Volume 9 Issue 3
July 2006    
application - The use to which a lighting system will be put; for example, a lamp may be intended for indoor residential applications. ballast - A device required by electric-discharge light sources such as fluorescent or HID lamps to regulate voltage and current supplied to the lamp during start and throughout operation. color rendering index (CRI) - A rating index commonly used to represent how well a light source renders the colors of objects that it illuminates. For a CRI value of 100, the maximum value, the colors of objects can be expected to be seen as they would appear under an incandescent or daylight spectrum of the same correlated color temperature (CCT). Sources with CRI values less than 50 are generally regarded as rendering colors poorly, that is, colors may appear unnatural. compact fluorescent lamp (CFL) - A family of single-ended fluorescent-discharge light sources with small-diameter [16-millimeter (5/8-inch) or less] tubes. high-intensity discharge (HID) - An electric lamp that produces light directly from an arc discharge under high pressure. Metal halide, high-pressure sodium, and mercury vapor are types of HID lamps. grid - The combination of electric power plants and transmission lines operated by an electric utility. lamp - A radiant light source. lumen (lm) - A unit measurement of the rate at which a lamp produces light. A lamp's light output rating expresses the total amount of light emitted in all directions per unit time. Ratings of initial light output provided by manufacturers express the total light output after 100 hours of operation. luminaire - A complete lighting unit consisting of a lamp or lamps and the parts designed to distribute the light, to position and protect the lamp(s), and to connect the lamp(s) to the power supply. (Also referred to as fixture.) correlated color temperature (CCT) - A specification for white light sources used to describe the dominant color tone along the dimension from warm (yellows and reds) to cool (blue). Lamps with a CCT rating below 3200 K are usually considered warm sources, whereas those with a CCT above 4000 K usually considered cool in appearance. Temperatures in between are considered neutral in appearance. Technically, CCT extends the practice of using temperature, in kelvins (K), for specifying the spectrum of light sources other than blackbody radiators. Incandescent lamps and daylight closely approximate the spectra of black body radiators at different temperatures and can be designated by the corresponding temperature of a blackbody radiator. The spectra of fluorescent and LED sources, however, differ substantially from black body radiators yet they can have a color appearance similar to a blackbody radiator of a particular temperature as given by CCT. efficacy - The ratio of the light output of a lamp (lumens) to its active power (watts), expressed as lumens per watt. halogen lamp - An incandescent lamp that uses a halogen fill gas. Halogen lamps have higher rated efficacies and longer lives than standard incandescent A-lamps. illuminance - The amount of light (luminous flux) incident on a surface area. Illuminance is measured in footcandles (lumens/square foot) or lux (lumens/square meter). One footcandle equals 10.76 lux, although for convenience 10 lux commonly is used as the equivalent. luminance - The photometric quantity most closely associated with the perception of brightness, measured in units of luminous intensity (candelas) per unit area (square feet or square meter). glare - The sensation produced by luminances within the visual field that are sufficiently greater than the luminance to which the eyes are adapted, which causes annoyance, discomfort, or loss in visual performance and visibility. lumen maintenance - The ability of a lamp to retain its light output over time. Greater lumen maintenance means a lamp will remain brighter longer. The opposite of lumen maintenance is lumen depreciation, which represents the reduction of lumen output over time. Lamp lumen depreciation factor (LLD) is commonly used as a multiplier to the initial lumen rating in illuminance calculations to compensate for the lumen depreciation. The LLD factor is a dimensionless value between 0 and 1. footcandle (fc) - A measure of illuminance in lumens per square foot. One footcandle equals 10.76 lux, although for convenience 10 lux commonly is used as the equivalent. lux (lx) - A measure of illuminance in lumens per square meter. One lux equals 0.093 footcandle. driver - For light emitting diodes, a device that regulates the voltage and current powering the source. illumination - The process of using light to see objects at a particular location. PN junction - For light emitting diodes, the portion of the device where positive and negative charges combine to produce light. fluorescent lamp - A low-pressure mercury electric-discharge lamp in which a phosphor coating on the inside of the glass tubing transforms most of the ultraviolet energy created inside the lamp into visible light. inverter - Also known as power inverter. A device used to convert direct current (dc) electricity into alternating (ac) current. irradiance - The density of radiant flux incident on a surface. light-emitting diode (LED) - A solid-state electronic device formed by a junction of P- and N-type semiconductor material that emits light when electric current passes through it. LED commonly refers to either the semiconductor by itself, i.e. the chip, or the entire lamp package including the chip, electrical leads, optics and encasement. photon - A small bundle or quantum of electromagnetic energy, including light. photovoltaic (PV) - Photovoltaic (PV) cells produce electric current from light energy (photons). PV cells are joined to make PV panels.
How is system efficacy calculated for PV lighting systems using various light sources?

