∵P=
U 2
R ,
∴灯泡电阻R=
P =
(220V ) 2
40W =1210Ω,
灯泡接在55V的电源上,灯泡实际功率:
P 实=
U 2实
R =
(55V ) 2
1210Ω =2.5W;
故答案为:55W.