Yes, your grid tie systems could see each other as a grid that is still running, but ONLY if they are putting enough power out to keep the voltage up. If the grid is down, and still connected, it is unlikely that you would be putting enough power out to keep the grid up, so if the grid is connected they should see it go down and act appropriately.
If the grid is disconnected, each unit should find the other is unwilling to accept any current, if they are not under load, on the other hand, one unloaded unit may prop up the loaded unit. In theory they should try to help each other, within the limits of available power. If they can't find a happy balance, one or both will shut-down using whatever fail safe they have.
It is possible that one will not like the quality of the power from the other, especially if the sine waye isn't as pure as they need for syncing up. In that case they should simply shut down.
However no matter what, you should discuss your intended design with the manufacturer, to be sure they don't have a problem with what you are trying to do. There may be some additional equipment, connections or precautions to be taken.
I have found grid tie systems to be incredibly intelligent, and reliable. I used them when I was building a critical lab that needed quality power, and I used them without solar inputs, rather I had one set being used to charge battery from the grid, and generator as backup, with a second set that simply ran off the battery. Clearly I had both grid-tie AND battery storage capabilities, I know many newer grid-tie systems don't support battery. No matter the issues around any power outage, my battery backed power source was solid with zero interruptions for 15 years straight. All maintenance, including periodic refresh of battery, installing new generators, replacing pole transformers, and even weeks long power outages were all handled by the system gracefully with no issues. I was using Trace inverters, purchased around 1996. Trace has changed names, and has newer models now, but they were so darned flexible, and so much cheaper than the more traditional systems for lab/datacenter power... They had three inputs, two AC inputs and one DC input and I could current limit each input to match the source capability. the DC and City connections were bi-directional, and the generator input was input only, at least that is how the software worked. In practice I had inter-tie functionality turned off, except when I was doing off-grid experiments that mimicked your scenario.
I did play with them when I first bought them, just to answer questions like yours, and I was very impressed with their capabilities. If I was building a power plant again today, I would very likely try to use the same type of equipment. Of course you have to be an engineer to know what you are doing, and know the electrical code well enough to stay safe.
Will your systems work as well as the systems I used? Who knows? So your mileage will vary. Not all systems have the same features that the Trace units had in 1996.
It's your ground wires. They're on top.
They're biffing onto a neutral or hot terminal.
Also, you need to use the #10-32 screw in the steel box and put a pigtail on that, and join that to the ground wires. Alternately you could use ground clips to attach to the sides of the box.
The key here, however, is to push the grounds into the back of the box behind all other wires. You should do this as the very first thing, and then never think about them again. This will greatly aid troubleshooting; often people wire grounds last after testing, and discover connecting grounds breaks the circuit. This leads them on a snipe hunt looking for a ground problem, when it's something else.
The next layer above grounds is everything that doesn't tie to the lamp, i.e. those black bundles one of which will soon be a black-white-marked.
Then finally the wires go in that actually attach to the lamp socket. That should solve it; there's plenty of depth in a 1.5" octagon box to keep the grounds far away from the socket back face.
Your switch loop is wrong
Code requires a /3 cable for a switch loop so neutral is present for smart switches etc. Use of a /2 switch loop is allowed if changing the cable is easy without tearing apart any part of the building, i.e. Conduit or unimproved work. However you must do BOTH these things:
- Use the white wire for always-hot, not switched-hot as you have. That is so the white wire will definitely test hot at all times, and it's not likely to be confused for a neutral. AND
- Mark both ends of the white wire with colored paint, tape or shrinkwrap.
Best Answer
Given you say the shed/house cable was completely disconnected from the input, and is being used the other way around, then there's something else going on.
You have not fully disconnected the link wire, and it is still connected to other wall jacks with load on them?
Your inverter is simply too small and the cable's length is causing too many losses.
But your underlying question is correct - wire does not care which way around it is installed so power doesn't have a preferred direction, or a "downhill". AC power alternates direction every 50th/60th of a second, so power is going "both ways" many times a second.
What else do we not know ?
Your safest solution is something more like this:
Off phase ? The grid has a frequency of 60 or 50 HZ. That's fixed and not going to change. If your inverter pushes out the same frequency but off-set a little, then the two wave forms are going to cancel out, resulting in something like this (not to scale)
The other options are
a grid-disconnector installed at your main distribution board. This is essentially a big master switch with two inputs (grid and inverter) and one output to the house. To safely chance source it literally disconnects one and connects the other. Everything in the house notices, though modern switchmode power supplies tend to be okay. Better quality disconencts are softer, but some are like a mad-scientist throwing a big knife switch.
Use equipment that has dual power inputs. Server computers often have two or more independent inputs for redundancy. (I'm not sure what happens if a server is fed phase-shifted AC on another input... it should be okay but check that)
A local disconnector - originally for computer kit without dual PSU options, a local version of the disconnector can be installed to supply items.
There is no good and safe way to backfeed AC power to your house from the shed, using the existing cabling. To do it right, you need to install a reverse feed cable, and connect it to a mains disconnecter at the home's input. Such an install would probably best involve a licensed and insured electrician, not bodging around.