Modern LED bulbs must convert the standard household supply (for example $240\text{V AC}$ in the UK) into a DC supply at a lower voltage (usually $12\text{V DC}$ I think) for the LED array. This is done on a per-bulb basis.
Does this waste much power in the conversion? If all of your bulbs are LED bulbs, would it make sense instead to have a $12\text{V DC}$ circuit for lighting around the whole house and LED bulbs without the per-bulb conversion?
Distributed DC power is actually used in some new construction. It's driven less by the efficiency of the transformer than by other logistics. Here in California, at least, we have a law (known as Title 24) which requires some fairly sophisticated controls of lighting, as well as fairly low power consumption per square foot. The control requirements include compensating for daylight by automatically dimming lights, occupancy sensing, and brownout usage reductions. This means that the controllers are fairly sophisticated and expensive, and that LED light fixtures are sometimes preferred in new construction. As a result, some systems are being sold with the controllers that output dimmed DC which directly powers the LED fixtures.
DC distribution reduces the number of wires that have to go to each fixture (DC+ and DC- instead of AC hot, neutral, and a separate control line) and saves some money on electronics. As far as I know though, there is still at least one controller per room. I'm not aware of any systems that distribute DC all throughout a building. I imagine this is because as lengths and currents increase, the advantages of high voltage in reducing wire size become more significant.
Yes: Something like this has been commonly done in the U.S. for decades. Both Incandescent undercabinet lights and exterior low-voltage "landscape" lighting are often run from a single 12V transformer over 12-gauge wires. So there is code for it, and anyone who has built a computer recently knows that high-efficiency DC converters carry an upfront premium so one doubts the converter built into every LED bulb is as efficient ... not to mention that distancing the transformer from the bulb will increase its lifespan by reducing heat exposure.
RV, camping, boating, and solar power industry might have had a significant influence in developing LED blubs powered by 12V DC. The automotive industry too might have had some influence. Some of the issues that make 12V DC lighting not be practical are:
Converting AC power to 12V DC power locally for LED (Light Emitted Diodes) bulb lighting
The fundamental building block of an LED is a Diode. A typical diode switches on at 0.7V forward voltage. Current/Voltage (I-V ) characteristic for a typical diode is shown below.
Depending on the LED color the forward voltage varies. The table below lists typical forward voltage for a different colored LED’s.
Forward voltage for a typical white LED is between 3V to 4V. Below is I-V graph for typical coloured LED's
Typical AC supply (120V in USA, 240V UK) need to be rectified to a lower DC voltage. Transmitting DC power over a long distance is inefficient. Thus AC supply is a far more efficient to transmit power. There is always an energy loss when you convert AC to DC, but current technology has overcome most of these efficiencies. There are small cost effective rectifier circuit modules available in the market.
Below is an example of one such device. (RECOM Power RAC01-05SC)
Click on image for a larger version of the image.
Summary
Although converting AC to DC is efficient, the other structural cost outweigh the benefits of a separate 12V DC line in homes. The growth in technology and markets it is very likely there will be universal AC LED bulbs where the conversion will occur seamlessly regardless of input AC voltage.
References:
I'd say it depends on the size of the 12V network and the number of transformers and their size. The problem is two-fold.
First of all, we have to think about the conversion losses incurred. If (and that's a big If) the efficiency is the same on the bulbs than it would be on a central transformer, then the losses incurred by having a central transformer are on par with the one incurred by the distributed light bulbs. However, usually a transformer has an efficiency peak (for example, if the peak power of the transformer is 500W, the efficiency peak could be at 460W. When the system uses more or less power, the transformer is less efficient). This has to be taken into account when calculation potential losses.
Secondly, there's a wiring problem. When you route 220V or 110V, the cable section takes into account the maximal current that will have to be transported. In France, you can route lighting wiring on 1.5mm² cable (AWG15 or 16). Using LEDs will reduce the power used, but yet, as you reduced the voltage, the voltage drop may increase if you use a cable too thin. For example, if you use 12W bulbs, they will draw 1A of current at 12V instead of 0.05A at 220V. A cable too thin may incur a voltage drop too important that will prevent the LEDs from lighting up.
