> isn't it complicated to have their output be in perfect sync with the frequency

Not especially, given that the inverter has a microprocessor in it. All it has to do is measure the phase of the existing grid.

I don't have references for how it's actually done, but one obvious approach is simply to wait at each zero-crossing for a new half-cycle to cross a voltage threshold before turning on the output. This also implements the requirement to drop out if the grid goes away. It is probably also possible to measure during the "off" side of inverter output PWM, in the same way that variable frequency motor drivers work.

The micro inverter (most of these balcony kits use micro inverters) uses the grid as the reference. Most of these inverters will actually do nothing when the grid goes down. Like, they shut down for safety so you are not back feeding the grid, but even if you had some sort of back feeding isolation going on, they would still do nothing because they don’t have the reference of the grid.

It’s a downside of many grid tied residential systems (even large ones). No grid = no solar.

The Enphase IQ8 series is one of the first mass market micro inverters based systems to have the ability to make its own tiny electric island when the grid goes down. Requires an isolation switch and a relatively power hungry controller to use that feature, though. I looked into them for a balcony solar setup but it would be way overkill to run a full on controller for 800w of solar!

The best way for a small setup is just have a small “solar generator” battery that can take MC4 connectors as input. Prolonged power outage? Unplug the inverter, plug in the battery.

Not a specialist, just from what I heard: There are two things that make it work. First they are not really "independent" like the title says. They sync with the grid frequency. If the grid is down they shut off for safety. The other reason it works is that the grid power inside the home is just what you get as incoming power 〜230V. For example, I think in the US you get 240V or so delivered to your house, but 120V from the plug.

Syncing them with electricity is easy. The hard part is preventing export to the grid which requires either a compatible smart meter that can communicate with your panels or a transformer clamp installed by an electrician. My understanding is some meters measure both directions equally so if you do end up exporting power you can conceivably increase your utility bill.

Why is preventing export hard? Just shut down production when the grid is down, which is how plug-in solar systems work.

You will still export when the grid is up if your solar generates more power than you use at any moment.

The grid doesn't need to be down, your solar just needs to produce more than you're using.

Why would you want to prevent export?

We just got rooftop solar in Canada. Our meter was old and had to be upgraded to bidirectional.

We were warned if we turned on the system before the meter upgrade the old meter would sum together power coming from the grid and power going into it from our solar and we would be billed for the combined.

So with some old meters you don’t want to put power into the grid.

Plug in solar must be zero export to bypass utility approval process.

Interesting, why? That is not the case where I live.

You must live in Europe. Not entirely sure but that's the way things are going in North America, as far as I know all the proposed legislation here is zero export. I guess one concern is local grid instability if too much power is generated this way. Also if for example you put a plug in solar on the same circuit as an appliance that uses its power then the breaker doesn't "see" this usage but it still heats up the wire just the same leading to fire risk, but not sure if this has ever been demonstrated or just theoretical. And also there's the old meters still around that don't measure bidirectionally, so those will just roll back or roll forward each way, neither desirable.

I guess if you can solve phase alignment then another big problem is grid capability?

If everyone plugged one in, could the transmission network reliably deliver the power generated where it's needed? I thought that was a serious long term challenge for utilities wrt solar.

Typically, you have "dumb" panels connected to a mppt-controller/charger/inverter box which is connected to batteries and and electrical plug. This controller tunes voltage/current that is taken from the panels, optionally manages the attached battery and measures and feeds into the grid connection.

Some systems are capable of running in isolation from grid (providing 230V AC on their own), but this is less common and often unnecessary.

My understanding is that plug-in solar inverters do sense what is coming from the grid and phase-sync to it with a PLL, and also adjust voltage accordingly.