Calculating Power Consumption of Lighting

This is a simple calculator to help you work out the total cost of electricity for Lighting. It can be used for your home or business to give you a basic cost of your energy consumption. To use this formula you will need to know the wattage of your light source and the cost of Kilowatt hours.

image-04Wattage (watts):

Is the power required to operate a light fitting or the basic unit of measure for electric power. It refers to the total electricity that is consumed which will include:

  • – The heat generated by the light source
  • – Operational aspects depending on the type of lighting device (such as gear control)
  • – The energy of the light consumed from the lamp.

Kilowatt hours (Kw/h):

Since the number of watts measured over a period of time (by hours) can accumulate into the tens of thousands, energy consumption is measured in kilowatt-hours – one kilowatt (1kW) is equal to 1,000 watts. We also pay for the power we consume in Kw/h of electricity. To help you determine the energy cost rate in your area just look at your last power bill from your power service provider this will give you the operating cost.

Lighting Electricity calculation

This formula can be changed to work for your requirements. The first step is to add up all the total wattage of the lights you are running or would like to setup.

+ Total number of lights = Total watts  

Multiply this by the number of hours the lights are in operation per day.

Watts x Hours per day= ___ watts per day

Then divide the total wattage by 1,000 to obtain the kilowatts.

Watts / 1,000 = ___ Kilowatt hours (kw/h) per day

and then times by the energy cost rate per kWh in your area to work out cost per day.

Kw/h x cost of Kw/h (cents) = ____ cost per day

Lastly times by the number of days of operating the lighting per year to work out annual energy costs.

x days per year (365 days) = $___ total annual energy cost per light

Future Power Costs

This formula only works out power cost at a fixed Kilowatt hour price and does not take into consideration potential increase for electricity cost which may occur. Therefore cost maybe even greater as future electrical price are unknown.

Ballast requirements

All ballasts actually consume energy within themselves while driving luminaires. Luminaires that require a ballast include Metal Halide (MH), Flurescent, High Pressure Sodium (HPS) and High Intensity Discharge (HID). For details, always consult the manufacturer’s specifications to determine input requirements for these components. So to adjust your calculation to include the ballast, the total energy (watts) being drawn from the line is the total of the energy consumed by the ballast plus the energy used by the Luminaire. This additional energy consumption can range from 1 percent to 25 percent of the total kWh. Incandescent and LED Luminaires do not require a ballast an therefore no additional energy is needed to drive the Luminaire.

An Example:

A carpark requires lighting for night time security and pedestrian movement it is recommended that they use 6 x 150 W Halogen Flood lights. These lights a required to be on all nights the current cost of Kw/h is $0.26

+ Total number of lights = Total watts  

6 x 150 watt Halogen Flood Lights = 900 watts

Watts x Hours per day= ___ watts per day

900 watts x 12 hours per day = 9,000 watts per day

Watts / 1,000 = ___ Kilowatt hours (kw/h) per day

9,000 / 1,000 = 9 kw/h per day

Kw/h x cost of Kw/h (cents) = ____ cost per day

7.2 Kw/h X $0.26 = $2.34 cost per day

x days per year (365 days) = $___ total annual energy cost per light

$2.34 X 365 days = $854.10 annual energy cost for carpark lightings.


Photovoltaic Lighting Group.



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