The other night, I caught an episode of the Canadian CBC show, The Nature of Things, hosted by David Suzuki. The episode was called "Build Green" and was a fascinating tour through Canadian examples of single dwellings, buildings, and even whole communities, that are "green" and more self-sustaining. I highly recommend it if you can catch a re-broadcast. Definitely makes you wonder why this kind of planning/building isn't done more often.

Anyway, one example was a very small home--a micro-home, you might say. I believe it was off the grid. The architect/occupier of it showed the sleek, small power plant just outside the house, and described it as a Stirling engine, which he fed with biodiesel. He didn't mention the manufacturer, but after some digging around, I'm quite sure he had a model made by WhisperGen of New Zealand. From their website, it seems these engines may be used in various situations, both marine and terrestrial, on-grid and off-grid.

In this particular case, this single small Stirling engine produced all of the heating, hot water, and electricity for the house, combined with a small bank of batteries, along with some solar.

Anyway, my main question is, if anyone has any first hand experience, how quiet are Stirling engines? I have read in various places that Stirling engines are relatively quiet, but I have no idea what that means. The sound of a running generator could be a liability, whether during normal times in a dense urban neighborhood or when everything is very quiet, like during a power outage. Are they noticeably quieter than your typical portable genny?

sterlings ARE basically silent, BUT have HUGE issues

1)VERY low power output - a nature of the beast. Different "working fluids" can help (the default is air)

2)The problem with keeping the cold end cold - Sterling engines work on temperature differential - you heat one end, the other end is kept cold, and the air expands on the "hot end", moves the power piston, the displace moves, and the air is now on the "cold end", where it is cooled, shrinks, the power piston is sucked in, which moves the displacer, and the whole cycle repeats. The thing is, you are moving heat from the hot end to the cold - and the cold end gets a little warmer with each stroke of the engine, unless you have some way to carry said heat away from the cold end. The Swedish subs have a nice coolant medium for doing this - the ocean, and the fact that they are moving through it.

Basically, Sterlings and similar temperature diferential engines were used in places where you needed low power, had simple fuel available, and could NOT be bothered with things like boilers, and all the fun they imply (clean water, safety, maintainence etc). The as 1/2 the power stroke of a sterling has a theoretical max pressure of 15 psi (and usually a LOT less), your basically dealing with a low energy source

Cogeneration (electricity/heat/steam/hot water) has been used extensively in industry for more than 20 years. The investment in a much more energy-efficient, dual-purpose unit offsets the initial cost in a few years, after which it's all gravy.

Stirling engines are an old technology that is getting a second, high-tech facelift. I had the pleasure of seeing an antique Stirling in action at a museum in Nova Scotia. Coal-fired (or any biomass) and absolutely fascinating. They were widely used before grid electricity to pump water, and when placed in an outbuilding like a workshop, the waste heat was not really wasted.

Enmax, the City of Calgary's electricity and natural gas utility, sells a co-generation unit, fired by natural gas, that uses a Stirling engine. They've scrubbed a lot of details out of the new website, but here's the link anyway:
http://www.enmax.com/Energy/Res/Greenmax/Technology/CombinedHeatPower.htm

EDIT: Regarding the OP's question about noise: I can't speak about the new units, but the antique Stirling I saw was very quiet, much quieter than any generator I've come across. I don't think they could sell the new ones if they were louder than a washing machine.