instead of the above this will do, later I realized...
var qtreqdperho = (from qt in ReqdMats
group qt by new { qt.ID } into g
select new ReqdQTYPerMonth
{
Anyag = g.Key.anyagszam,
Hó1Menny = g.Where(ho1 => ERPConnect.ConversionUtils.SAPDate2NetDate(ho1.datum) < DateTime.Now.AddMonths(1) && ERPConnect.ConversionUtils.SAPDate2NetDate(ho1.datum) >= DateTime.Now).Sum(qt1 => qt1.reqmennyiseg)
}).ToList();
So Where().Sum() moved into query select part.