Windy Dankoff

Three Types of Electric Lights

(1) INCANDESCENT (the common light bulb): Electric current passes through a thin tungsten metal filament causing it to heat white hot and emit light. The absence of oxygen in the glass bulb prevents rapid oxidation (burning) of the filament. The tungsten evaporates gradually, causing thin spots on the filament (while clouding the glass) leading to reduced efficiency &eventual failure.

(2) QUARTZ-HALOGEN (also called Quartz-Iodide, Tungsten-Halogen): An improvement on the incandescent bulb, works on the same principle except the tungsten filament is run at a higher temperature resulting in brighter, whiter light and higher efficiency. Ordinarily, this would result in short bulb life, so the bulb is (1) filled with "halogen" gas which slows the rate of evaporation of the tungsten (2) made smaller so the glass temperature is much hotter (this helps prevent tungsten from condensing on the bulb) and (3) made of a special "quartz" glass to tolerate the high temperature. Quartz-Halogen light is a very bright white (less red component) which aids the eye in perceiving detail. The most common applications are vehicle headlights, projectors, and spot-lights for displaying art work and merchandise.

(3) FLUORESCENT: Electric current flows thru a gas-filled glass tube, generating ultraviolet light (invisible). The tube is coated on the inside with a phosphorescent material which absorbs the ultraviolet and glows white. Very little heat is generated and efficiency is high. All fluorescent tubes require over 100 volts to operate, so low voltage fluorescents use a transistorized "ballast" to step up the voltage. LOW VOLTAGE DC FLUORESCENTS USE THE SAME TUBES AS 120 vac LIGHTS.

Incandescents, quartz-halogen and fluorescent lights differ in five major ways: (1) efficiency (2) life expectancy (3) installed cost (4) light quality and (5) light dispersion. Consider each separately.

(1) EFFICIENCY: Quartz-Halogen bulbs average 30% higher efficiency than incandescents. (Higher efficiency claims are based on comparison with worst-case incandescents.)

Fluorescent lights average 3 times the efficiency of low voltage incandescents (5 times compared with 120V incandescents!). We are assuming high quality fluorescents. (Some cheap ones are dim and less efficient in comparison).

Efficiency may vary widely even within the same class of light. For instance, low voltage (12 or 24V) incandescents are more efficient than 120 volt (common household) bulbs. This is because the low voltage bulb has a shorter, thicker filament (to pass higher current)

so it is physically stronger, allowing a higher operating temperature. Just the shift from 120 volt incandescent bulbs to 12/24V bulbs (inexpensive RV and automotive bulbs) can reduce energy usage by an average 40%! Within the same voltage, incandescents vary. Long life and rough service bulbs run a cooler filament and have the lowest efficiency.

(2) LIFE EXPECTANCY: Incandescents have the shortest life, typically 1,000 hours (about a year of every-evening use.) Quartz-Halogen bulbs last longer--about 3,000 hours. Unlike incandescents, quartz-halogen bulbs do not blacken over time. They retain peak efficiency until the end. High quality DC fluorescents last longer yet--up to 10,000 hours, which can be 10 years of living room use!

(3) INSTALLED COST: Most fluorescent lights come with their own fixtures, ready to screw right to the wall or ceiling. The installed cost of a quartz-halogen or incandescent bulb must include the cost of a light fixture. Quartz-halogen bulbs cost 3 to 10 times as much as incandescents. However, their superior performance make them popular in renewable energy homes. Good fluorescents also cost 3 to 10 times aa much as incandescents (when you count the cost of incandescent fixtures). But, their cost is easily justified by radical gains in efficiency and life expectancy.

WIRING COST (FOR DC CIRCUITS): 1/3 the power requirement means wire may be two sizes smaller. Smaller wire costs less and requires less labor to install. Undersized wire causes voltage drop and reduced light output. For fluorescent lights, a voltage drop of 10% will cause a 10% drop in light output. But, in incandescent or quartz-halogen light circuits, BEWARE! Light output will drop by 25% because lower filament temperature causes further reduction in efficiency! Where wire runs are long (or existing wire is small) fluorescents may be clearly economical even for lights that are seldom used--their INSTALLED cost is less.

