If you have a UPS powering some load in an off-grid situation, and you have a solar panel with a grid-tie microinverter, is it possible to connect the grid-tie inverter to the UPS output, such that the microinverter assists with powering the load plugged into the UPS, and if there's an excess of power available, actually charge the UPS batteries?
I've read that it's possible with certain kinds of inverters, and that Tesla's "Powerwall 2" is apparently one of them. It accepts power from grid-tie inverters on its output and uses it to charge its batteries, or if they are full to feed back into the grid.
I thought this kind of function would require additional circuitry to extract the extra incoming power, but from what I've read, it sounds like there are some inverter designs that inherently have this capability.
Is this true? What sort of designs would work this way? Is there any quick way you could tell whether a given UPS is capable of this, without getting hold of a schematic?
Best Answer
Yes, As long as the voltage criteria are satisfied a mosfet will happily allow current flow in either direction. So it's not difficult to design an inverter that can feed power in both directions. Indeed APC made some UPSes which they called "delta conversion" that explicitly exploited this, the battery bank was connected to the output through a single converter, then a separate "delta converter" was used to push power from the input side of the UPS to the output side.
Having said that though, the devil is in the details. I would not connect a grid-tie inverter to the output of a UPS that was not explicitly designed to support it. I would be concerned about the potential for bad interactions between the control systems of the two converters, and about the possibility for excessive voltages on the DC bus of the UPS.