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?
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.