low voltage operating costs

[sitelights]

In general, halogen lamps are more efficient than ordinary incandescent lamps. Watt-for-watt the apparent brightness of halogen vs. incandescent sources is primarilly a function of the "whiter" light produced by halogen lamps. The marginally higher efficacy (lumens per watt) of halogen vs regular incandescent sources is not the reason for lower operating costs. It is the precision of the beam patterns and higher operating temperatures of these halogen lamps that accounts for their increased performance. By lowering the wattage and increasing the precision of the beam shape, 12v halogen MR lamps can be twice as efficient as their 120v halogen counterparts thus halving the operating costs.

If one studies the photometrics i.e. light output quantified by the size of the beam, distance from the object and footcandles (FC) delivered and compares the performance of 120v halogen lamps and 12v halogen lamps it is readily apparent that the lower voltage, lower wattage lamps are more efficient. As a typical example, the precise beam of a low voltage 20w ESX MR-16 (Mirror Reflector) lamp can almost match the footcandles achieved by a 120v halogen 50w PAR30 (Parabolic Reflector) NSP on the surface to be lighted. The low voltage narrrow beam (12 degrees) 20w MR lamp (ESX) delivers a nominal 24 FC at 12' while the 50w PAR NSP (Narrow Spot) lamp (12 degrees) delivers a nominal 28 FC at the same distance. "Nominal" since the photometric tables vary somewhat between manufacturers. If one extrapolates the X2.5 (20w vs 50w) difference by dividing watts into footcandles the 20w MR lamp delivers 1.2 FC/W while the 50w PAR lamp delivers about 1.4 FC/W. The result is about twice the light for the same energy cost.

In residential landscape lighting applications it is seldom necessary to exceed 20w per fixture. This contention is rejected by some low voltage landscape designer/installers who are fixated on high surface brightness as a design element. [See "grazing vs washing" on the Design forum for an example.] Subjectively it is the contrast ratio between what is illuminated and what is not that determines perceived brightness. High light levels in residential low voltage outdoor lighting applications are actually detrimental to seeing the lighted features of the landscape as a cohesive whole. Brighter is not better; it is worse. An excessively bright landscape lighting experience diminishes the appreciation of the modeling and perspective effects that add dimension to the view. Bright lighting ups the ante (brighter and brighter) as, similarly, voices in a room full of talking people rise (louder and louder) in order to be heard.

This paragraph was added 11.04.07: In the almost 4 years since the original post "Dark Sky" considerations have entered the equation. It is true that the Dark Sky guidelines and model regulations being promulgated are intended to correct major offenders such as parking lot lighting and signage. It is also true that the guidelines concentrate more on unshielded sources than on FC output.* However landscape lighting applications invariably direct a significant amount of light skyward; this intentional effect is not without unintended consequences. It is difficult to imagine a landscape lighting design that uses no uplighting and so a reduction of wattage aimed skyward is simple good sense and also good citizenship.

An outdoor low voltage lighting project becomes increasingly more expensive in initial as well as in operating costs as the wattage increases. A system comprised of 20 luminaires lamped at 50w costs 2.5 times as much as a 20 luminaire system lamped at 20w to operate; it is also more expensive to install. A 1000w (20 X 50W) system would require two 600w (2X300w) transformers plus two controls; a 400w (20 X 20w) system requires only one 600w (2X300w) transformer with a single control and has reserve capacity for an additional 10 luminaires lamped at 20w. The 50w system has only enough reserve capacity to handle 4 more luminaires. It is my belief that the misguided trend toward higher (up to 22v) secondary outputs is a function of the difficulty in achieving 12v at the lamp in these high load conditions. It is regrettable that those wedded to high output lamps do not recognize the unintended consequences of their folly. In low voltage outdoor lighting applications higher wattage increases the hazards of fire [See "fires" on the Safety forum.], increases the cost of a system, increases the operating cost and has a negative impact on the design. Additionally, the perceived necessity to use higher and higher voltages and larger and larger transformers (coupled with the use of inadequate ampacity distribution cable i.e. 12 gauge or even 14 gauge), contravenes the intent of UL1838 which is to guard the safety of the consumer.

The manufacturers continue to ignore, or worse, add to this trend toward increased system loads and the attendant increased acquistion and operating costs ultimately borne by the consumer. The makers have abdicated any leadership role they may have had in bringing the low voltage outdoor lighting business to maturity. In every other aspect of lighting the manufacturers are promoting energy conservation as a "green" alternative; low voltage landscape lighting equipment manufacturers have not seen fit to act intelligently and responsibly in this regard. In this leadership vacuum it is even more important that the designer/installers with professional interests in this field recognize the irresponsibility of continuing this trend. Without self-regulation it is only a matter of time before regulations will be imposed. It is in the best interests of professionals in this business to conform to an emerging set of standards and adopt methods and procedures that will enhance the safety and success of their endeavors.

*Search "Dark Sky" for more information.

[LowVoltSparks]

Can anyone tell me if there are energy savings using Low Voltage Lighting beyond being able to use a lower watt bulb to achieve the same lumens or foot candles?

Does the power used on a 50 Watt Line Voltage (120VAC) = energy used on 50 watt low voltage (12 VDC)?