Non-low voltage (line voltage, typically 120V) under-cabinet lighting offers advantages like ease of installation and the potential for higher brightness, while also avoiding the need for a transformer.
Why not hardwired ?

Well, I'm not an electrician, but I'd expect line voltage lights to take a lot more current and require Romex wire to be run to each light position. Low voltage LED lights use much smaller and easier to route wire. LEDs do rely on a transformer but I only need one AC outlet for most of the under cabinet lights in the kitchen. How are line voltage lights easier to install? My point about not hardwiring the lights is that I want to place a couple AC outlets then plug in off-the-shelf modules rather than needing to hardwire the light strips/fixtures into the house wiring. To be honest, I never thought about line voltage lights because I haven't seen them mentioned in any of the articles I've read or videos I've watched. I gather that low current low voltage LED lights are the current state of lighting technology replacing incandescent or halogen lights in ceilings, under cabinets and everywhere else. So I'm surprised to see a recommendation of line voltage lights under the cabinets. It may be a viable option I've just not considered. Can you give me some examples of the kind of fixtures or devices you're suggesting?

Not an electrician but do much of the wiring in my house including kitchen.

I think low voltage LED lights driven by a plug-in power supply are more practical and flexible than hard-wiring 120v lighting, fishing NM cable, running conduit, etc. You don’t need big lumens for under cabinet lights. If you change out the lighting, it’s just a matter of unplugging the old power brick, not futzing with hardwiring.

I would make the receptacle switched so you can turn all the undercabinet lights on/off with a single intuitive wall switch, rather than teaching Aunt Millie to use some weird little wireless remote that gets misplaced every few weeks.

I would also consider using a short extension cord, like 3’ with a flat plug, so the power brick can be tucked away somewhere instead of being an ugly wall wart.

A wall wart! Too funny John Liu.

Here's installation options for WAC products. Link.

Link.to pdf installation instructions.

Based on your description of your needs, what you want is hardwired LED tape lights. They are considered line voltage because it’s line voltage to the transformer but then to the fixture everything is Low Voltage in a wire roughly the size of yarn. I typically install the transformer either on top of the cabinet if there’s access or inside a cabinet, the transformer is the size of a toothpaste box. The LED tape lights are cut to length and are mounted at the front of the cabinet behind the light rail and it’s fed with wires that you can hide inside the cabinets or between fillers. Super easy and clean installation.

Thanks for everyone's comments but my question is really about making the connection from the undercabinet lights to the AC power. I agree with John Liu that LED lights are the best and easiest option. The system I have in mind uses light strips like what WAC offers but they're enclosed in aluminum channels with diffusers. They're already assembled into lengths that fit under the cabinets and they plug together end to end under a run of cabinets. No soldering or crimping. Then there are the molded connecting cables that run from the light strips to the transformers. Again, similar to what WAC offers (thanks, dan1888), the transformers are low profile enclosures that accept the cables from several sets of light strips. The transformers (or "drivers" or "controllers") need to plug into regular electrical wall outlets. I do not plan or need to hardwire them into the electrical boxes with wire nuts or solder or crimping, just plug them in.

The question is where to put the transformers and electrical outlet boxes. I have no room above my cabinets. I do not want to take up counter outlets with these connections. I don't like cutting holes in the cabinets to access outlets behind the cabinets. I think that leaves putting the equipment under the base cabinets on the floor. I can install switch controlled outlets in the walls at floor level, plug in the transformers and let them just set on the floor under the cabinets behind the removable toe-kicks. I don't think I need to mount the transformers, they can just set there on the floor much like a power brick on a laptop computer. The problem is that the wall outlets are 2' back from the front of the cabinets and hard to see and reach to plug or unplug the transformers. One way to fix that is to NOT mount the box in the wall, but instead pull the Romex cable out of the wall a couple feet under the cabinets and connect the box to that cable. I have read that it's acceptable to extend Romex out of a wall like that as long as it's protected (like under the cabinet) but the box would be unattached to anything but the cable itself and that might be a problem. What is your opinion on that? Terrible idea for some reason or perfectly fine?

