Disadvantage is that you'd have to use thick wires due to the high current. A 50W 12V light needs over 4A whereas a 50W light at 240V needs only about 0.2A. If you didn't use thick wires, you'd have a potential fire hazard. Using standard 240V wiring would soon run into current problems. The benefit of running 12V would be that electric shock hazards would be greatly minimized. But I'd say the disadvantages outweigh this small advantage - particularly with the price of copper these days.

Paul.

I looked at a 12V ring a couple of years back and the wiring sizes were alarming! Star/radial was the only way to achieve what I needed, but then then the sheer mass of the cables made them difficult to route through the house. It's a shame though as the efficiency of the 12V appliances would be greatly improved with a real PSU. I'm surprised that no-one has come up with a more efficient local transformer.

48 or 50 V is an industry standard for all battery supported kit. Things like your local Vodaphone transmitter and all its input kit runs from 50V as do telephone exchanges. There are a lot of standard inverters 50V DC to 230Vac available.
Toroidal transformers are extremely efficient at about 97% and they don't have any of those silicon based fuses in them.
frank

In response to Guest I know that toroidal transformers can be efficient if they are made correctly. But the normal plug-in type transformer to charge phones or run various electronic equipment don't seem to be very efficient. Generating lots of heat and is this because the core doesn't contain enough iron?

In response to Paul in Montreal; I have also thought that a DC motor is more efficent than a like for like AC monitor. Therefore, could it be that motor based appliances such as washing machine and tumble dryers are more efficicent if fed from a DC supply? I think the statement that all appliances are more run as DC maybe taking this a bit far.

After a bit of googling, it seems that small motors operating under partial load are relatively inefficient, but large AC motors have efficiencies in the 90-96% range. The magnetization losses in the core are easily mitigated in the same way they are in transformers: the iron is laminated to prevent eddy currents. Transforming AC to DC does not have much loss in the transformer, it's the subsequent electronics where the losses are. AC to AC transformers also have efficiencies in the high 90s%

Most computers don't use transformers in their power supplies so it's hard to see where efficiency improvements can come from - switching power supplies are also pretty efficient. Small fans and motors (like in the disc drive) and relatively inefficient and I guess could be improved, but to talk of a 6-10x improvement is fallacious IMHO as most of the power in a computer is dissipated in the chips themselves, not the fans and motors.

Your example of water pumps is also mistaken since power is measured in Watts, not Volts. So it is meaningless to say a 20V AC pump could be replaced by an 8V one. From what I've read, it seems that is is easier to make small DC motors more efficient than small AC ones, but large motors are almost universally AC - mainly because they need a moving magnetic field to provide the motion which is already supplied by AC whereas has to be "commutated" in a DC motor.

As for having a DC supply in a house, I do buy the argument that it is better to have only one AC-DC converter rather than having every device have its own converter. However, 48V DC is a much larger voltage than most electronics operate at (3V or less these days) and so some conversion is still required. DC-DC conversion is no more efficient than AC-DC conversion so it's hard to see how much gain there could be. Large appliances would still need 240V AC (as they do in North America where the supply is at 120V for small appliances) so it's hard to see how having an extra supply at 48VDC can save wire.

Paul in Montreal

Yes should have been watts not volts... I'll claim a typo there...

Paul in Montreal, if you were to consider PV or Wind turbines as the electrical source would it then make more sense to run at lower voltages in DC? I believe most off-grid solutions are 12v / 24v and stored in lead acide batteries. They then require an invertor to step up to mains voltage.

Transformers can only do AC-AC conversions (and can do it with close to 100% efficiency). Invertors convert DC-AC (sometimes the other way) using high power electronics with somewhat lower efficiency (I think 70-80% is typical).

For PV sources that are DC, then if the DC can be used directly, this is a big advantage. If mains voltage is required, invertors do cause some loss, and high power invertors are expensive.

