To me it just sounds absolutely bonkers that you can feed electricity _into_ a power outlet.
I passed high school physics with good grades, so … I’m not surprised other people instinctively say this can’t possibly be a good idea.
I passed high school physics with good grades, so … I’m not surprised other people instinctively say this can’t possibly be a good idea.
But it’s AC, the electrons are shuffling in both directions at 50/60hz
Voltage goes forwards and backwards and odd but current also goes forwards and backwards, for both appliances and generators.
The difference between consumer and producer is only the phase angle between the voltage and current waveform.
Because power is VxA, do that across the wave cycle and the power direction changes for consumers and producers.
At every junction, Kirchhoff’s current law will sum the currents so you get a “net current” waveform on a wire that might have both a kettle and a plug in solar panel.
That summation means the kettle will chomp as much current as the panel can deliver and than take more from the grid. Swap the kettle for a small LED which takes a fraction of the current and the sum gives a NET supply, and on to the next junction.
It took me a while to get my head round it too.
Take power out of the flywheel? Sure, why not -- that's exactly what we've done with it at home since we had an AC grid to speak of.
Try to impart higher voltage to put power back into the system? Fine, no worries. Flywheel just accepts and absorbs whatever you put into it as long as it has the right shape.
Try to change the flywheel's AC frequency or waveform, though? Hah, fat chance: Flywheel will trip your breakers or reduce your gear to ash -- whichever comes first. We play by the flywheel's rules, and those rules aren't optional at all. It may be just a big dumb flywheel, but the flywheel's enforcement is absolute.
Anyway, it's a big mythical flywheel. We get charged money to take power out of it, and (with the right arrangements) we get paid money to put power into it.
And by design, grid-tied inverters are quite good at following the flywheel's rules. They match the flywheel's waveform very well [lest they go poof]. They can only put power into that flywheel, and they reject power from that flywheel. The inverter is a one-way thing that, from a high-enough viewpoint, looks kind of like a check valve or a diode does.
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But it's not an ideal, perfect system. It does not exist in a world of frictionless pulleys and ropes that don't stretch.
That imperfection shows up in the wires and transformers connecting the house to that flywheel system, in that these parts have a non-zero impedance. (For the sake of this discussion, impedance can mean about the same thing as resistance)
And that impedance is useful. It means that we can increase voltage a very tiny bit on our local leg, and thereby succeed at using our puny little solar power system to help cover our own local base load.
Suppose our plug-in solar widget can produce 800 Watts today. The plug-in kit increases the voltage that it drives that plug with until 800 Watts of power output is achieved (which is only a very tiny amount of extra voltage, but it's real anyway). This is its purpose: Where some robots pass butter, this robot puts 800 Watts into a power socket.
Since the wiring between the plug-in solar and the local household baseload stuff has a lower impedance than the path to the mythical flywheel does, our baseload stuff (the fridge, the router, the fan by the window) gets first dibs at that 800 Watts of minutely-enhanced voltage.
If we produce more than we use, the excess local power production gets provided to and soaked up by the flywheel (maybe we get paid for that).
If we use more than we produce, then the deficit is made up for by the flywheel (and we definitely pay for it).
If our generation and consumption is perfectly balanced (which never really happens), then the flywheel plays no real part in the program except to provide a waveform for our solar inverter to synchronize with.
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Anyway, it's not new tech at all. It's safe in that it won't work at all without a connection to the big flywheel in the sky. And folks have been using grid-tied inverters for quite a long time, but that was done on a dedicated circuit (with a dedicated breaker or fuse or RCBO or whatever they're called wherever you are).
But there's still some new stuff going on: It can be a plug-in system that works without individual planning or permitting; in some places, a person can legally buy and use a kit without obtaining extra permission.
The safety of sharing a circuit is also important, since enhanced potential exists for overheating things. Germany limits their plug-in solar to 800 Watts, or about 3.5A @ 230V, while their circuits are usually 16A.
That means there could be as much as 19.5A of nominal current flow within a circuit before some combination of circuit breaker or inverter limit is reached.
I learned a long time ago to never question a German about their electrical standards, though, so I can only assume that very smart people have worked very long hours to see that this situation is safe-enough.
Current load pulls the voltage down, sourcing current pushes it up. No bother, as long as your sources cut out if the voltage goes too high, and your loads cut out if it goes too low.
In terms of electricity and physics, this is unremarkable, the bit that probably scares people is the fact that you have a plug connected to something that's _outputting_ AC, particularly for those of us with 240V supplies.
If it were any old AC source on the other end they'd be correct, but current technology allows the output to be cut in milliseconds, faster than the period of a single AC wave, once the plug is removed.
Tangentially; if you have solar (mine's roof-mounted) don't assume that flipping the breaker means there's no power; I live in a 1930s UK home with all sorts of interesting wiring and flipped the breaker to change a plug socket, only to be shocked; literally and figuratively when it was hot.
