[acomjean]

We have a solar array on the roof. It’s great. But the dc->ac converter needs the grid to be active to do its conversion, so when the power goes out no solar. (I suspect it syncs with the grids 60hz...)

If you include a battery backup in your system, you can run without grid power.

When I worked at a power monitoring start up we had a demo house that was charging an electric rav4, so it’s possible.

[WrtCdEvrydy]

(I suspect it syncs with the grids 60hz...)

Yeah, that's the actual reason. I've seen someone use a small Honda generator to generate the frequency required to trick the solar generator into going... it's a weird setup because you basically tie in your generator using a breaker, so both generator and solar can be enabled at the same time.

[Toutouxc]

That sounds borderline stupid, is there a technical reason you can't just generate the "clock" internally? Do most of the devices require grid AC?

[toast0]

Most home solar installs are grid-tied. No utility power, no solar power.

It simplifies the system a lot, because the grid provides "instantaneous" load matching for your house. If the inverter is sourcing more current than your house is sinking, the grid will sink it. If the house is sinking more current than your inverter is sourcing, the grid will source it.

When there's a utility outage, you need something else to provide the load matching. Either a local generator, or a battery is common; but either way, it's a lot more expensive.

[rhodozelia]

It is a more complex and expensive inverter that can operate both isolated from the grid and in sync with it I think because it has to know when to do which and if it is outputting power how can it tell if the system is live because it is grid tied or because it is keeping the system energized itself.

I think they need external signals to tell them which mode to operate in.

Maybe the external system sees the frequency and voltage dropping or lower than normal range and decides to disconnect the load from the grid and opens a breaker and tells the inverter to operate in isolated mode.

When the external system sees the utility power is good again it could tell the inverter to shut off, close the breaker to connect the load to the utility, and then the inverter can go back to grid tie. The outage can be very short unless the load is large motors they don’t like the instantaneous change in phase when switching between different sources that are out of sync.

[pjc50]

It's a safety reason. If the company disconnect the grid upstream to work on it, and your house is still powering it, they have a problem. You need extra equipment to automatically disconnect the house from the grid too.

[throwaway2a02]

I understand why grid-tied systems are built this way, but from an engineering standpoint this is really inefficient. Converting from DC to AC and back to DC, for no reason. In fact, most solar controllers are already designed to use and charge batteries.

It doesn't help that most car manufacturers use locked battery charging protocols that don't allow you to easily change this process.

[yarcob]

Only Tesla uses a proprietary charging protocol.

AFAIK other car makers in the US use either CCS or ChaDeMo.

In Europe, all car makers (including Tesla) support CCS.

CCS allows DC charging and has support for load balancing, so I assume it could be feasible to create a home CCS charger that directly charges an EV with DC from solar panels.

However, considering how expensive power electronics are, and how difficult it would be to integrate such a charger into an existing solar installation, I have doubts that many people would buy such a charger. Also, I don't really know how CCS works in detail, so I may be totally wrong about this.

[bluGill]

solar panels are the wrong dc voltage, so some sort of conversion is needed. I'm no EE, but my understanding is you go through AC and a transformer anyway in most cases.

[throwaway2a02]

Virtually all off-grid solar installations use some kind of an MPPT controller with a battery setup. The controller is always charging the battery (when there is power output from the panels), and the consumer power output is supplied by the batteries. This way you get much higher peak current output than the panels could provide, you also ensure stability.

In fact, many cabin installations don't even use inverters to convert into AC. They instead use DC appliances. So no, going through AC makes no sense in this scenario.

As for DC voltage levels, that of course is dependent on solar panel configuration and battery cells configurations/packing. Virtually all electric cars have a built in charger with a switching power supply that's already doing the DC voltage conversion. What I was remarking was that there is a lot of duplicate electronics that perform the same function and that a lot of intermediary steps can be eliminated.

[bluGill]

Off grid is going to be designed to fit and probably cheap. Not always best. If you panels are near the batteries and near the load what you say can make sense (though I wouldn't be surprised if the charger had a AC step internally, that is clearly optional)

That isn't what you would do for anything tied to the grid. This is far more common (economies of scale mean the parts are cheaper). There you would take advantage of AC's ability to change voltage easily to allow the panels to be farther from the rest of the system. You would also want to use the grid as a backup for any system failures in the solar setup.

Eliminating steps isn't always a good idea.