根据公式P= U2 R 可得，灯泡的额定电压UL= RLP = 5Ω×500W =50V，串联电阻两端的电压U1=U-UL=220V-50V=170V，灯泡的额定电流I= P UL = 500W 50V =10A，电阻R= U1 I = 170V 10A =17Ω．故答案为：17．
