To calculate energy consumption follow this simple formula. Energy (E) in kilowatt-hours (kWh) is equal to the power (P) in watts (W) multiplied by the number of hours used (t) per day divided by 1,000 watts per kilowatt.
E(kWh/day) = P(W) × t(h/day) / 1000(W/kW)
You can find more information about watts and kilowatts here.
Understanding your energy usage will help you understand how your habits affect your pocketbook in the form of your electricity bill and how large your carbon footprint is. Making the right choices, even on small energy consumers like light bulbs, can save you money and reduce your carbon footprint significantly.