I hope that makes sense and clarifies what I'm trying to sort out.

I would not do that (#3) - I would do option #2 - put the outlet inside the cabinets.

When we do undercabinet lighting, we put the transformer/outlet on top of the cabinet, facing down into the cabinet for the plug. Then run the wires down the inside corner of the cabinet. If the cabinets are very close to the ceiling, then we do the outlet inside the cabinet ( use a shallow box recessed in the wall ) . It really doesn't take up that much space.

We use the LED tapes.

I don't like loose NM cable, box, receptable, transformer all sitting loose on the floor, even if concealed by the toe kick. Messy, sloppy, and probably not to code (code requires boxes be securely fastened to something - wall, cabinet, etc)

Here are two options.

"A" (on left) is easiest - install a combo switch-receptacle in a single-gang box in the wall under the upper cabinet, mount transformer to underside of cabinet, run a short extension cord from transformer to receptacle (you can find 1', 2' etc flat cords in white with "flat plugs", on Amazon). Yes the short bit of cord will be visible on the wall - if that is unattractive, see "B".

"B" (on right) is slightly more complicated, but tidier and what I'd do personally - switch in box in wall (can use a two-gang box and put an unswitched receptacle there for general use), NM cable in wall to receptacle in the top-most part of the upper cabinet (where no-one will ever see it, unless they are on a ladder), plug transformer in there and run the thin low-voltage wires down to the under-cabinet light (those wires can be dropped down behind the cabinet, or run them down the corner of the cabinet)

I did "B" in my pantry cabinets. There is a switch (in my case, inside the cabinet) and a switched receptacle in the upper-most part of the cabinet (totally out of sight). So I can easily add in-cabinet lighting later, if I ever feel like it.

You'd cut a rectangular opening in the back of your cabinet (which is probably just 1/4" plywood or MDF, no problem) and mount the electrical box to that, with a faceplate - looks tidy. Behind that would be an opening in the wall, to make room for the back of the box. As long as the receptacle box is directly above the switch box, it should be easy to just drop the NM cable down between the wall studs.

Use the plastic "old work" boxes that can simply be pushed into the appropriately sized opening and secured with the screw/pivoting tab mechanism.

If you follow NEC, it allows for outlets inside cabinets, and for under the cabinet base (toekick space) would allow a protected whip (like MC cable) and a mounted outlet to the floor. But, I don't suspect any inspector would allow an outlet behind the toekick as it is not easily accessible like it is within a cabinet with hinged doors or drawers. Your toekick face has "clamps" which fit tightly on the legs, and would need tools to remove sections easily.

It isn't worth reinventing the wheel here, I don't know why the hesitation to core through a cabinet. You don't need outlets and holes everywhere, just a single outlet for a 16' run of 12V LED tape. So, inside one cabinet is an outlet, either face mounted or flush within wall. You plug in the transformer and run the "cord" through the cabinet bottom to the UCL. Depending on kit chosen, you may use 18ga red+white wire if it is not a prefab length of cord. The UCL then is a single long (interconnected diffusers) piece across the underside of that cabinet run, up to 16' long. Repeat for separate cabinet runs.

With IKEA boxes you will need a light valance. UCLs go on front minimum 1" away from the valance, unless you get 45degree diffusers which can mount against the valance.

As much as I like putting everything on the floor where I don't expect to access it for a long time, if ever, I don't want to do something I'll have to undo later. The biggest problem is probably the loose box.

The next best option, as you've pointed out, is to put the outlets in the cabinets. I've hesitated to do that because there's a gap between the back of the cabinet and the wall. I don't want to put the outlets in the wall behind the cabinets then cut an oversized hole in the cabinet to "reach through" to the outlet. I know it's a short "reach" but it leaves an unfinished hole sawed in the back of my brand new cabinets. Even though most people will never see it, I'll know it and it will bother me. OTOH, I can extend the box into the cabinet and install the faceplate from inside the cabinet. All things considered, that's probably the best solution.

