15 g is a very small amount of beans, which is rather unsuitable as a single dose for grinding. You may need to try out what quantity (depending on the selected grinding degree) is ejected by the machine. If you mainly need small quantities of powder and do not want to pre-grind them, you could also switch to a grinder with a beater mechanism. This makes it possible to measure the exact amount of beans, grind them and remove them from the machine yourself. A good choice for this is our EKM 100 model.