When calculating the total efficacy of a photovoltaic (PV) lighting system, take into account the efficacies and/or efficiencies of all of its individual components (Zhou and Narendran 2005). This calculation is relevant to sizing the PV system (see What information is needed to specify a PV lighting system?). One of these components is the light source. When selecting a light source for a stand-alone PV lighting system the following factors should be taken into consideration:

  • Lumen (lm) package (total light output of the light source)
  • Power requirements (total watts required to power the light source)
  • System efficacy (efficacy of the light source and any ballast or driver needed to operate it)
  • Environmental issues (impacts the system will have on the environment such as disposal of components and reduction in atmospheric pollution from lower power plant emissions)
  • Climate (effect of high or low temperatures on the light source's light output and life)
  • Lumen maintenance (ability of the light source to sustain its light output over time)
  • Life of the light source (amount of time over which the light source will operate reliably).

The total system efficacy of a PV lighting system is associated with the efficacy of the light source and the efficiencies of the other system components. The typical luminous efficacies of various light sources are:

Note: LED lamp efficacy has improved rapidly in recent years, and a series of premium white LED products are now achieving efficacies about 35 to 40 lumens per watt (at time of publication).

The formula below can be used to calculate the overall system efficacy for a stand-alone PV lighting system:

E = f / P

     f: Light output from PV lighting system (in lumens)
     P: Input solar power (in watts)


f = P*hPV * hbat * hele * Esrc * hlum
We have
E = P*hPV * hbat * hele * Esrc * hlum / P
  = hPV * hbat * hele * Esrc * hlum
hPV: PV panel efficiency, assume hPV = 15%
hbat: Battery efficiency, assume hbat = 80%
hele: Product of efficiencies of all electronics, which may include efficiency of charge controller (hcha), inverter (hinv), and ballast/driver (hdri)
Esrc: Efficacy of light source, as lumens per watt
hlum: Luminaire efficiency, as the ratio of total output lumens from the luminaire to total lumens from the lamps

We assume that hPV= 15%, hcha, = 90%, hbat = 80%, and the battery is large enough to meet the maximum power demand of the system. The PV efficiency of 15% is on the high end, which may be achieved with premium level PV panels available on the market. The battery efficiency of 80% is also on the high end, which implies high-quality batteries and little conduit loss. Table 5 shows the remaining assumptions used to calculate system efficacies for different system configurations.

Table 5. Assumptions used to calculate total system efficacy

White LED   Halogen   CFL   LFL

ηdri 85%  No inverter or
regulator necessary
  ηinv 80%   ηinv 80%

Esrc 25 LPW  Esrc 20 LPW  Esrc 65 LPW  Esrc 85 LPW


ηlum 85%   ηlum 75%  ηlum 60%  ηlum 70%


ηlum 85%  ηlum 80%  ηlum 70%  ηlum 80%

Note that in this table, CFL and LFL (linear fluorescent lamp) luminaire efficiencies are higher in general lighting applications than in directional lighting applications. This is important to consider because outdoor lighting applications, especially those using low light levels, tend to favor directional lighting and allow a user to aim or focus the light where needed.

