Another question!! ":O}
Why are genies rated at 50% load, why is 50% load considered a full load?
Why would anyone make a product called Banana brittle? Why would my wife buy it?
If you have any questions other than those place before us by me, save yourself some clicks and ask them here!!
Ok I've had time to read the thread properly.
Load balancing: generators, including ones in power stations, typically produce 3 phase power. To get the maximum performance out of a generator, the phases need to be balanced as close as possible, in terms of power consumption, otherwise the energy produced in the windings for the under-used phases manifests itself as heat, which may damage the generator. To mitigate this, power companies do two things (in the UK/Europe at any rate. American power standards are weird*). First, in any street, each house is connected to each phase in sequence - you might be connected to R phase, your neighbour to Y phase, next house to B phase, then back to R for the third house along. On average, people use roughly the same amount of power, so in any given block, the substation should be roughly balanced. The next is load shedding - unwanted power from under-utilised phases is passed through a resistive load (basically an electric heater) to get rid of it outside the generator.
Some better domestic generators can manage a certain amount of automatic load shedding - typically not enough for the full power off a winding, though.
It's useful that you plan to supply 3 houses as then your best bet is to put each on a separate phase. If you need to keep your 220V sockets running, that's also simple. (house 1: R+Y, 2: Y+B, 3: B+R)
50% is maximum load: as noted above, phases need balancing. If you have unbalanced loads, the output must be derated to avoid overheat and/or over-current. Domestic generator manufacturers know they're not selling to engineers, so they de-rate it first, in order to prevent you from letting the magic blue smoke out of your generator the first time you use it. In principle, with a properly balanced load you could get the full 15kW out of it, assuming the breakers are up to it.
Winterisation for propane: check the datasheet, or label on the side of the cylinder - if it's 99% propane you're fine. If it's 90% or less, insulate the regulator and hoses. Do not put the bottles next to the generator - they don't like getting too hot.
Now get back on eBay and look for an Automatic Transfer Switch with generator start contact and you could have the generator fire up automatically on power failure