A correspondent writes:
I have mini-track lights underneath the kitchen cabinets.
Instructions say that replacement bulbs should be
R14 (E17). What kind of code is this? How do I
interpret it when buying new light bulbs?
When you read the labeling on a package of light bulbs, something that you need to know, in addition to the power rating (measured in watts), is how to be sure that you are buying the size and shape that will physically fit into the fixture.
In the case of your mini-track lighting, if a package of light bulbs, or the specifications listed on a web site, says R14, you know that a bulb will fit properly into the housing. R14 is sufficient information for you to be sure that you have the right size.
The designation E17 would not be sufficient information for you to purchase the exact product that you need. The E17 describes only the size of the threaded base that screws into the socket. As you have probably noticed, the E17 threaded base, which is sometimes called an "intermediate base", is a lot smaller than the screw base of an ordinary light bulb.
The threaded base of an ordinary light bulb, a base about an inch in diameter, commonly used in living room lamps, is called the E26 base. It is also known as the "medium base" or "standard base." Any bulbs which have this size threaded base will be labeled E26. For example, the smaller "appliance bulbs" that many people use in refrigerators, and which also fit into lava lamps, are called the A15 size. They also have E26 bases. The decorative three-inch-wide spherical bulbs, which many people use for lighting around bathroom vanities and mirrors, are called the G26 size. They also have E26 bases. These various kinds of bulbs are noticably different in sizes and shapes, but their bases -- the part that screws into the socket -- are the same size as that of a common light bulb, namely, E26.
Now that you know what size to buy, also remember never to exceed the power rating, measured in watts, that the manufacturer of the fixture recommends. The power is the rate of energy consumption, for example, a 40 watt bulb consumes 40 joules of energy per second. Some fraction of that energy is converted to heat instead of light. The manufacturer has probably determined, on the basis of lab tests, that exceeding the specified power level can produce a fire hazard.
What about LED bulbs, instead of the traditional incandescent bulbs?
Suppose that the power rating for a fixture is, for example, 40 watts. In that case, it would be safe to install LED (light-emitting diode) bulbs having labels similar to "5 watts, equivalent to 40 watts incandescent." Such a label means that, through the use of LED technology, the total power dissipation, light and heat added together, has been reduced to 5 watts, while giving you as much light as an older-style incandescent bulb dissipating 40 watts of light and heat. Switching to bulbs that generate LESS heat than before is economical as well as safe. Switching to bulb that generate MORE heat than the amount approved by the manufacturer would be a big safety hazard.