Most people assume that in order to save on energy costs, you should turn off the light whenever you leave a room, regardless of what type of light it is.  Of course, there are those that drive you crazy, always leaving every light in the house on!  It all boils down to the type of bulb you are using, whether incandescent, halogen, CFL, or LED. Who knew you would need a lesson as to when to turn off your lights?

Here’s a quick tutorial.

Incandescent Lights

The least efficient type of lighting, incandescent lights should be turned off when not in use.  Surprisingly, only about 10% of the energy used to result in light; the other 90% results as heat.  During summer months, turning incandescent lights off will help keep those rooms cooler.

Halogen Lights

Halogen lighting uses basically the same technology as incandescent lighting, although they are more efficient.  CFL and LED lighting are still far more energy efficient lighting options, so turn off halogen lights when not in use.

CFL Lighting

CFL lighting is highly efficient, and because their operating life depends in part on how many times they are turned off and on, it may not be necessary to turn the light off every time you leave the room.  How do you know when to turn a CFL light off?

If you will not return to the room for longer than 15 minutes, turn the light off.

If you will return to the room in 15 minutes or less, leave the light on.

Switching a CFL bulb on and off less often is more important in extending the life of the bulb than turning the light off every time you leave the room, or basically using it less.

Many people are under the assumption that it takes a substantial amount of energy to get a CFL bulb started, therefore it’s best not to turn the light off for short periods of time.  The amount of energy it takes to start a CFL light varies between models and manufacturers. Bulbs that are ENERGY STAR rated hold up to frequent switching on and off, because they are required to hold up to rapid cycling for intervals of five minutes.

Regardless, the relatively higher “inrush” current lasts for 1/120th of one second or half a cycle.  Ultimately, with CFL bulbs the real issue is the value, whether in terms of energy saved by turning the light off or the cost of replacing the light bulb because it burns out quicker when turned on and off frequently.  With fluorescent lighting, more energy is saved when the light is turned off for as few as five seconds than is used when turning a fluorescent light back on.

Factors That Determine the Value of Energy Saved By Turning CFL Lights Off

Rates charged by electric utilities are often different depending on whether the customer is industrial, residential, or commercial.  Depending on the rate schedule, the price can affect the value of energy saved.

During “peak” periods when the consumption of power is highest, it costs utility companies more to generate electricity, which usually results in higher rates to the consumer.  Some utilities also charge different rates depending on what time of the day it is.

Average kWh (kilowatt-hour) consumed, base charges, and/or taxes per billing period and other miscellaneous charges may be added to your bill by your utility company if these charges are not factored into the rate.

Commercial and industrial customers may be charged more per kWh during peak periods by some utility companies.

Customers who consume a certain amount of power may be charged a base rate by some utilities; this base rate may increase as blocks of consumption increase.

LED Lighting

An LED or light emitting diode’s operating life is not affected by switching it on and off.  With fluorescent lamps, the more frequently they are turned on or off, the shorter the lifespan; this is not the case with LED lighting, which gives LED’s numerous advantages in terms of operating.  LED’s, unlike fluorescent lighting, come on almost instantly at full brightness without delay.  This type of lighting is also effective for use with daylight or occupancy sensors that rely on on-off operation, considering switching them on and off has no impact on their operating life.  LED lights also have no glass enclosures or filaments, so vibration doesn’t usually affect them.

How to Calculate Energy Savings

In order to determine the energy you save by switching a light bulb off, you must first know how much energy the bulb uses when in use or “on.”  Every type of bulb has a watt rating printed on it, such as 40 watts, 60 watts, etc.  If a bulb is rated 40 watts and is turned on for one hour, 0.04 kWh is consumed.  The same is true if the bulb is turned off for one hour – 0.04 kWh is saved.  It is important to keep in mind that fluorescent fixtures may have more than one bulb and that in some cases, one light switch controls several fixtures such as a lamp, lighting in a ceiling fan, etc. – an “array.”  To determine total energy savings, you will need to add the savings for each fixture.

Next, look over your electric bill to determine how much you pay for electricity per kWh, both in general and during peak periods.  Notice what your electric provider charges per kWh, and multiply that rate by the amount of electricity you determined you saved to learn the value of your savings.  Using the 40-watt example above, let’s say your utility charges 10 cents per kWh.  The energy saving value for a 40-watt bulb left off for one hour would then be $0.004, or 0.4 cents.  The higher the watt rating of the bulb the higher the value of the savings.  Savings also increases the more bulbs controlled by one switch, and with a higher rate per kWh.

Ultimately, the type and model of bulb and ballast used will determine how long a light or set of lights should be turned off to be the most cost-effective before the savings is more than the cost of bulb replacement due to a shorter operating life.  Of course, the cost of replacing a bulb or ballast can vary, depending on the type of bulb and any labor costs to replace it.

Generally speaking, a highly energy efficient light bulb can be left on for a longer period of time before it becomes more cost-effective to switch it off. Manufacturers should also be able to supply you with information regarding the duty cycle of their products.

Once you know when it is more cost effective to turn your lights off, you may want to consider timers, sensors, and other lighting controls that will turn your lights on and off automatically.

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