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LED Light Power & Efficiency

LED Light Power

The power (P) of any electrical device including a LED light is measured in Watts (W), which is equal to the current or electricity drawn (I), measured in ampres, multiplied by the voltage (V).

                P = V x I

Therefore, the power of the LED light is proportional to voltage and/or current such that a device may have a low voltage but may still draw a very high current and have high power consumption. For example, the traditional 50W dichroic halogen downlight is only 12V AC but draws 4.167 Ampres.

LED lights by their nature are low voltage but also relatively low current making them lower in power and more efficient that traditional incandescent bulbs and halogen downlights. Generally we are talking about between 100 to 750 milli-amps depending on the forward voltage required to turn on the LED. In this regard, just because a LED light uses a higher current, it does not mean that it will be brighter. Rather it is dependent on the power which is proportional to the increase in voltage and/or current. There is some advantage to having higher voltage LEDs where long distances occur between the LED and power supply such as in Strip LED lighting. However, for most applications it does not really matter.

Typical power ranges for general purpose residential and commercial bulbs range from as little as 3W to 15W. Generally the higher the power the greater the current and therefore the greater the light output. However, this is not always the case and brings us to the concept of efficiency and power factor.

LED Light Efficiency

The efficiency of the LED light is measured in lumens per watt (Lm/W) which refers to the total quantity of light the LED lamp produces per 1 W of energy. 

                Efficiency = total lumen output / total power

Older LED chips found in older generation LED bulbs from as recent as 2008 – 2010 produce less light per watt than the 2011 – 2012 LED chips found in more modern LED bulbs. For example, a 2012 7W bulb with a CREE XT-E chip can produce more light or lumen output than a 12W bulb with an older CREE XP-E chip. More modern LED bulbs also have better heat sink designs which enable higher light outputs.

The important message is that higher power does not always mean more light and "bigger is not always best". Ultimately for the consumer it is important to do your research or to "try before you buy". Consider referring to our LED buying guide checklist in the section on LED Life Time as a way to screen out potentially inefficient or unreliable products. 

LED Efficiency versus Lamp Efficiency 

As discussed in the section on Lumen levels in "Understanding LED Lighting" you must also be careful to ensure that the retailer information is specifying the Lamp Efficiency rather than the LED efficiency. Due to the inherent loses in the bulb the lamp efficiency will always be less than the LED efficiency depending on the design. This includes thermal effects, driver losses and optical inefficiencies which all combine to reduce the overall efficiency of the LED bulb or luminaire compared to the internal LED package or chip. Collectively, these losses can decrease the efficiency by more than 30%. In such cases a manufacturer may specify a LED MR16 bulb has having 720lm but in reality for the LED lamp it is only approximately 500lm.

LED Lighting and Power Factor 

A further complication is Power Factor (PF) which is a value less than 1.0 which measures the efficiency of the LED driver or power supply. Essentially an electrical device may be rated at a Power of 100W but actually consume more than 100W due a phase delay between the instantaneous voltage and the instantaneous current. Remember that Mains power is AC or alternative current and it is made up of sunusiodal wave forms of oscillating voltage and oscillating current. Ideally these two wave forms are synchronous (PF=1) but due to the nature of electronics or inductive loads such as electric motors a delay emerges between the voltage waveform and the current waveform leading to a wasteage of electrical power or a Reactive Power which is incapable of doing any work. Therefore, a device may be rated at 1000W Real Power but consume 1500W Apparent or Active power due to a PF of 0.67 and end up wasting 500W or 1/3 of the total power consumed due to the current being out of phase. Noting that for the electrical device to make use of the current it must be in phase with the voltage given power is equal to voltage x current or P=VI.  

PF is generally only a problem in commercial applications in inductive devices which use very high power such that the delays between current and voltage add up to produce significant power losses. Other components which will cause delays betweeen current and voltage include transformers and voltage regulators and ballasts in flourescent lighting. In residential settings such loses are relatively minimal and electricity companies will only charge for the real power anyway. However, there is still a loss so those energy conscious or green individuals may wish to examine the power factor of their LED lighting power supplies to ensure that have a PF of greater than 0.8 to ensure minimal energy loss. In fact US Department of Energy (DOE) Energy Star program mandates minimum acceptable power factors or 0.7 and 0.9 respectively for domestic and commercial LED lights. 

Most power supply devicies these days will have some form of either passive or active power factor correction leading to PF of > 0.9 so minimal power losses can be achieved. One exception is ultra high dimming drivers which dim down to 1%. Due to the high capacitive loads required to stablise the current at very low dimming levels to avoid flicker PF is poor, generally approx 0.65 meaning that a 10W rated LED bulb will consume approximately 15.4W (or VA, apparent power) near full load. However, in practice this is not a big problem given these drivers are generally used in applications where the lamps will be dimmed down to low levels for most of their life such that the real power is 2 or 3W and the Apparent power still very low at up to 4.6W.

If dimming is only going to be occassional then we suggest comprimising on minimum dimming effect with a driver which has a PF > 0.9. Although as noted this is mainly an issue in commercial lighting where lamps are on for 8 to 24 hours per day. If you are a residential user you can be less concerned. Neverthless please note the PF ratings on the speicfications tab of our products to get an idea of efficiency. 

If you are a commercial customer with a business that is particularly dominated by highly inductive loads such as electric motors or a large collection of capacitive loads with poor PF, then you should consider power factor correction (PFC) and visit our section on government rebates and schemes where such projects can be subsidised. If you are a very high energy user then PFC can leading to very large savings in electricity and power bills. 

References:

Energy Efficiency of LEDs. Building Technologies Program. Solid-State Lighting Technology Fact Sheet. US Department of Energy. www.eere.energy.gov

Renewable and Efficient Electric Power Systems. 2004. Gilber M. Masters

Power System Analysis. 2007. PP Deo

Power Factor Correction Techniques in LED Lighting. August 2011, Electronic Component News 

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