Lighting
Electric lighting, on average accounts for about 15% of residential electric usage, but that number varies with both user behavior and lighting efficiency. The rule of thumb is to use efficient lighting everywhere, but the energy conscious person will find that changing as little as 25% of bulbs to efficient lighting will achieve 90% or more of the possible savings, because only a handful of bulbs are on for any length of time. Occupancy sensors can reduce lighting energy, but can be extremely annoying, and for the energy conscious, will not result in any savings.1
Lighting has come a long way in the last twenty years or so, and there are now a number of high efficiency bulbs, and CFLs are now also much better. Unfortunately this new era of choice, means your now faced with buying bulbs labeled not just in watts drawn, but with their CRI (color rendering index) and color temperature, and lumens. Plus you need to know whether it is dimmable or not dimmable.
Daylighting
It is not that common to have to use electric light in houses during the day, but it is still more common than it should be. If the site is dark for some reason, you usually have to live with it, although when the cause is trees, at least some pruning or removal often happens.
Because houses are small buildings, getting daylight in every room is generally easy, especially since even north windows on a somewhat cloudy day let in much more light than a 100W bulb generates. In spite of that a huge number of buildings have daylighting problems in at least one area of the house.
Not every room needs the same amount of light, and in general the more public spaces tend to want more light, while private spaces like bedrooms need less light. Lighter colored walls reflect much more light than darker ones, and so also create a more dispersed light. The contrast between a bright window and a dark corner can be hard on your eyes (which is why in a bathroom with one window, you need to turn on the light to see in the mirror), but is easily remedied by making sure there are windows on two different walls, creating a more even dispersed light. If necessary, add "crinkles" to the exterior to create places to put a second window facing a different direction. When bright sun is an issue, indirect light (for example, bounced off a wall, instead of directly through the window) is much easier on your eyes. Since light doesn’t penetrate more than about 12 feet into a room, any area further away than that is at risk for being dark. Skylights provide proportionally more light for the amount of glass, but in general skylights should be oriented so that the summer sun doesn’t shine directly in them to avoid overheating, so their advantage is somewhat diminished. Because warm air rises, and skylights are directly exposed to cold night skies, they tend to be a bigger heat loss per unit area than windows. When a window is in a wall that is relatively thick (twelve inches or more), tapering the walls around the window will increase the amount of light coming through it: something that can be especially important for skylights. An alternative to a skylight is a sun pipe, which is just a round small diameter skylight that feeds light down through the ceiling via a highly reflective tube.
Electric Lighting
The remaining part of this discussion pertains only to electric lighting.
Lighting Terminology
New labeling laws are requiring bulbs to be labels with the following characteristics, so although it makes picking more complex, at least you will now know what you're getting.
Lumens - a measure of the light output. This is not the same as wattage, which is the amount of electric the bulb takes to produce its light.
Wattage - the power the bulb draws. This translates directly into your power bill, not necessarily into the amount of light.
Efficacy - a measure of the efficiency the bulb produces light, as lumen per watt. An incandescent would generally be under 20 lumens/watt and a fluorescent over 60.
Color temperature - a measure of how skewed the light is toward one or the other color in the spectrum. Technically, its temperature a "standard black body" would have to be to put out a light spectrum that is similar to what the bulb puts out, and it measured in degrees Kelvin. A standard incandescent is skewed toward the yellow/orange side and has a temperature of 2700K, which means the filament in the bulb is effectively that hot. A temperature of 4000K would be "cool" light, and 5000-6500K would be blue like daylight. Many people like the warm feel of 2700K bulbs, but beware of the CRI also.
Color Rendering Index (CRI) - is a value from 1 to 100, and is a score of how well a set of eight standard colors appear under the light. A score of 100 means the colors appear exactly as they do under a standard incandescent bulb (the is by definition), and any score above 80 is good. A poor CRI is often the reason a bulb makes everything ugly, even if its "warm", or supposedly 2700K. In fluorescent lighting this is generally caused by a poor mix of phosphors that emit only a limited spectrum. The only way to tell is to look at the CRI value.
Beam spread - a measure of how wide the light is distributed. Standard incandescents are generally very wide, but other bulbs may not be. Bulbs may be spot or flood, with flood sometimes subdivided in how wide.
