Rowan -

The legal requirement that any auxiliary/emergency power generator that powers circuits that also are, or can be, powered from the grid must be isolated when the generator is on also applies here.

Most "whole house" generators would be large enough to require permits for installation, which gives the ability/opportunity to assure qualified installation, inspection and proper isolation. The normal situation would be that any time the local generator is running, the entire circuit it supplies is just disconnected from the grid. Many such "standby generators" are rigged to disconnect from the grid (when grid power drops) before they start up and remain off the grid as long as they're running. They can be set up to sense when power returns, shut down, and reconnect to the grid automatically; but I don't know whether doing so is prevailing practice.

Most "fixed installation" auxiliary/standby generators, are intended for "emergency" use only, since fuel costs and efficiencies make it uneconomical to try to feed back to the grid. If the grid is up, power from the grid is cheaper than you can generate.

Small generators, portable and/or trailer mounted (esp. welding rigs), are very common here however, and it's not unknown for people to crank one up when there's a power failure and jury-rig a connection that may or may not be isolated. Failure to isolate is, of course, illegal - not to mention incredibly stupid - but does happen. These smaller generators are generally much too expensive to run at all when the grid is up, and most of them aren't designed/built to be used continuously for long periods and won't last long if you try.

I've heard rumors of photovoltaic systems that claim to be able to produce excess power to feed back into the grid, but haven't seen convincing reports that any "home systems" actually do produce net returnable power. Safety/isolation methods used haven't been documented where I've run into them.

Even for fairly large industrial installations, few "solar" systems for which I've seen any details produce enough electric output to be worth selling off any excess, and most reports indicate that (usually) the "solar" component of the systems is used mostly for heat storage/distribution. Avoiding the purchase of power for heating has a much better payback than producing electricity to sell back to the system, in the situations I've seen described.

In order to use your own solar-electric power while still drawing some from the grid, you would of course have to figure out how to connect the two systems while both are in operation; but I haven't seen details of how it's being done here, or much indication that power suppliers are willing to allow it in very many places. Wind generators give more variable power, and "peaking" by feeding back to the grid when the winds are up is perhaps a little more common.

The hazard from an isolated grid that can be hot from an isolated input appears to be a very real one, and one that would require intentional fail-safe systems; but I haven't seen any discussion of this, and have no idea what, if any, requirements are demanded or used by anyone here. Unless your system has enormous excess capacity, the two neighbors down the line who think the whole grid is up because of what you're supplying probably would bring your system down in fairly short order, so a simple circuit breaker might be all that's really necessary, if it isolates when it trips and at least requires a manual reset. If you're the only one on the grid segment that's isolated though, the power your system is supplying could make for a long string of hot wires that aren't supplying power to anything but the maintenance/rescue/fire-eater guy who picked up a piece of it.

About all I can offer is that I'll plan to watch for discussion if I run into a place where it might come up.

John