Having 21 panels requires a rectangle 17ft by 24ft (each panel is 5.4ft by 3.25ft). That's far from an estate: if it's a two story home with a flat roof you're talking 816sqft compared to a US median of 2,300sqft.
1. Most roofs aren't flat.
2. For pitched roofs, you're only going to want to install on the south or west facing sides. So only some percentage of your roof area is ideal.
A pitched roof and installing on only half doesn't change it that much. If your roof is 45deg then you're talking about a 24x24 building. If that's two stories this is 1152sqft, still less than half the size of the median US house.
There are roof designs where even on a large house 21 panels won't fit, but looking around my neigborhood most houses could easily do that many.
