Consider this topic for a new build, not retrofit. Many efficient lighting technologies are DC in the 5v- 20v range, and as you think about it, many things like computers, displays, cordless phones, TV equipment... are all DC but require an AC-DC adapter. Of course, there's no way you can get away from AC completely (you'll always need a 240v circuit for your welder), but has anyone successfully implemented a fully DC lighting and switching solution in their home? Low-voltage circuit breakers are readily available in marine applications, and the wiring would be similar, and fancy lighting controllers would, in theory, be easier to use. Also, if you had solar or wind as an energy source, maybe the low voltage lighting could be powered on the DC side of the inverter, further improving efficiency. Interested in others' thoughts.

You might save 25% on a few things in cases where the home runs off of solar charged batteries. I doubt it adds up to much. In general, US homes should move to more 240VAC use. Less copper and less heat lost in the wires.

I agree with you, jonr. My goal is to standardize on 24v and 240v throughout the house. Because I want LED lighting throughout, I didn't want to have to deal with (and purchase) so many rectifiers to convert AC to DC. So for most electronics, I can use DC-DC converters, and for all appliances, simply spec 240v.

LEDs typically need a current controller anyway. So a pulse width modulated reduction in voltage isn't more circuitry. Nor can I think of any advantage to using a DC-DC converter instead of an AC-DC one (diodes are really cheap).

I've seen a lot of DC systems in houseboats. One of the things working for them there is that houseboats are quite compact and the wiring runs are short.

Something to look at

I considered running 12-V DC wiring around an observatory site that I have since it has a solar PV system and batteries. I was surprised at what large diameter wiring would be required to avoid significant voltage drops in such a system. Going to 24-V instead of 12-V would help, but you still need to consider the length of wiring runs with the high cost of copper.

http://solar.led-guy.com/ 12vdc works best, yes wire is a lot bigger for 12vdc,  but if you do use 12vdc,  its much better with 12vdc than an inverter, no loss, and its a big gain, it tried it

I recently attended a 1-hour seminar presented by Armstrong (yes the ceiling people) on the use of low-voltage power in office and data centers. Apparently there is a push to have central DC systems as it is more efficient than lots of smaller DC power supplies. It was stated that over 30% (way over if LED lighting) of the energy use in an office is converted to DC. Computers, servers, small electronics, lighting, etc. all running on DC. Armstrong is involved as they have a system to run DC on the ceiling grid t-bars. Then lights can clip onto the t-bars to receive power. Personally I am still unsure of the viability of the idea as it required many different types of manufacturers to buy in to the same standard. However if it does work we can expect the technology to eventually be brought into the residential market.
http://www.armstrong.com/commceilingsna/article55189.html
http://emergealliance.org/Home.aspx

Armstrong has some good ideas, but their systems are not flexible enough to be used everywhere. There has been a huge push for DC lighting solutions for residential and commercial applications. Distance is not an issue because there's no high-speed data being transferred(no 300 foot rule, DC lighting can be run over 24 gauge wire for thousands of feet). People worry about the efficiency of AC-DC conversion. We cannot lose the AC to our homes, but if you transfer the AC to DC only once and make the process around 95% efficient, it is worth it. This company has something worth checking out. Their system is simple and much easier to install than regular AC systems. It's a start up, but it exploded at Light Fair International a few weeks ago. lumencache.com

DC lighting can be run over 24 gauge wire for thousands of feet

Maybe you could explain to us how that works, because length of run has limited us in a number of lighting applications and we are nowhere near "thousands of feet".

What kind of applications are you working with? What kind of wire? What voltages/current? What kind of lights/drivers are you using? Ideally DC should be run with lower currents.Think about this: the lower the current the lower the voltage drop. A commercially available Sylvania LED is current controlled, around 225 mA (that's low) for a 4-inch and 360mA (also low)for a 6-inch. Also, many times the drivers used are not capable of being far away from a light (one of the many problems with the drivers currently used with LEDs). At this exact moment I am running an LED over 788 ft of wire. The voltage drop is about 6V (37.9V to 31.9V exactly), and the current is around 275mA. If I adjust the current up to 315mA, the voltage drop stays around 6V (38.7V to 31.9V).It works perfectly, and even dims nicely. If you'd like I can show you. I have pics!

It's magic wire that doesn't follow E=IR!

There is a little bit of legitimacy to centralized DC. For example, instead of a computer plus 10 wall warts, I take the DC from the very efficient (and not oversized) power supply in the PC and bring it out to the misc small devices I use.

The voltage drop is about 6V (37.9V to 31.9V exactly), and the current is around 275mA. If I adjust the current up to 315mA, the voltage drop stays around 6V (38.7V to 31.9V).It works perfectly, and even dims nicely. If you'd like I can show you. I have pics!

Yeah, that's nice. I'm not sure where you actually leave the domain of "low-voltage", but at 38 Volts, you are already out of the range of just about all the residential products I've seen. And, you have one lamp out there. Don't think you can drive another one at 315mA on that wire. Like jonr says, there is no magic. Low voltage lighting systems run into real limitations.

Well fortunately, the "magic wire" is mass producible, and cheaper than copper! You are correct, there is no wire that defies any of the laws governing electrical behaviour.
According to the UL standard, low-voltage(class 2) is considered to be anything less than 60V(see UL 2108). So say you have 4 15V LED fixtures. You can string at least three of them together and remain class 2, possibly even all four. Remember, 38V was the voltage over 788 ft. I would never recommend someone run a 788ft wire for this application, it was simply a test. Besides, when was the last time you saw residential(or commercial) Ethernet wire run over a few hundred feet? How many residential products have you seen like this? The Sylvania fixture used in the test is also a variable. Any LED array can be used, whether it is constant current or constant voltage. So, instead of a 30V 275mA fixture, there could be an LED with completely different requirements. 15V LED's have also been used in testing, and worked great. They were 350mA, and when put in series, they worked great.
This system is already installed, and working very nicely in several residential properties.

24 gauge wire is 25 ohms per thousand feet. Put a small 10 watt/33V bulb on it and 20% of the energy is being wasted in the wire. Bigger bulbs waste much more. Not very efficient unless you are trying to save on wire costs (a reasonable goal for short runs to small bulbs).

Yes, you're right. LED arrays have low wattage like that(Sylvania's 4-inch, 550 lumen light uses 9 watts). Most LED arrays that you would use in a residential setting have very low power requirements, and are still much brighter than an incandescent(Sylvania's 6-inch, 850 lumen lught uses 11 watts). And like I said, I would never recommend running one thousand feet of wire for a light(not overly practical, or realistic in any kind of application), because that is a waste of energy. And why install a highly efficient system when you're just going to bog it down with long runs like that?
If you install a properly designed DC system, things like that shouldn't be an issue.

That's how I would pitch it - well distributed lighting that is less expensive to wire and has very long bulb life. Total cost and efficiency - probably about the same as 120VAC and fluorescents.

we are in the process of planning a new home, 4000 sq ft slab on grade, single story. WE are wiring the house both ac and dc. Ac more for resale as most people get a little wigged out when something seams different, none the less we will be powering 95 % from solar collection. Given the voltage drop on dc systems we plan to divide the house into zones that are smaller dedicated solar systems. We have a lot of roof space and placing the panels in clusters to control the zones reduces the heat collection factor on the panels and should provide a more stable system.

That oughta be interesting. Hope you do a writeup or a website.