Lighting

Different lightbulb types:
(1) Light emitting diode (LED) shown with DC polarity marked (2) Compact Fluorescent Lamp (CFL) (3) Incandescent

Lighting is the most common use for an off-grid PV system. Choosing the right type of lighting is not only important to ensuring that a system functions properly, but also to making sure that whoever is using the system is satisfied as the type of lighting, brightness and color can have a dramatic impact on user experience. There are three types of light bulbs commonly found on the market:

• Light Emitting Diode (LED) - this design uses what is called a diode which creates light when electrical current passes through it. LED lighting is the only appropriate option for use in an off-grid PV system. LED lighting as it is far more efficient and durable than its predecessors - incandescent (traditional bulbs) and compact fluorescent lamps (CFL). LED bulbs cost slightly more than these other two options initially, but are cheaper when their longer life cycle is considered, not to mention the importance that their reduced consumption contributes to reducing overall PV source and energy storage system size. Reductions in the cost of LED lighting, as much as reductions in the cost of PV modules, have contributed to increasing accessibility of PV systems for people in rural areas that lack electricity. Commonly available in both alternating current (AC) and direct current (DC).

• Compact fluorescent lamp (CFL) - this design creates light as current passes through a gas filled glass tube. These bulbs contain mercury - a hazardous element - which requires special recycling facilities. If found in remote areas, these bulbs should be returned to a proper electronics waste recycling facility. CFL bulbs should not be used with off-grid PV systems due to their low efficiency.

• Incandescent – The original lightbulb design. It creates light by passing current through a thin wire filament. Incandescent bulbs should not be used with off-grid PV systems due to their low efficiency.

Characteristics

LED lighting is the right choice for off-grid applications, but it is important to distinguish between the different designs to ensure that there is a clear understanding of the advantages and to highlight the important role that energy efficiency plays in PV system design.

Light output

Higher energy consumption for a particular lighting design (LED, CFL, incandescent) will translate into improve light output, but a 40 W incandescent bulb will produce less light than a 10 W LED lightbulb. This is because the LED bulb can more efficiently turn energy into light than an incandescent bulb which loses a significant amount of energy in the form of heat. Lumens are the proper unit of measurement for determining the brightness of a light source – a higher lumen rating means a brighter light.

	200 lumens	400 lumens	700 lumens	900 lumens
LED	4 W	6 W	10 W	13 W
CFL	6 W	9 W	12 W	15 W
Incandescent	25 W	40 W	60 W	75 W

Lifespan

LED lights last far longer than CFL or incandescent lights.

	Lifespan (hours)
LED	25,000 - 35,000
CFL	10,000 - 15,000
Incandescent	750-1,500

Color temperature

Visible light spectrum with key Kelvin values and corresponding colors.

Different light sources (the sun, candles, different light bulb types) emit light of different colors. A candle emits a soft yellow light that is pleasing to many people whereas the light from the sun can be quite white and harsh. The color of visible light can be classified using a measurement called Kelvin - a higher Kelvin rating means a whiter light. Different lighting products will use Kelvin to classify the color of the light that they will emit. In addition to considering what color of light may be most appropriate for a given application (home, clinic, community meeting hall), different cultures have varying lighting color preferences that should be taken into account. Thankfully For the same brand and model of LED lightbulb there are often various color choices to choose from.

Color temperature (Kelvin)	Source
1850 K	Candle flame, sunset/sunrise
2400 K	Standard incandescent lamps
2550 K	Soft white incandescent lamps
2700 K	"Soft white" CFL and LED
3000 K	Warm white CFL and LED
5000 K	Tubular fluorescent lamps or cool white / daylight CFL
6000 K	Vertical daylight, electronic flash
6500 K	Daylight, overcast