Calculating Energy Savings

To calculate the exact value of energy savings by turning a lightbulb off, you need to first determine how much energy the bulb consumes when on. Every bulb has a watt rating printed on it. For example, if the rating is 40 watts, and the bulb is on for one hour, it will consume 0.04 kWh, or if it is off for one hour, you will be saving 0.04 kWh. (Note that many fluorescent fixtures have two or more bulbs. Also, one switch may control several fixtures—an "array." Add the savings for each fixture to determine the total energy savings.)

Then you need to find out what you are paying for electricity per kWh (in general and during peak periods). You will need to look over your electricity bills and see what rate Eskom currently charges per kWh. Multiply the rate per kWh by the amount of electricity saved, and this will give you the value of the savings. Continuing with the example above, let us say that your electric rate is 150 cents per kWh. The value of the energy savings would then be 6 cents (R 0,06). The value of the savings will increase the higher the watt rating of the bulb, the greater the number of bulbs controlled by a single switch, and the higher the rate per kWh.

The most cost-effective length of time that a light (or set of lights) can be turned off before the value of the savings exceeds the cost of having to replace bulbs (due to their shortened operating life) will depend on the type and model of bulb and ballast. The cost of replacing a bulb (or ballast) depends on the cost of the bulb and the cost of labor to do it.

Lighting manufacturers should be able to supply information on the duty cycle of their products. In general, the more energy-efficient a lightbulb is, the longer you can leave it on before it is cost effective to turn it off.

In addition to turning off your lights manually, you may want to consider using sensors, timers, and other automatic lighting controls.

Estimating Energy Use for Appliances

If you're trying to decide whether to invest in a more energy-efficient appliance or you'd like to determine your electricity loads, you may want to estimate appliance energy consumption.

Formula for Estimating Energy Consumption

Use this formula to estimate an appliance's energy use:

(Wattage × Hours Used Per Day) ÷ 1000 = Daily Kilowatt-hour (kWh) consumption

1 kilowatt (kW) = 1,000 Watts

Multiply this by the number of days you use the appliance during the year for the annual consumption in kWh per year.

Estimating Annual Cost to Run an Appliance

Multiply the annual consumption in kWh per year (that you calculated above) by your local utility's rate per kWh consumed to calculate the annual cost to run an appliance. Note: To estimate the number of hours that a refrigerator actually operates at its maximum wattage, divide the total time the refrigerator is plugged in by three. Refrigerators, although turned "on" all the time, actually cycle on and off as needed to maintain interior temperatures.


Window fan:
(200 Watts × 4 hours/day × 120 days/year)  ÷  1000
= 96 kWh × 147.4 cents/kWh
= R141.50/year

Personal Computer and Monitor:
[(120 Watts + 150 Watts) × 4 hours/day × 365 days/year] ÷ 1000
= 394 kWh × 147.4 cents/kWh
= R580.76/year


You can usually find the wattage of most appliances stamped on the bottom or back of the appliance, or on its nameplate. The wattage listed is the maximum power drawn by the appliance. Since many appliances have a range of settings (for example, the volume on a radio), the actual amount of power consumed depends on the setting used at any one time.

When measuring the current drawn by a motor, note that the meter will show about three times more current in the first second that the motor starts than when it is running smoothly.

Many appliances continue to draw a small amount of stand-by power when they are switched "off." These "phantom loads" occur in most appliances that use electricity, such as VCRs, televisions, stereos, computers, and kitchen appliances. Most phantom loads will increase the appliance's energy consumption a few watt-hours. These loads can be avoided by unplugging the appliance or using a power strip and using the switch on the power strip to cut all power to the appliance.