Have solar cells become so good that a roof of them can power the whole house?
The answer depends on what exactly you mean by "powering the whole house", but "yes" is a reasonable approximation. I had an array installed in 2013 which covers about a sixth of my roof; every year it produces about 40% of the electricity we consume. The story has only improved since then as efficiency of solar cells has continued to rise and cost of installed panels has continued to fall.
Oh wow... how much do they cost to cover a roof?
This is designed for new houses. That means to calculate the cost of the electricity, you first need to subtract the cost of an equivalent-quality conventional roof.