A 12 volt DC home using incandescent lights must use AVERAGE #10 wire, which is stiff and awkward to work with. The smaller #12 and #14 wire used in conventional ac homes can cause 12V lights to burn dimly. High efficiency lighting allows use of these smaller wire sizes, at least for some of the wiring in a 12V home, but NOTE: A 24 volt system requires one quarter the wire size of 12V, so conventional home wiring can handle nearly all 24V lighting. See wire size charts in most alternative energy catalogs and reference books for specifics. (24V systems may also run 12V lights and appliances using a "Battery Equalizer"--See HP#6.)

(4) QUALITY OF LIGHT: "Warm Spectrum" light is rich in the red/ orange end of the light spectrum (like candle light). "Cool White" is rich in the blue/violet end of the spectrum. Warm spectrum light is the most pleasant in the home. Incandescents generally produce a warm to medium spectrum, depending on bulb design and voltage at the bulb (beware, an overly warm orangy looking incandescent indicates very low efficiency, as low as %5!). Quartz-halogen bulbs produce medium to cool, best for reading and seeing fine details and colors. Fluorescents may be cool or warm, depending on the tube you select. Because low voltage DC fixtures use the same tubes as ac fluorescents, you may choose from a wide variety of tubes available on the market, including color-enhancing, full spectrum and plant-growing tubes. (Check with a well-stocked lighting supplier. In small stores you may find nothing but the standard "cool white" which many consider harsh and unpleasant.)

In the past, fluorescents have been notorious for harshness, color distortion, flicker, and poor life expectancy. The strobe-like flicker (caused by 60 cycle/second ac power) and unnatural spectrum have been blamed for behavioral disorders, nervousness and eye strain. But, use of DC power and recent advances in fluorescent light technology have overcome these problems. The human eye can detect the 60 cycle per second flicker of ac fluorescents. The DC fluorescents are being driven at 1,000 and 30,000 cycles per second, far too fast for the human eye to detect. Compact fluorescents now fit into bulb sockets. Better phosphors produce full-spectrum, color true light. We have customers who are artists and they PAINT under them! Problems with radio interference have also been solved. Many PV users who have rejected fluorescents in the past, now use them extensively with complete satisfaction. We use them in our living room, kitchen and shop too!

Full spectrum fluorescent light has been found to alleviate wintertime depression that some people experience. If you are not pleased with the quality of your fluorescent lights change to better, more modern tubes. (Reference: HEALTH AND LIGHT by John Otte.)

(5) DISPERSION OF LIGHT: Incandescent and quartz-halogen bulbs are small, intense light sources. This suits them to localized placement and use of reflective fixtures to concentrate light where it is needed. The quartz-halogen bulb is extremely small, practically being a "point source" of light. This makes it easy to reflect in a tight spot or flood beam. (Reflectors can multiply the intensity of light MANY times.) Point source light is good for "task lighting" of small areas but produces sharply defined shadows. Most fluorescent tubes are long and produce a highly diffuse light (from many directions)) good for lighting medium to large sized areas with a minimum of shadows. Diffuse fluorescent light is also perfect for kitchen counters, sinks, and work benches because your hands and tools will cast a minimum of shadow.

To be effective, light must shine onto the surfaces to be seen! Light that is absorbed by the surroundings or that shines into your eyes is wasted. Factors influencing overall lighting efficiency include positioning of lights, fixture design (reflective properties) and the color of ceiling and walls. A 5-watt quartz-halogen spot lamp can light the pages of a book better than a 100 watt bulb hanging from a dark ceiling! Placement of switches is also important in determining how handy it is to turn lights on and of as needed.


Renewable energy systems that depend on storage batteries (photovoltaic, hydro-electric, wind-electric) produce low voltage DC power. Utility companies supply high voltage ac power (more appropriate for mass-distribution). We live in a world of two electrical standards. Neither form of power is "best". What's important is to use the available form in an efficient, simple and reliable manner. Every step of energy conversion (ie. inverters) involves both a loss of energy and extra complexity. If you are producing DC power, it is best to use DC lights.