I'm curious about the comment regarding UCLs being at least 1" away from the valance. My plan has been to use Ikea UCLs and make my own valances. My valance would be a painted-to-match MDF 1x4 attached to the underside front edge of a run of cabinets. The UCL strips would be mounted to the cabinet bottom flush against the back edge of my valance. When you say the lights must be a minimum of 1" away from the valance is that to avoid blocking the light with a thick valance or is there some heat issue I'm not aware of in placing the light strips against a wooden valance?

I'm not married to the Ikea MITTLED lighting system. I do like their compact size, modularity and plug and play features, and I like the finished look of channel enclosures and diffusers. But if there's another brand that works better, I'll consider that.

Thanks for everyone's comments. I'm not ignoring any of this. It's been very helpful.

UCL's are held off the valance min 1" to avoid the hard shadow line the valance causes on the counter. Since there are so many different diffuser profiles, I usually play with the exact diffuser to find the best distance for diffused light on the counter/backsplash.

Another detail is stop the horizontal run 1" from the end next to a wall. Butting it all the way to the end would create a sort of "hot spot" there. Again, play with the exact fixture to adjust the distance.

I believe the Mittled are fixed lengths. The open market has multiple choices for alum diffusers you can cut to length and just use tape LEDs inside the diffuser. That gives the most flexibility IMO, you can cut the tape, put 'tees' and splices anywhere that fits your cabinet cable runs, etc.

As far as DIYing a valance on prefab finished cabs, especially if it's melamine, thermofoil, even lacquered, be mindful you probably won't be able to match the finish.

A box set into the cabinet back with receptacle and cover plate is very tidy looking. The transformer might be an eyesore, but you’d put all this in the very top part of the upper cabinet and, since your cabinets seem to go almost to the ceiling, that will be out of sight for any but an NBA center.

3onthetree: Yes, the MITTLED lights are fixed lengths to match the widths of their cabinets (except no 21" for some reason). They can connect end to end. The channels are 5/8" thick so butting them up against a 3/4" valance should give a built-in appearance, I hope. Our cabinets will be their Lerhyttan style with solid birch framed doors and birch veneered panels all painted light gray. I'm hoping Sherwin Williams can color match the paint. Ikea sells matching end panels that could be cut to form the valances but a narrow end panel costs $165. Or I can buy a primed MDF 1x4 for $10.

Instructions for the MITTLED say: "The low-voltage Class 2 power supply is located outside the cabinet and is not concealed". I ran into this a while ago and took it to heart. I only have about 1.5" clearance between cabinet and ceiling. My work-around was to mount receptacles in ceilings over each bank of cabinets, then install access panels in the tops of the SEKTION cabinets under each of those outlets to access the plug/cord/power supply on top of cabinet.

In new IKEA kitchen I'm doing, I'm going with a non-IKEA, hardwired power supply (and tape-COB lights) so it can be dimmable from wall switch. (I've not seen any restriction for mounting in-cabinet on any of these.) I'm thinking of routing bottoms of cabinets to recess tape light channel to avoid need for light valance.

I don't see why the MITTLED power supply cannot be in the cabinet. Heat would be the only issue and I doubt the PS gets hot - interested if you could check that.

"I like putting the outlet plate on the inside of the cabinet."

This electrical box is only 1-1/4" deep so you could mount it into a hole cut in the underside of the top of the cabinet and attach a cover plate to it.

Joe Schmo: "I'm thinking of routing bottoms of cabinets to recess tape light channel to avoid need for light valance."

Or you could use a slim surface-mount channel or a recessed channel. In either case I would think you'd want a diffuser to reduce the LED "stars" that might otherwise be reflected in the countertop.

I would not recommend routing a channel for either a stand-alone LED tape or diffuser in a 3/4" particle board bottom that may also have cam locks in the way at the sides.

I like what Joe Schmo has there with the removable panel and ceiling outlets. Even if you leave the panel itself off and just use the frame, it cleans up the cutout which is my chief complaint about cutting a reach-through hole. Nothing to stop you from using that on the back, either. You'd just have to put the driver inside the cabinet due to lack of space behind.

Is there a path to route the low voltage cable from the UCL to the cabinet top without running it inside the wall?

There is often a bit of space between cabinet back and wall, easy to stick something down there and check if there are obstructions.