They are nice machines though.. just a bit pricey for my budget.

I think the point of cogen devices like this is to pick up otherwise wasted energy and put it to use.

In this case you're going to burn the gas to fire your heater anyhow.  The gas burns a LOT hotter than the max water temperature.  Running that heat across that temperature difference increases the entropy of the universe without doing anything useful.

So instead you run the same heat through the heat engine, and end up dumping it in the hot water anyhow.  And you generate some power in the process.

$10/watt is comparable to solar panel prices - at least it was a few years back.  Maybe a bit high, but what the heck.

You'd need an inverter, of course.  But if you have net metering you wouldn't need a battery bank (unless you want backup power).  You use the grid as a big battery - "charging" it by feeding it power when the heater is running, "discharging" it when the heater is off.  If you discharge more than you charge you pay for the excess.  If you charge more than you discharge you probably get zilch for it, but you'd get zilch if you had to use a dump load anyhow (unless you used it to heat the water some more and thus cut off the flame sooner  B-)  ).  Meanwhile you don't pay for self-discharge.