For the independently powered home, we design lighting circuits especially for low voltage DC, using larger wire than usual and maintaining isolation from ac appliance circuits. This results in the best overall economy in spite of higher installation cost. DC/ac dual wiring is simple enough for the average electrician when wiring a new home. If you are adapting alternative energy to a conventional ac home (retrofitting) you may choose to use ac power from your inverter to run all of your lighting. If so, be aware of the following:

(1) INVERTERS are complex high-tech devices, not usually serviceable locally (they are also expensive). Modern inverters are highly reliable, but anything can fail as the years go by. Running DC lights from a DC source requires two wires. Running ac lights efficiently requires microprocessor chips, transistors, transformer and other complexities within the inverter. We prefer to use inverters primarily for "luxury" appliances and leave essential lighting, well pumping and refrigeration to DC power, both for peak reliability and efficiency.

(2) LOW VOLTAGE DC LIGHTS are more efficient than ac lights, the exception being "electronic ballast" fluorescents which are the same either way (see below). Low voltage incandescents use half the power of ac bulbs for the same light (see efficiency analysis above). Quartz-halogen are also superior on the low voltage forms, so much so that ac quartz-halogen fixtures (like track lights) use 12V bulbs powered by a transformer! The use of an inverter to convert 12V to 120 only to have it converted back to 12V again (with additional losses) is a technical absurdity ala Rube Goldberg!

(3) INVERTERS loose energy, generally about 10% (that's 90% efficiency). Efficiency can be much lower for a large inverter running just one or two lights. An ac incandescent requires almost twice the power of a DC bulb, causing the inverter to waste still more.

(4) INVERTERS only approximate the properties of utility power. Most ac fluorescent lights work less efficiently than normal on inverter power and may emit an annoying buzz. This is because utility (or generator) power produces current that alternates smoothly (like a swinging pendulum) producing what's called a "sine wave". Inverters produce alternating current (ac) by switching, which produces a choppier waveform often called a "modified sine wave". Common fluorescent lights contain a "magnetic coil" ballast which does not respond well to non-sinewave (most other appliances work fine).

ELECTRONIC BALLAST FLUORESCENTS offer the best solution for efficient ac lighting from inverter power. Screw-in versions are available from many lighting suppliers. They may be bulkier and heavier than standard bulbs and cost about $15 each, but last about 7 times as long and use 1/4 the energy of ac incandescent bulbs. They produce a pleasing warm light. Electronic ballasts are also available for common long-term fluorescent fixtures, but you will need to contact an industrial lighting supplier. They are more efficient on any ac power source and they eliminate the strobe-like flicker that conventional fluorescents produce, so we recommend them to everyone. Editor's Note: In the PV system featured in this issue, Roger and Ana Murray power GE "Compax" fluorescents via their inverter. These miniature fluorescents have a standard candela base (like a lightbulb). The General Electric "Compax" model FLG15 consumes 15 Watts and produces the equivalent light output of a 40 watt 120 vac incandescent lightbulb. These "Compax" fluorescents produce a warm natural light, not the harsh cold stuff we normally associate with fluorescents. They also run very quietly from inverter produced 120 vac. RP


EFFICIENCY may not be critical for lights that are not used often. You may have closets, storage rooms or outbuildings where lights are seldom used. You need not spend extra money on energy-efficient light there unless line loss is a factor. Likewise, you may wish to run only ac to a garage or outbuilding rather than dc, if the distance is more than 100 feet, especially if the lights are not used for long periods. Some of our customers have generously sized PV systems to run summer irrigation pumping. In winter, they have so much excess energy that they don't need to spend a lot of money on efficient lights.

OUTDOOR LIGHTS: Some fluorescent fixtures will not work at low temperatures. For unheated spaces where temperatures may drop below 40°F., special fluorescents are available.. "Low Pressure Sodium" lamps are even more efficient, but have poor color rendition and need long warm-up time. They are frequently used for yard and security lighting. Any incandescent or quartz-halogen bulbs will work fine outdoors if protected from moisture.


Lighting is the biggest electrical load in many PV homes. It is needed the most when there is the least amount of solar energy available! High efficiency lighting design reduces generating, storage and distribution costs so much that it can make PV power more affordable than most people realize.

Windy Dankoff is owner/visionary of FLOWLIGHT SOLAR POWER, P.O. Box 548, Santa Cruz, NM, 87567. 505/753-9699. High efficiency lights are available by mail from Flowlight Solar Power and from other Home Power advertisers. An earlier version of this article originally appeared in the PV NETWORK NEWS ($15/year from PV Network, Rt 10, Box 86PV, Santa Fe, NM 87501).

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