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Question: I have equipment with about 600W of consumption. My plan is to connect 3 solar panels with MicroInverter to have a GridFree installation. However if the equipment turns off for some reason, the inverters will keep working and will feed the unused energy back to the grid. How to prevent this?

Answer: The current monitoring relay is the solution. This type of relay will connect the switching contact when the current flow will over-reach a certain defined level. When the current flow falls down again the relay will disconnect. 

The relay allows to set the operation so that the solar panel AC circuit will connect only after the equipment is taking sufficient energy. After connection the MicroInverters will start working and they will compensate the need for energy from the grid. In case the equipment turns off suddenly, the relay will disconnect the  solar panel AC circuit and the flow fill be fully from the grid again.  This installation will prevent from the overflow to the grid.

See the design of the installation on the picture above. (The picture is only a components design, not a complete wiring)

Hint: it may be possible to make a cascade of the current monitoring relays (more relay switching one by one) and this way to turn off/on the AC circuits based on the changing levels of the power in the monitored line. This way it is possible to have more steps of the power regulation to avoid overflow back to the grid.

See the PRI-32 current monitoring relay at EV-POWER