How to calculate monthly electricity bill?_Electrical Power or energy calculation

The unit of energy measurement shown on an energy meter is Kilowatt-hour(kWh).
Therefore, When we say 1 unit, it is 1kWh.
(i.e.,) 1 unit = 1kWh.

Now, to calculate the energy consumed in joules for using an equipment rated at W (say, 40W) for a period of T (say, 1Hr).

We have,
1 watt = 1 joules/second = 60 joules/minute = 3600 joules/hour
Similarly,
1 kilo-watt =  3600*1000 joules/hour = 3.6MJ/h

Therefore, 40W = 40 joules/second

If a 40W light is lit for 1 hour,
Energy = 40 * 1 = 40Wh
For this 40W light to consume 1 unit of power, it should be lit for t = 1000/40 = 25 Hrs.

Approximate power rating for various domestic equipments are listed below. (Ratings may vary)

Appliances/Device	Approximate Load (Watts)
Lamps (Bulb)	60
Lamps (CFL)	15
Tube Lights	40
Electric iron	750
Immersion heater	1000
Water heater/Geyser	2000
Instant Geyser	3000
Refrigerator (165 liters )	150
Window air -conditioner ( 1.5 ton)	2650
Room cooler ( medium)	200
Table fan/ceiling fan	75
Exhaust fan	150
Washing machine	400
Television	100
Mixer	500
Wet Grinder	300
Microwave oven	1200
Computer	200
Mosqutio Repeller	9
Pump motor (1 HP)	740
Battery Charger	7

Similarly you can calculate the energy consumption for home appliances individually for a day and the sum will give energy consumption for a day and multiplied with 30 gives your monthly consumption. Multiplying with 60 gives you bi-monthly consumption.
You can calculate monthly consumption here.