Types of bulbs
Incandescent types: these are all variations on Edison's original bulb, using a filament heated to a very high temperature in a glass bulb with the oxygen removed. They run directly on the standard 120VAC electric.
Running an incandescent on a dimmer not only saves power, but extends the lifetime. Just dimming 10% can double the lifetime, and 20% will quadruple it. The caveat, is that turning the bulb on and off causes heat stress, and ultimately failure.
Incandescent - most everyone likes the look of them, but they use a lot of power: they convert only 5-10% of the power consumed into light. In summer, that excess power turns into heat that the air conditioning system must then remove. The major advantage of them is that they're simple, cheap, dimmable, and have a relatively low environmental impact.
Halogen - these are improved incandescent lights that are slightly more efficient, and produce a whiter light. Instead of having the filament in a semi-vacuum, the filament is in a halogen gas. They burn hotter, also last longer than standard incandescent lights.
Halogen IR - these are up to 30% more efficient than standard incandescents. The tricks is that the quartz capsule that holds the halogen has a special coating that reflects infrared (IR) back to the filament, allowing it to operate at the necessary temperature using less power. Because these are more efficient, you buy them by the lumens needed, not watts used.
Fluorescent types - there are two general types: hot cathode and cold cathode. In either case, current flows thru the bulb, exciting atoms of mercury vapor, which in turn emit UV light, which in turn hits the phosphor coatings on the glass to emit visible light. In the hot cathode type, electrons are release by the heat, in cold cathode is its a combination of heat and voltage. The cold cathode versions are longer lasting, but generally used only is special applications, like exit signs and LCD backlights. While old bulbs used a single phosphor that produced a very greenish light, new ones use multiple phosphors. The color temperature and CRI are dependent on what mix of phosphors are used. Newer CFLs use a much better mix than older ones.
Fluorescent bulbs require a ballast, which is a circuit used to operate them. This circuit must "start" the bulb, which is to get the initial flow of electrons going off the cathode, and then maintain the proper flow. In a cold cathode bulb, a high voltage is also needed. Older bulbs used magnetic ballasts that often hummed, buzzed and produced a flickering light, while newer ones use electronic ballasts that have none of these problems. There is, however, often a delay of 1-2 minutes before the bulb reaches full brightness, which is the time it takes to fully heat the cathode up.
Fluorescents generally last much longer than incandescents, anywhere from 5 to 30 times longer, although turning it on and off frequently will shorten its lifespan (as it will to incandescents as well). Although they are still more expensive per bulb than incandescents, they are cheap enough (especially when you buy a six-pack), that their lifetime cost (including the electric used) is usually much cheaper than a standard incandescent. They use only 1/4 to 1/3 of the power as an incandescent.
The problem with dimming Fluorescents has to do with the interaction of the dimmer and the ballast, and most standard dimmers don't work with most ballasts. Even when they do work, don't expect to get the range of dimming available to an incandescent because at lower power levels, it becomes impossible to maintain the current flow to ionize the mercury atoms, and so the lamp turns off.
Because fluorescents contain a small amount of mercury in the bulb, and CFL additionally have electronics in the base, disposal is a serious concern. In some communities CFLs are considered hazardous waste, and must be disposed of specially, but most allow then in the trash. It is the major environmental argument against fluorscents.
Linear fluorescent - these come in various lengths, typically 2, 4, or 6 feet, and in various thickness' also, with typical sizes being T12, T8, and T5, where the number is in eights of an inch (so a T8 is 1"). Unlike CFLs, in linear fluorescents, the ballast is part of the fixture, and it is this ballast that largely determines the efficiency of the lamp.
Compact fluorescent (CFL) - there come in various shapes, although the "twister" bulbs have become the standard for residential usage. A CFL contains both a lamp and a ballast in one package. At one point pin-type CFL (as compared to the standard screw base) were meant to be used in fixtures with built-in ballasts, but the newer GU24 pin type CFL are exactly the same as the screw base, except you can't ever put an incandescent in the fixture.2
Light Emitted Diodes (LEDs) - are solid state devices similar to transistors and computer chips that use a direct current (less than 120V)3 to produce light. Like fluorescents, they don't run directly on 120VAC electric, but require a power supply (a variation on the little black wall warts used to charge various electronic devices). Unfortunately the mechanism that produces light in an LED, produces monochromatic lights (one color) in an extremely narrow frequency range, so to get white light, either multiple color LEDs must be mixed, or phosphors put on the LED that convert the monochromatic light to white light. Although the phosphor method is less efficient in principle, it is the most common method in use. In this system, the efficiency of the bulb is a combination of the efficiency of the power supply, the LED itself, and the phosphor conversion.
