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I'm currently sending grid power from my house to a shed, and I'm now trying to send solar power from an inverter back to the house. I'm confused because the house can power the shed (120V AC displaying from an outlet in the shed, but when I connect an inverter to the shed outlet, I am reading negligible AC output at the house outlet (60V AC) after disconnecting the outlet from the grid. Why would it be that AC can go from the house to the shed and not vise versa? Is it possible there is some diode underground that blocks current from going the reverse direction?

I wish I could provide more details about the wiring, but I'm kind of working in the dark because it's underground and inaccessible. I've provided the crude drawing below based on my most likely flawed understanding.

EDIT

Sorry I remeasured the AC voltage from inverter to house and am reading 60VAC (not 6VAC) on the house end. Seems like only half of the sine wave is being sent for some reason.

I've included some pictures of my setup. The first one shows the outlet on the home, the power comes from the attic down the tube into the box, which feeds the outlet. The cord in picture 2 plugs into this outlet and powers the shed through underground wire with 120V AC. When it's not connected, the extension cord has no power. What's weird is the cord has 60VAC when connected to the inverter (nothing else) so something must be going on underground since the shed has 120VAC from the inverter.

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  • Are you intending to supply your house with solar power from the shed while the grid is down, or...? – ThreePhaseEel Dec 26 '20 at 0:25
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    Is it a grid-tie inverter? Or more generally, please provide make and model details of the inverter. – Ecnerwal Dec 26 '20 at 0:28
  • What synchs the AC frequency output of your inverter with the grid ? I'm guessing nothing ? – Criggie Dec 26 '20 at 0:30
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    Holy smoke, I'm not touching this one with a 10 foot pole. This whole deal is a sea of child-killing safety problems, shortcuts, code violations, huge skill gaps in the technology, extreme cheapness and shoddiness. This is far, far beyond what can be handled in a single Q&A. There are just too many "elephants in the room"... – Harper - Reinstate Monica Dec 26 '20 at 4:10
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    @st4rgut your inverter is intended to run an appliance in a caravan or RV. amazon.com/WZRELB-1500W-Solar-Inverter-Camping/dp/B07SPZWYY4 – Criggie Dec 26 '20 at 5:20
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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.

  1. You have not fully disconnected the link wire, and it is still connected to other wall jacks with load on them?

  2. 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:

enter image description here Where powering something off the inverter means a physical disconnect and replug. This will keep the off-phase AC separate

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)

enter image description here


The other options are

  1. 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.

  2. 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)

  3. 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.


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    I'm pretty sure that it's possible to feed solar power to the house from the shed, but it'd either require living with the limits of a GT inverter, or using a multimode battery inverter or other energy storage system with microgrid capabilities at the house, AC coupling with the GT solar inverters at the shed – ThreePhaseEel Dec 26 '20 at 1:01
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    @ThreePhaseEel excellent point - Feeding DC from the panels/batteries back to the house, with the inverter in the house is another possibility. Distance/losses could be an issue, and depends whether the inverter sits between batteries and solar panels. – Criggie Dec 26 '20 at 1:06
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    though on second thought the amount of current needed at 24V may be more than the underground wire can handle – st4rgut Dec 26 '20 at 5:04
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    > I'm not sure what happens if a server is fed phase-shifted AC on another input... it should be okay but check that -- it should be fine, as the two are independently rectified to DC before reaching any other component. – Bob Dec 26 '20 at 9:55
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    It's OK to generate the AC near the panels. Better for voltage drop actually. But for power-outage support, there MUST be TWO separate cables between house and shed. One from house to shed for use at all times to power things in the shed... and the other for panel to house during outages. Your current wiring is cordage which is not proper at all, and with the plugs is outright dangerous. If you're into doing it all the right/safe way, let us know. – Harper - Reinstate Monica Dec 26 '20 at 19:14
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This is something the inverter's most likely doing, not your wiring

Grid-tie solar inverters are designed to feed power into an operating electric grid, and thus need a grid AC source to synchronize to. If you deny them that grid source, they will refuse to invert. This is to prevent the inverter from trying to power up a dead grid, which poses hazards to the linesfolk out in -20°C, 20m/s winds, and 20 cm of snow trying to get your power back on, as well as the health of the inverter. Furthermore, grid-tie solar inverters don't have the ability to stably regulate grid frequency and voltage, or provide something called reactive power in order to keep the grid functioning properly, for that matter.

Note that if the shed is connected normally to the grid, and the grid is functional, a grid-tied solar inverter will supply power to everything: the shed, your house, and if anything is left over, up through the meter and service cable to the utility for somebody else to use. It's doing this by synchronizing its own output to the grid-supplied AC waveform, though, hence the limitations described above.

For what you want to do, you'll need a proper inverter, not some Cheese-pipeline special, though

The bad news is that for what you're after, your inverter is likely the wrong tool for the job. Your average cheap inverter purchased on Amazon/eBay/AliExpress/... is meant for RV(caravan)/portable power usage, and thus isn't made or tested for grid-tied service, or for long-term off-grid use for that matter. For what you're doing, though, you need an inverter that's been independently tested to meet the applicable national/power utility standards ("grid code") for interconnection to the electricity grid, even if you're running an off-grid system. Given that you're outside North America, I would recommend a Victron MultiPlus multimode inverter. These have good support for worldwide grid standards, extensive testing to verify that they comply with those standards, and are also going to be more rugged with regards to motor starting and such than your average cheap inverter will be. Furthermore, they have models in the line that support 120V operation (vs. 230V), and can support off-grid operation either in addition to or instead of grid-tied service, as they have an internal switch in them that lets them automatically disconnect the inverter from the grid when mains power goes away while still providing inverter power to a set of standby loads wired to a second set of AC terminals on the inverter.

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