One of our main uses for electricity is for lighting and the choice of lighting is very important
in a solar powered system.
Low Voltage Lighting
This will obviously be the choice for those with a small system without an inverter to produce "mains" voltage AC power. The modern bulbs used will be more efficient than the "traditional" incandescent mains voltage bulbs not as efficient as "low energy" bulbs. When this page was originally written, LED bulbs were not in general use for lighting as they are now. These can be low voltage.
Incandescent Mains Voltage bulbs
These are the filament bulbs invented by Thomas Edison (or at least he developed the first commercially viable bulbs) which are in general use today. They are however not as efficient as alternatives and do not there for have a place in a solar powered system.
These bulbs produce light by using electricity to heat the filament (in an inert atmosphere) to a high enough temperature to glow and emit light. However, only about one third of the energy is emitted as light, the rest is heat.
Low Energy Light Bulbs (CFLs)
Compact Fluorescent Light bulbs (CFLs) are actually miniature fluorescent bulbs which use approximately one quarter of the energy of an incandescent bulb and should also have a much longer life. They work by exciting mercury vapour to produce ultra violet light, which in turn causes a phosphorus coating on the inside of the glass to fluoresce and produce light.
One of the disadvantages of CFLs is the mercury content (only about 5mg per bulb) which is a concern in dealing with breakages or disposal.
Light Emitting Diodes (LEDs)
Light emitting diodes have been around for some time but it is only relatively recently that they have become practical for general lighting in the home. Although currently more expensive than CFLs, LEDs use approximately half the energy for the same light output and have a number of other benefits including less heat output, can be dimmed in some circumstances, do not contain mercury, and potentially have a longer life. LEDs are solid state devices where a semiconductor emits light when a voltage is applied across it, the colour of the light depending on the semiconductor material used.
Both of the above two types of bulbs are available in a choice of light output (slightly different colours) though this information is not always available on the box. The colour is quoted as a colour temperature (no connection with the actual temperature of the bulb) quoted in degrees Kelvin (°K)
An attempt to illustrate these colours is shown below.
The colour temperature relates to the property of matierials to give off light when they are hot.
Steel glows red when it is at around 600oC (873oK). As it is heated to higher temperatures, the light given off becomes more white and eventually has a blue tint.
A degree Kelvin is the same as a degree Celcius but 0oK is at -273oC