LED promise very high efficacies (twice CFLs), but many of the currently available products are less efficient than CFLs. Currently, the main advantage is that they're dimmable and more efficient than Halogen-IR bulbs.
LEDs are very sensitive to heat, and burn out easily, so they are usually mounted in big heat sinks. Care must be taken in where they mounted, so that heat can dissapate.
Currently LEDs are very expensive, typically $50 to $100 per bulb, but promise extremely long lifetimes (30-50,000 hours, compare to 1000 for an incandescent, or 10,000 for a CFL).
LEDs are dimmable by the standard wall dimmers.
Lighting Design
There are two primary kinds of light fixtures, ambient lighting and task lighting. Ambient lighting is for walking around in the room and doing anything that doesn't necessarily involve seeing in great detail. Task lighting is a spot lighting, providing a localized brighter light for tasks like reading which require more light. In homes, traditionally ambient lighting is bright enough to read by, which obviously uses more power than necessary. Every room will require ambient lighting (unless you want to walk in the dark!), but only a very few areas will require task lighting: for example, kitchens, offices, a living room chair, or a bedside table.
Ambient lights generally need flood type widespread bulbs, while task lighting may be either flood or spot, depending on whether ambient light is also present.
What bulb to use? Many lights in the house are rarely on, and so using more expensive high efficiency lights will not result in measurable energy savings. It is often the case that only a small handful of lights in the house are on for any length of time, and so those are the prime candidates for high-efficiency bulbs. Because CFLs are now so cheap, fixtures receiving a medium amount of use can profitably be changed to CFLs. For the environmentalist who leaves only a minimum amount of lighting, the mercury added to the environment may not be worth the small incremental savings of a CFL that is only on 20 minutes a day in the winter.
The best way to save energy is to just turn the light off when you're done with it--done meaning you have no current intention of returning to the room, or the likely return is long enough off (say 10 minutes). If you want to leave lights on for the evening, then 13 to 15W CFLs (or even lower wattage LEDs if you can find an efficient one) are the best choice.
Light fixture amount - not only have houses grown in size and number of electronic gadgets, but the number of light fixtures (in particular can lighting) has also gone up, in my view totally unnecessarily.
Recessed downlights (can lights): although they're relatively cheap and hidden in the ceiling, these are not the best way to light a room, because of the inherently narrow beam.
Energy Star labeling - energy star bulbs are not only the more efficient ones, but also have a CRI of at least 80, so will produce decent light.
Resources
www.energysavers.gov - US DOE lighting info
Seattle lighting Guide From the city's green building program
Seattle City Light's energy efficient lighting website
http://www.americanlightingassoc.com/info_lighting.php American Lung Assn. lighting guide
www.cee1.org Consortium for Energy Efficiency (CEE) lighting info
Notes
1: Occupancy sensors are intended mostly for commercial settings, or in residential uses, they are to cover up for the dumb and lazy, and in those cases they save energy. If you're good about turning lights out, occupancy sensors will cost you a bunch of money ($20-$40 ea) and will buy you nothing. They also might end up causing you to use more energy and/or have bulbs burn out early due to excessive cycling. Nowhere could I find how much phantom load they create, apparently quite small, but if the world goes the way of California Title 24, I imagine 100 million of these things, and that adds up to real power, just because people don't turn out the lights.
2: in my not so humble opinion this is a really stupid idea, driven by California Title 24 with the intention of prohibiting people from replacing the CFLs with standard bulbs. At the moment, it also keeps you from screwing more efficient LEDs in there. And, of course the intrepid rip the fixture out and sent it to the landfill anyhow, or simple buy pole/table lamps and put their beloved incandescents in them.
3: old style low output LEDs were generally 5VDC, and the new white LEDs in flashlights clearly run on the similar battery voltage, but I was unable to find the typical voltage of the high-output white LEDs used as light bulb replacements.