It sounds as though you are doing what is called an energy audit - a review of how much energy you use.
For everyday electrical devices you need to determine how much power they use and how many hours do you use them. The power is measured in kiloWatts. The way the electric company measures the consumption of electrical energy is by kiloWatt-hours or kW-h.
Let's take an average PC. They typically use 100 Watts of power to run. To find kiloWatts we need to convert this number (remember metric conversions?). Just divide the wattage by 1000 to get kiloWatts. Thus the PC runs at 100/1000 = 0.1 kW. (If the decise is rated at kW you don't need to do this. Just use its kW.)
How much do you use your computer per month? Let's say you use it for 8 hours per day. That would be 8 hours x 30 days = 240 hours per month.
The energy consumption is just the kiloWatts x hours = 0.1 x 240 = 24 kW-h per month.
The electric company adds up the kW-h from all the devices you use - TV, stereo, lights, refrigerator, AC - and bills you for it at a fixed rate. My electric company bills me at 13 1/2 cents / kW-h. It may be instructive for you to look at your household electric bill.