[Doxin]

Or you have your inverters run a phase locked loop. When the grid is there it'll keep perfect sync, when it's not there it'll keep running. Switching back to grid power should be as easy as waiting to reconnect until your inverters are back in sync, which depending on the PLL design shouldn't take long.

[zaarn]

Waiting to be back in sync could take forever, plus you need a Sync Check Relay. Price of those is usually "contact sales team" and they don't work reliably if you don't need more than a couple dozen kW of power. (They're intended for 1MW+ installations).

Just shut it down, flip the switch and restart. Everything else will just be prohibitively expensive because it needs to be very safe.

If you get the phase wrong then you'll either reduce the lifetime of your components or the components explode after the nearest power plant tries to pump all available power into your poor inverter.

[Doxin]

> Waiting to be back in sync could take forever

Only if you design it to. The phase locked loop would "listen" to the mains frequency and slew to match. Slew speed is simply a design parameter you can set to any value.

[stephen_g]

A phase-locked loop would fairly quickly come back into sync with the reference frequency - that’s exactly what they’re designed to do.

I’m pretty sure companies like Victron Energy already make these kinds of systems - combination solar inverters and battery chargers that have transfer switches to be able to seamlessly switch to UPS mode when the grid drops out but can still export excess energy when it’s up.

[zaarn]

Of course these systems are made but they're just very expensive and usually for customers of theirs that don't play around with a couple 100W panels because they want to save a buck or two in the summer.

[namibj]

The relay you need is a normal breaker with a solenoid to trigger the spring and a small geared DC motor to re-engage/reset the breaker.

And that's for what you need to allow your inverter to handle this automatically (you might need another voltage sensing channel to sense the grid-side of this breaker).

[lostapathy]

If this is such a hard problem, how does the $150 UPS hooked up to my home computers pull it off???

[jrockway]

Your UPS doesn't save money by discharging its battery back to the grid when it's not in use.

Your UPS handles an order of magnitude (or more) less power than a whole-home solar installation.

None of these problems are intractable of course, but you are oversimplifying the problem a little.

[lostapathy]

I'd be willing to accept not discharging my solar system's battery to the grid when not in use, I can use it at night.

One can get a UPS affordably that will power 1500 watts of continuous power. Being able to supply just that much, or twice that much, from solar panels in a grid-down scenario would be tremendously useful, even if it's not enough power to fire up my welder.