To illustrate the calculation listed above, consider the case of a PV lighting system using white LEDs as the light source. The system efficacy in directional lighting applications, expressed as lumens per watt (LPW) of solar power reaching the PV panels, can be calculated as follows:

ELED = hPV * hbat * hcha * hdri * Esrc * hlum
                                   = 15% × 80% × 90% × 85% × 25 LPW × 85%
                                   = 2.0 LPW

In comparison, a lighting system using a CFL as the light source does not need a dc current regulator, but it does need a dc-to-ac inverter. The system efficacy can be calculated as follows:

ECFL = hPV * hbat * hcha * hdri * Esrc * hlum
                                   = 15% × 80% × 90% × 80% × 65 LPW × 60%
                                   = 3.4 LPW

Note that the efficacy values above were calculated for every watt of solar power that arrives at the PV panels. The ideal solar radiation power is 1000 watts per square meter at sea level, which is approximately 100 watts per square ft.

Using similar calculations, NLPIP obtained the results found in Table 6 (in "What factors should be considered when selecting a luminaire for PV lighting?"), which summarizes the system efficacy (LPW) for PV lighting with different light sources in directional lighting applications. Since LED technology is rapidly evolving, the efficacies of LED are expected to increase significantly over the next 15 years. According to U.S. Department of Energy's roadmap on solid-state technology (U.S. Department of Energy 2005), for example, the efficacy of white LEDs is expected to reach 100 lumens per watt by the year of 2010, and 140 lumens per watt by 2015. Therefore, Table 6 contains projections of system efficacy using white LED lamps in future years.

Table 6. System efficacy for stand-alone PV lighting with different light sources (with projection for future years)

Year White LED efficacy
PV System Efficacy
(lumens per watt received from solar)
dc light sources ac light sources

White LED Halogen CFL LFL

2003 25 2.0 1.7 3.4 5.1
2006 40 3.2 1.7 3.4 5.1
2010 100 7.9 1.7 3.4 5.1
2015 140 11.1 1.7 3.4 5.1

In the survey conducted by NLPIP (see "What are some common beliefs about PV lighting systems?") respondents indicated that LEDs were the most suitable light sources for PV lighting applications followed by fluorescent lamps, while halogen light sources were considered not suitable for PV lighting applications. However, given current lamp efficacies, PV lighting systems using fluorescent lamps are the most efficient. PV lighting systems using white-light LEDs are considerably more efficient than systems using halogen lamps, but neither is as efficient as those using fluorescent lamps.

In addition to system efficacy, the impact of the outdoor environment on the PV lighting system also needs to be evaluated. For example, even though ac light sources such as fluorescent lamps have a higher luminous efficacy than LEDs or incandescent lamps, they may have starting problems and/or reduced light output in low temperature environments.

Lumen maintenance and lamp life should also be considered when choosing a light source for a PV lighting system. CFLs and LFLs tend to have very good lumen maintenance (up to 90% or greater) and long average rated lives (ranging from 6000 to over 20,000 hours). However, these projections are based on lamps operating under ideal power conditions. Because the power conditions provided by PV lighting systems may vary much more than those provided by grid-powered lighting systems, it may be reasonable to assume that both the life and lumen maintenance of these sources would be reduced. There are no data available to end users concerning the performance of these light sources under PV power conditions, so the risks of using them in PV lighting systems increases. LEDs and incandescent light sources, on the other hand, are generally less affected by variable power conditions than their gas discharge (fluorescent and HID) counterparts.

CFLs and LFLs also have relatively high total lumen output per lamp and are relatively high wattage (e.g., 9 watts or greater). LEDs and incandescent lamps, on the other hand, are available in very low-wattage versions, providing very low light output. White-light LEDs, for example, are available in versions requiring as little as 0.2 watts. These low power requirements and low total light output are well suited to low light level PV lighting applications because they allow for the use of low-mounted luminaires with small PV panels and small battery capacity. Therefore, assuming low light levels are acceptable for the application, PV powered luminaires using these light sources (LEDs or incandescent lamps) are a much more viable option than larger, high-mounted luminaires using CFLs or LFLs.


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