Is there a device to only allow AC power to "flow" one-way?

Scenario: I have a circuit in which I would like to draw AC power from two sources: One source is good ole wall (grid) power and the other source is a tie-in inverter. The problem is is that if the tie-in inverter ever exceeds the power needs of the circuit, the excess power will "leak" out and be available throughout my house. I need to prevent this.

I cannot simply throw a couple relays between my circuit and the wall and only switch them on when the draw of the circuit exceeds the capability of the tie-in inverter because the tie-in inverter won't do squat by itself--it can only feed into an existing AC system.

Is something like a DIAC what I need? I read a little about it, but was confused as to how that would actually fit my needs. Is there a device that prevents AC power from escaping a circuit, but allows it to flow in? What if I take grid power from a UPS, would that prevent "back flow"? If not, what must I create to be functionally equivalent?

edit: Motivation:

The reason I want something like this is because after talking with the power company, it is very expensive to perform an interconnect with the grid and would be super pointless in an apartment. If you want to provide any power locally (solar, wind, etc) you have to interconnect so the grid doesn't explode with (rare) excess power. Now of course, this isn't much of an issue if it were just me occasionally spewing out extra power, but I understand how serious the issue becomes if lots of people did it (without interconnect), so I do want to play by the rules.

I was informed that even with an itsy bitsy tie-in inverter (we're talking ~30Watts), my power company would eventually find me out and I'd be forced to stop or pay for an interconnect installation. I'm still not entirely sure how they would find out since our fridge, for example, is plugged in all the time and draws way more than 30 Watts, but they said eventually power will leak out and they'd see it.

So my end game is a single powerstrip that draws from my own generated power (granted a very small amount) supplemented by the grid in a safe way that will not ever leak out in the obscure event that my load is less than my locally produced power.

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    \$\begingroup\$ You show no understanding of electricity and are proposing modifying a mains system. Walk away now and hire a qualified electrician before someone gets hurt. \$\endgroup\$ – RoyC Nov 30 '17 at 16:08
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    \$\begingroup\$ @RoyC: I deleted your second comment because it was dangerous and misleading.You clearly have no understanding of how a grid-tied inverter works. It simply converts the available input power to grid power -- it can't distinguish one grid load from another. \$\endgroup\$ – Dave Tweed Nov 30 '17 at 16:41
  • \$\begingroup\$ @Dave Tweed I have a very good understanding of how a Grid Tie inverter works although I put things in a deliberately simplistic manner . How exactly was my statement misleading?. The inverter will create a voltage waveform at or slightly higher than the line in. If the local load is not enough to soak up that power it is fed back to the grid it does not leak out into the OPs house or put simply the inverter deals with the excess power. \$\endgroup\$ – RoyC Nov 30 '17 at 17:06
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    \$\begingroup\$ @RoyC: As I said, the inverter cannot distinguish loads inside the house from loads outside the house (the rest of the "grid"). You are still oversimplifying. \$\endgroup\$ – Dave Tweed Nov 30 '17 at 17:20
  • \$\begingroup\$ @RoyC, The electrons don't know the legal definition of the "grid" stops and the "premises wiring" begins. \$\endgroup\$ – Solomon Slow Nov 30 '17 at 20:20

It's going to be almost impossible without a deep knowledge of how that particular inverter detects the incoming mains supply. Block the mains, and the inverter will shut down, as it's designed to do. Leave the mains connected, and it will start exporting, as it's designed to do.

It's not clear why you'd want to do this anyway.

  • \$\begingroup\$ Hey Simon, thanks for the answer. I updated my question to include why I would want to do something like this. Thanks! \$\endgroup\$ – Paul Nov 30 '17 at 19:31

Here are some thoughts.

As Pieter said in his answer, this would be easy for DC: you would use a diode to prevent current to flow back to the grid.

But for AC, you can design something that behaves like the diode in DC:


simulate this circuit – Schematic created using CircuitLab

You would connect your inverter and your home equipment to the “OUT” side.

But I would not recommend this anyway, since you don’t know how your inverter will behave if the equipment does not drain all the power the inverter tries to push…

  • \$\begingroup\$ One could even have problems with this circuit if there are reactive loads in the network. \$\endgroup\$ – Vovanium Jan 31 at 13:57
  • \$\begingroup\$ @Vovanium Good point, one more reason not to recommend it. \$\endgroup\$ – user2233709 Jan 31 at 14:36

Disclaimer: I am just an amateur, get professional advice.

If you have two DC sources it would be easy, you just add a one way (diode) and power/current can only flow in one direction.

You might be able to simulate this by feeding grid into UPS(double conversion AC->DC->AC) and then having your grid-tied inverter connected to the output of the UPS.

Thus if you draw low power from the UPS output the grid-tied inverter will elevate the voltage slightly but nothing will flow back through the UPS AC->DC

It might also provide the additional benefit of extending the runtime of the ups, and allow the grid-tied inverter to operate when the mains power fails.

By connecting the grid-tied inverter to the output of a double conversion UPS you have effectively removed it from the grid, and created your own isolated grid.


You got the wrong kind of inverter. A grid-tied inverter is never used for one specific load; you use it to convert the available power to grid power to supplement anything that might be connected to the grid.

If you want to supply power to a specific load only, you need the other type of inverter, which simply converts DC to AC with no tie to the grid at all.

  • \$\begingroup\$ Well the thing is, 90% of the time the inverter will not meet the load requirements of the circuit and I'd like to supplement from the grid. What if I take grid power from a UPS? Would that prevent leaking back out to the rest of a house/grid?d \$\endgroup\$ – Paul Nov 30 '17 at 19:06
  • \$\begingroup\$ So why is it important that when the inverter output exceeds the load requirements, you don't allow power to flow to the rest of the house? You can't have it both ways. \$\endgroup\$ – Dave Tweed Nov 30 '17 at 19:08
  • \$\begingroup\$ Hey Dave, thanks for the comments. I've updated the question to reflect why I want this. Thanks! \$\endgroup\$ – Paul Nov 30 '17 at 19:32
  • \$\begingroup\$ I find it hard to believe that even a small apartment would ever use less than 30W, so the only thing that the power company would ever notice is that your average net power consumption drops by 30W, which for all practical purposes is undetectable. You would never actually deliver power to the grid. If there's a power outage, your grid-tied inverter will also shut down. \$\endgroup\$ – Dave Tweed Nov 30 '17 at 22:26

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