Some Basic L.E.D. Info:
L.E.D. stands for Light Emitting Diode. L.E.D.s have several sizes. Five milimeter is the original size, 3mm for HO gauge train signals, and the newer 10mm truck and signal ones being used today. There are also the super flux which are both bright and wide angle. At 9" one white led will light a book. The industry has run out of adjectives to describe brightness and ultra bright, mega bright, super bright, ultra ultra bright  don't mean much. I may go to "wicked bright 1-9" for my own use. I do NOT stock the 1, 3, and 5 watt L.E.D.. They only burn the indicated power. They do not produce 5 watts of light. They require Heat sinks. While they may last longer than incandescent bulbs, when treated with kid gloves, they burn more electricity and the extra expense isn't justified.

Technical Mumbo-Jumbo For Techies:
L.E.D.s are a short circuit one way and an open the other. The long lead (anode=arrow)
is positive the same as an electrolytic capacitor After you clip the leads, you can still determine polarity. The lower rim beside one lead is shaved off : this is negative (=cathode=line).

Red, Yellow, Orange L.E.D.s have a forward voltage of 1.9 to 2.4 (2.1 typical) volts.

White, UV, Green, and Blue have 3.2 to 3.6 (3.4 typical) DCV.

Infrared is in a category by itself with very low voltage. One point two vdc is typical and some are lower. These work well with six in a row and one resistor.

Every commercial L.E.D. light made has a current limiting resistor. To figure yours add the forward voltage, subtract from 12 (for 12 v system) and divide by .02 (20ma). This is simple Ohms law. For 3 super-bright leds there is a 10.2 forward voltage drop (series wiring). So 1.8/.02 gives a resistor of  90 ohms. For 13.8 volt supply a 3.6/.02=180 ohms. Super-flux leds are slightly higher voltage so 3.6(typical) is used for each led. For 2 on a 9v battery it is 1.8/.02=90 ohms. If you don't have the exact value, always go to the next higher fixed resistor. This will work fine with 100 ohm resistor or even higher, and gives a small margin for safety. For questions, consult ohms law or Uncle Cy.

For parallel wiring use 3 -1.5 volt AA penlite batteries or even AAA for light duty. AA has about 2400 ma hours and AAA has around 900. For Battery life In hours, divide by 20 for each L.E.D.: 3 leds = 60.  2400/60=40 hours. W,G,UV,B color leds figure out to 1.1 v/.02 a=55 ohms . For  R,Y,O,A  use 2.4v/.02a=120 ohms. For several parallel L.E.D.s you may need to go to half watt resistors. Quarter watt resistors are ok for series L.E.D.s mentioned above.

For (12v) Christmas lights I used 4 R/G w a 100 ohm resistor (all in series). I put a flasher in with 3 W/B and 47 ohms. Four L.E.D.s is pushing it, but the extra resistance is working so far. I'd use this same formula for energy saving camp/cabin/RV/off grid lighting, or night lights.