calculating a percentage of a selection

Is the window in your image one contiguous area (no mullins or more than one window), and is it straight on, no perspective? If so, you can try this script that will show the percent of a selection.

var doc = activeDocument;
try{
    var selArea = (doc.selection.bounds[2]-doc.selection.bounds[0]) * (doc.selection.bounds[3]-doc.selection.bounds[1]);
    var docArea = doc.width * doc.height
    alert(selArea / docArea)   
    }
catch(e){
    alert('there is no selection')
    }

If you merge the layers and have a single layer with transparency where the windows are, this script will tell you exactly the percent of transparent pixels. you need to have the merged layer selected for this to work.

var doc = activeDocument
var curLay = doc.activeLayer
var trans = true;
getSelc ();
try{var b = doc.selection.bounds}
catch(e){trans = false}
var his = doc.histogram
var hisCount = 0
for (var i=0;i<his.length;i++){
    hisCount+= his
    }
var perc= 1-(hisCount/(doc.width.value*doc.height.value))
doc.selection.deselect()
if(trans){alert (perc)}
else{alert('The layer is transparent.')}
function getSelc(){
       var idsetd = charIDToTypeID( "setd" );
        var desc7 = new ActionDescriptor();
        var idnull = charIDToTypeID( "null" );
            var ref5 = new ActionReference();
            var idChnl = charIDToTypeID( "Chnl" );
            var idfsel = charIDToTypeID( "fsel" );
            ref5.putProperty( idChnl, idfsel );
        desc7.putReference( idnull, ref5 );
        var idT = charIDToTypeID( "T   " );
            var ref6 = new ActionReference();
            var idChnl = charIDToTypeID( "Chnl" );
            var idChnl = charIDToTypeID( "Chnl" );
            var idTrsp = charIDToTypeID( "Trsp" );
            ref6.putEnumerated( idChnl, idChnl, idTrsp );
        desc7.putReference( idT, ref6 );
    executeAction( idsetd, desc7, DialogModes.NO ); 
    }