That 1.414 Figure is ONLY For TRUE SINE WAVE.
Your Inverter is NOT True Sine Wave, It is Most Likely SQUARE WAVE.
You Could take the Transformer Apart and See What Size of wires are on the Primary and Secondary.
Than Based on a Reasonable Value of about 700 Circular Mils per Inch, You could Calculate a Typical Current rating.
(A Circular Mil is 1/1000 of an inch.)
And are you ONLY interested in the Transformer Spec's?
Or the Actual Output of the Inverter?
If for the Inverter, There are other things to Consider:
Is this transformer Wound Correctly for an Inverter Circuit?
EXAMPLE: The Primary Winding NEED to be Bifilar Wound, For Maximum Efficiency.