When we had our solar installed, we had to make sure we had a newer power meter that could handle power both ways, so it probably is a problem for some situations, but the power flow for a single solar panel into a plug isn't enough to trip this, or maybe newer power meters are better?
We definitely think of it as somewhere a power comes out of, but I don't think it's the name, just that it's only ever been that way.
Clearly some people think of it differently if they're plugging generators into wall sockets to power the whole house, dumb as that may be.
My home is 1930s, it's fuse, in the black Bakelite housing typical of the era was insufficient and we were missing a incoming breaker to/from grid, those were updated. Our meter is much more recent, though still dated; allegedly it would have worked but it wasn't a smart meter, which we wanted so we had that updated.
I've also heard through my many hours of watching Solar videos on YT before our install that some breakers, RCDs(?), RCBOs(?), whichever it is, aren't always suitable for bidirectional current flow, so that's a possible issue too.
All of that is aside the "suicide cord" danger of the old generator method, but that's gone now with modern tech, the output is cut before you can get your hands on the contacts on the end of a plug.
It's just been legalised here in the UK, and I think it will help people with their bills, but the key is to keep installation costs down; if it's DIY you can look at a decent ROI time, but with the cost of trades-people these days, if you have to call out a sparky to inspect before you plug it in, you've added potentially years to your payback period.
At ~800W, I think on the whole we'll be fine; the equipment in our homes has got much more efficient over the years, and our homes were mostly built for times where we were using higher powered devices. If you can draw over a kW to power an electric heater or kettle, I don't see why feeding a mere 800W back in would be a huge risk.
Disclaimer: I'm not an Electrician, but I do have a degree in Electronic Engineering.
The solar is the same way, but with an auto cutoff.
Yes, but that's dangerous for reasons[1] that people often don't bother explaining -- and that aren't important for solar grid-tied inverters[2].
[1]: The seemingly-ideal way to plug a big fat generator into an American house in a hurry is to use a double-ended suicide cord to feed into the 240v dryer outlet. That provides plenty of power to get a lot of things done, it's a single-outlet circuit, and it lights up both sides of the split phase panel. But that's bad.
The most obvious way that it's bad is because we shouldn't have energized male prongs. Like, ever -- that's just extra shit to get hurt with. But this suicide cord idea relies upon their use.
The other way that it's bad is that the generator doesn't know whether the house is still connected to the grid or not. So unless we deliberately disconnect from the grid, there's a wonderful chance that we feed it even though that grid is presently non-functional. This can fuck people up: The transformer on the pole outside (or in the box down the way, or whatever) will cheerfully convert the sleepy 240v from the generator into thousands of volts on the wires outside that were completely dead until someone decided to get clever. Now, the guy cleaning up storm debris down the block or the lineman doing the repair gets the fuck shocked out of them in spooky-dead kinds of ways. (Yeah, yeah: We shouldn't ever do anything near downed lines of any kind, and the lineman should always have appropriate PPE. And we shouldn't plug our generators into our dryer socket, too. Death is permanent.)
Panels with interlocks and wall-mounted male sockets that accept generator input both exist, and the combination of them mitigates both issues. The combination also prevents a third problem: Power coming back on while the generator is running.
[2]: But a proper UL-listed plug-in solar inverter doesn't have any of these problems. The issues are sidestepped because it outputs nothing when the grid is down, as a baked-in function. So there's no energized male plug to be a hazard, and there's also no interlock required to ensure that the grid isn't backfed while it is down. You can hypothetically unplug the solar inverter from the wall on a bright sunny day and lick the prongs, and nothing happens.
This has very little to do with the power meter itself, and more that the power company wants to be able to monitor if the meter is 'running backwards' (i.e., if power is flowing out rather than into the meter). Older meters could only run in one direction.
> Clearly some people think of it differently if they're plugging generators into wall sockets to power the whole house, dumb as that may be.
There's frankly nothing wrong with this if you do it properly. The 'proper' way would be to have a generator input socket near the main breaker panel, and the main breaker is interlocked with the generator input (i.e., only one can be on at a time). You absolutely can just plug a regular generator into a wall, assuming you aren't overloading the wiring or breakers... the safety issue is overblown. The reason why it's 'super dangerous and bad stuff will happen' is because people are stupid and forget to turn off the main breaker. Which, admittedly, is a reasonable concern because... people do forget.
What's new is cheap and ubiquitous micro-inverters. Before it wasn't worth putting a $800 inverter on a small 2-panel 600 or 800 installation. Now with micro inverters being so cheap, this tech becomes viable. And with loads so limited (less than a microwave) the regulation is equally light to allow this convenient plug-in solution.
On an AC circuit, all connections are duplex.