A normal fan, you could see at the back of it the max power of it. 55W for example. Same goes for lightbulb, 18w or so on. Just divide with 1000 then times how many hours you using, then times the tariff. This is for AC, your mouse is DC, doesnt count.

For example, a 100W bulb. Used for 100 hours per month

100w/1000*100hours = 10kwh
10kwh * 0.10sen/kwh(tariff) = rm1 per month.