function onEdit(e) {
var range = e.range;
var sheet = range.getSheet();
var activeCell = sheet.getActiveCell();
if (activeCell.getA1Notation() === "YOUR TRIGGER CELL ON THE ACTIVE SHEET") {
var sum = sumH15AcrossSheets();
sheet.getRange('YOUR RESULT CELL ON THE ACTIVE SHEET').setValue(sum);
}
}
function sumH15AcrossSheets() {
var sheets = SpreadsheetApp.getActiveSpreadsheet().getSheets();
var sum = 0;
for (var i = 0; i < sheets.length; i++) {
var sheet = sheets[i];
var range = sheet.getRange('YOUR TRIGGER CELL ON THE ACTIVE SHEET');
var value = range.getValue();
if (!isNaN(value)) {
sum += value;
}
}
return sum;
}