UPS systems are highly efficient when powering a designated load. The problem is the grid, it's far from a designated load. This is because the grid power is highly variable, and the UPS system must constantly adjust its output voltage and frequency to match it. This can result in additional losses and reduced efficiency. Additionally, the reactive power necessary to push out KVARs can also contribute to losses, especially when there are extra reactive elements in the mix.

Another challenge that can affect the efficiency of grid-connected UPS systems is the overall power demand on the grid. If the UPS system is connected to a heavily loaded grid, it may not be able to provide sufficient power to its designated load, which can result in decreased efficiency or even failure of the UPS system.

I'm not saying it's impossible, but with our current grid demands, it's unrealistic to think that we can power the world by solar and wind alone. That's what I'm saying.