Paul in Montreal

As far as I know the series connection will be OK as far as voltage is concerned but you could run into problems with current if your application is what I think it is. I really think that if you want to move heat around, you're better off doing it directly as in the Deer Lake solar community in Alberta. All that said, series connections may work, but you're right about the Christmas tree lights syndrome. At least this would avoid any conversion losses though (except for the really poor light to electricity conversion in the PV array in the first place compared to what can be extracted directly as heat).

Paul

Dear sir or Madam,

Thank you, for giving us the opportunity to ask questions concerning electricity. As of now, I am doing my further studies in Electronique Telecommunication.

There is an house and an office that are being supplied electricity by using generators (gensets) 24hours/day and 7days/week; I was asked to investigate what will be required to charge batteries during the day and then at night time convert DC-AC for only 10bulbs x 100watts, a tv and a refrigerator in order to reduce the cost of keeping the gensets running 24hours. Please do advice as appropriate.

Thank you in advance for your assistance.

JNV.

very interesting lads this is what I like about this forum not to technical but understandable. thanks.

We have been installing dual voltage systems for a number of years now.
It is great to have the choice and resilience provided by ELV.
In the average house there are already a plethora of ELV devices nearly all of which are plugged into micro charging transformers.
The life cycle analysis for the myriad of these little transformers usually indicates a far greater cost than the products themselves.
Renewable and conventional electricity distribution are complimentary and well established perhaps the best source for newcomers would be researching the marine industry.
The ELV (Extra Low Voltage) level should not be the prime concern ie whether it is 12V, 24V, 48V unless you have high power requirements.
Submarines and Milk floats being a good example of 48V.
The most important data to collect is what your load profiles are (ie power and number of appliances x time used)
We recommend doing 3 profiles for an average year (summer / Autumn & Spring / Winter).
There are a large number of domestic low power appliances available including fridges / washing machines etc.
The fridges are even tri input (ELV / LV / gas) and the washing machines DC motors etc.
Our experiences are as follows:
if using 12V distribute in a radial manner (Star) directly to appliances and seperate lighting circuits into logical groups
Limit circuit lengths.
Always use a different colour sheathed cable for ELV than those used in LV and run them segregated.
Avoid daisy chaining for DC and centralise all circuit protection (fuses)
Allways split your battery storage and for back up use a sensing battery charger (pulsed type).
replace batteries in pairs.
Minimise the use of incandescent lamps (halogen/tungsten etc).
Nearly all domestic appliances convert AC to DC usually more than once.
Standby (parasitic losses) can be fairly onerous over time.
Use DC fans for vent and heat recovery.
Use DC voltage converters for all those portable appliance battery chargers rather than invertors.
If you wish to have ELV DC as the primary distribution use a 230V LV AC step down isolating transformer.
If using renewables specifying DC pumps / heating elements can be useful way of shunting your excess supply in time of inclement weather.
Over larger distances (or as the previous writer described) it is better to provide isolated clusters of ELV DC of smaller power.

Saying that though if you are really keen on the subject consider one of our solutions....
Get your laughing hats on..
We allready have very cheap and reliable CHPs running at ELV available (LV AC mains alternators can be retrofitted also).
It is very simple to plug them in for both heating / hot water and electricity.
If you are going to use one anyway and are keen on biofuels it is probably the best solution allround.
Voila...
the motor car used as a back up domestic CHP...

best regards and remember to fuse ALL your ELV & LV electrical circuits
www.thamesrenewables.com

I'm planning to build a garage on a parking space I own a couple of hundred yards down the road from my house. I'm intending to convert the back of it into a home office and will need some power for a light and to recharge my laptop.

Do you think I could provide the power I need off a 12v car battery? Eventually I could charge it with micro-renewables but initially I'll just carry it up to the house and charge it off the mains. Assuming this is possible, what tackle would I need? The transformer for my laptop gives an output of 19.5v, 3.34A.