A small local generator will certainly be behind what a big commercial generator will do, but if the waste heat is also being harnessed ("Combined Heat and Power," or CHP), combined efficiencies of 60%-80% are possible. Commercial electrical generation is typically around 40% efficient, so CHP really can offer very big benefits.
Of course, there's nothing fundamentally efficient about a single family living in a huge mansion with a 10KW energy demand and a big heated swimming pool.
I would have guessed these things would be most attractive to hotels, universities, airports, truck stops, apartments and condos, and other institutional-scale places that have a significant round-the-clock demand for electricity and hot water.
I wonder what it says about the device and the market that they're planning to sell to well-heeled homeowners first.
I think the key is in where he says a unit with a fifth the output wouldn't be a fifth the cost. If he can use the luxury end of the market to drive the manufacturing costs down, I'm sure general retail won't be far behind.
The waste heat will be roughly 50% of the energy involved. The difference between the generators won't be that big. So each unit of fuel provides more energy to the end user (which is what I am calling efficiency, the energy utilized for something vs the energy consumed making it available).