I am not grid tied except through the ATS.
ATS priority is 1st solar. 2nd PEA.
Mine is a single phase ATS. If I have a need to use PEA at any time, all I have to do is disconnect my inverter output and the ATS will automatically switch over to PEA.
Transfer time is between ½ second to 1 second.
How does the ATS work?
There is a relay permanently energised by priority 1 input.
Relay contacts are arranged to connect rectified AC power to a small motor which controls, via a mechanical linkage, two mcb's in an OR logic way.
If power is off from the inverter then the relay is de-energised, the motor runs one way and switches the mcb's over.
If power from the inverter is put back on then the relay is energised and the motor runs the other way to switch the mcb's back to priority 1 input.
The mechanical linkage is arranged so that it is impossible to have both mcb's in the same state.