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Comment by singron | Hacker News Reader

Solar panels on home roofs aren't necessarily configured to power the house without the grid. Also you would additionally need batteries and related hardware to have a usable source of power at any time, so unless your house is set up for off-grid solar, you still need the grid to be managed well.

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Found this out in the house we are renting with solar panels installed by landlord. Bright sunny day, someone hits a pole nearby, power goes out.

There are two kinds of solar: one that adds to the existing signal from the mains, or one that is self-contained. The second kind is more complicated and therefore expensive. I wasn't aware of the distinction before, but I definitely am now.

Aren't there setups that can do both as well? i.e charge battery first and send excess to the grid?

Sure but they are more expensive. You need a rock solid controller with beefy and redundant electronics to make sure you're never going to be connected to the grid when you shouldn't be.

Yes, but any setup that's capable of feeding power back to the grid requires costly safety interlocks to allow linemen to make repairs at the street without your house unexpectedly energizing the line. Some jurisdictions just outright disallow it.

The setups have to be separate (e.g. different outlets for mains and solar) to protect the grid.

Even a small home solar installation puts out enough power to easily kill unsuspecting maintenance personell working on the grid, for example.

So the utility company has to have 100% control over he mains at all time for safety reasons (i.e. your meter must never run backwards). If they cut the power (e.g. for maintenance or repairs), no one must be able to feed power into the grid.

That's why even if you have a battery, said battery can either feed into the grid, or be connected to separate outlets in your house, but never both at the same time.

Meters run backward intentionally when selling solar back to the utility. You need a switching system that will isolate immediately if the grid fails, and resync with the grid AC before reconnecting, if you want to be able to operate on and off the grid.

That depends on the country then - in my country, you need to install a secondary meter instead.

> So the utility company has to have 100% control over he mains at all time for safety reasons

This is controlled independently by the solar inverters's software, not the utility via any sort of signal. That software is mandated by regulation as you say for the safety of line workers. Recent batteries allow for an "islanding" mode, which allows continued backup power delivery to the house during a grid outage but cuts off the grid connection.

> your meter must never run backwards

... during a grid outage. During normal operation it's fine for it to run backwards. That's what net metering quantifies.

> said battery can either feed into the grid, or be connected to separate outlets in your house, but never both at the same time.

I think this is a technical limitation, since it would require a current divider and 2 inverters, which is a level of complexity not worth it for the application.

> During normal operation it's fine for it to run backwards. That's what net metering quantifies.

Hm. Different countries, different regulations I guess. In my country you have to install a second meter instead.

That's not true, you can have an inverter setup which fails over to a battery backup when the grid goes down. It's also done all the time with natural gas generators.

The setup is more expensive, and it does have to meet certain requirements, but it is definitely an option.

True but, this has been made illegal in many places, California among them.

I don't actually know whether that's legislatively illegal, or simply banned by PG&E; but the effect is much the same.

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