They put out a good amount of light compared to LED bike lights. I have a LED light on my helmet for extra backup which is an excellent idea. The bike-mounted light points in your direction of travel and is very visible to vehicles and other travelers. The LED helmet light is a backup and points in the direction you are looking at, like your bike lock at night or a car in your vicinity.
More light is better for anyone traveling at night.
Step 1: Get the parts
This unit is a slightly higher end model at $15 compared to the $9 units I've used previously. The advantage of this unit is that the cheapest Malibu has an extended light barrel on the end of the light which is a stylistic add-on I think. The barrel has an extended lip on top. If you hang the light upside down on your bars, the barrel can be rotated but the lip won't be at the top of the light. Annoying.
Also, the light below is using an MR16 bulb and seems to put out more light at the same wattage than the lower end Malibu. Plus the body has ribs.
The light also has an adjusting screw that you can loosen then rotate the light vertically. That's a big help when you're dialing it in.
So, one Malibu light.
Some lamp cord. Long enough to go to wherever you plan to put your battery plus some extra slack.
I'm using a lighted switch below. You can also use a lamp cord switch for $1. That's cheaper but has the drawback that there's no visible signal to you that your light is on. You'll find out why that's important the first time you have your light on in somewhat bright conditions and forget to turn it off, draining your battery.
The connectors are called Powerpoles. They are genderless and work pretty well. I don't like the look particularly when they are exposed but that's what I'm using until I find another way.
Some folks use stereo plugs.
The switch is mounted on a conduit hangar. It's basically a metal u-shape that has a oval in the middle that fits most handlebars handily. The screw is behind the lamp cord in the picture. Very cheap and easy to use.
12V lead acid battery. I use these because they are inexpensive with good capacity.
The 7.5 amp hour batteries have thinner profiles than the 4.5ah or 12ah ones. I use them because they are a nice middle range between the 4.5ah (easy to mount with lower capacity) and the 12ah (big, heavy bricks).
I fuse my batteries because there's nothing like getting electric current through delicate body parts to ruin your whole day, not to mention catching on fire or getting a nasty burn.
So, one blade fuse available just about anywhere. I use a 10amp fuse. The fuse should be 1.5x greater than the amps in the circuit. so I guess it should be 20watts=12Volts x Amps = 1.6Amps in the circuit times 1.5 = 2.5 Amp fuse nominal.
The switch is a lighted switch for 12V circuits. i found one at an electronics store.
You'll also need a charger for your battery. I found an inexpensive one at an electronics store. The charger has to match the type of battery you are using. It's better to pay a little more for a charger than get the cheapest one you can find.
Light - $15
Switch - $3.50
Conduit hanger - .50
Lamp cord - $2
Battery (ebay) - $20 w/delivery
plus a couple of crimp on connectors for your battery posts, plus maybe some electrical tape.
Charger - $15 - $50 depending on what type you get.