I used a 35 lumen per sq foot guideline for the kitchen. The number of lights you need depends on the output per light, type of light.

For under cabinet, xenon will give off more heat than LED. I'd recommend LED.

While LED certainly consumes less power it has other drawbacks. Many are not dimmable, but they may have pre-set power levels (hi, lo, off). Like CFL's the tend to put out whitish/blueish light. Even the warm light versions are not what you are used to with incandescent and the warm light LEDs put out less light. Bare LEDs are very bright as point sources, if you have dark granite you are going to get reflections in the countertops. If you have a sample of your granite I'd take it with you when you shop for lights. If you spend a ton on a nice multi-color granite and then wash it out with 'blue' LED lights you may not like the results.

I'm struggling too between LED and Xenon. I like the light qualities of Xenon but want the efficiency of lED. I just think LED is not quite there yet. Maybe in another year or two the quality of light and price will be better. Meanwhile I need lights now. I may go with 12v Xenon for now and see if I can 'upgrade' in a few years to LED. I found the mini track lighting from Juno. It runs on 12v (or 24v) and they have both Xenon and LED fixtures which can be placed anywhere on the track. The problem is to convert from Xenon to LED may need a different dimmer or transformer. If the wiring is no different than I can handle that. Once the backsplash goes up I don't want to be changing the wiring! I'm still reseaching exactly what I'd need to install this and later upgrade to better LED fixtures. But this is looking promising and while I have the old backsplash torn out it makes wiring much easier.

Other companies sell this type of lighting normally called linear lighting. The other advantage is that you can put fixtures anywhere along the track as long as you don't exceed the power or min spacing limits.

Here is a link that might be useful: Juno Trac 12

Texaskitchentoo, I like this idea but do you have to have the lights now? I know the cost of the xenons is minimal when compared to quality leds but why not just wire for a future install of an led system. This is what I'm thinking if I can get the DW to agree. I also plan install a low voltage recessed dimmable halogen lighting system to be upgraded to leds in the future to take care of the lighting needs.

In our kitchen, I see the uc lighting being used more for a mood setting even though I know it will be used for task duties also. I just expect newer and better led technology in the future and hopefully lower pricing.

My xenons are HOT. Be aware of that if you keep food/medicines in the cabinet above a xenon fixture. The light quality is great, but I don't use them as much as I wanted to because of the excess heat. I would not install them if I had it to do over. What good will a fixture do if you don't like to use it?

I like footwedge's suggestion.

The backbone of any undercabinet lighting system is the wiring.

For low voltage, there is a transformer (or 2) located somewhere in a cabinet or other hidden spot(s).

Changing from 12V to 24V or 24V to 12V would entail replacing the transformer & lighting strips.

Changing from standard AC to low voltage could be more involved.

We are going LED. There is a lighting show in Las Vegas - Lightfair International, May 12-14, that will demo a huge amount of residential lighting. Although locals are not always up to speed, there are many manufacturers making excellent, effective LED lights right now!

It's difficult to wade through the exagerations and false information presented by the dozens of "me too" vendors at that show. Buyer beware. It's hard for the real technology leaders to stand out because they tend to be honest about their product's performance.