110417-1547 EDT
Your question is not at all clear. A formula does not do the conversion. The components in the inverter box do the conversion. Therefore, within component limitations you work from any desired input voltage to any output voltage.
If your question is how efficient is the inverter? Then the answer is maybe in the range from 70% to 95%.
If you want to know how much power output you get for a given input power, then use efficiency and some other fudge factor to determine this ratio.
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