有關計算用電的時間(f.2 i.s.)

for AC220V power supplying,

0.99kW=0.396kWh
h=0.5

time in use is 2.5 hours.

since the power(W,kW) times times in use(h),equal to kWh,

so the definition of kWh is 1000W electrical appliance operating continuosly in 1 hour.

2010-02-26 22:38:53 補充：
here a one mistake in my answer,

h=0.5 should to written as h=2.5

Since 990 W = 0.99 kW, hence no of hours during which the oven is in use
= 0.396/0.99 hours = 0.4 hour = 0.4/24 days = 0.0167 days