There are about 125 million households in the US (includes those in apartment complexes), so a million of a hundred million is not exactly something huge, but consider this: the country had less than 30,000 houses with panels by end of 2006 but by 2013 end this had risen to 400,000 – 13 fold in just seven years. In just about 2 years, this would have grown two and a half fold to reach 1 million.
This growth has been – and is being – driven by fast falling costs for solar (For comparison, in 2006, it cost $9 per watt of power generated by solar panels. Today, it’s about $3.5 per watt in the US)
I would say this is quite creditable, given that rooftop solar cost has not yet reached grid parity for the US residential sector (see this post for the levelized cost of rooftop solar power in the US).
Just imagine what the growth could be once the cost of rooftop solar in the US reaches grid parity – which, according to Solar Mango analysis, should happen by 2017 for many states.