unfortunate as it may be a 7-year old A/C unit will be more expensive to run because it is less efficient than a new one. That applies to all of the appliances in the household. Washer, dryer, dishwasher, A/C, refrigerator, that’s a mountain of efficiency.
So why Is solar better for a new house, because everything in the house is more efficient. A new home has LED lighting throughout the house which is extremely efficient.
The more power a home consumes the bigger solar system required. This suggests that making any home more efficient before going solar is best. A new house is ready to go, operating at peak efficiency which will use a smaller solar system compared to a less efficient older home.