Friday, 4 April 2014

Low voltage circuit... is it worth it?

They were selling double adapters with USB ports in the supermarket the other day, so you can plug in your phone to charge and still use the socket for something else. Great idea. It made me think about something I wanted to do in our house.

Since we have solar panels on the roof and since a lot of electronic appliances work on DC, I wanted to have low voltage outlets. One problem with this was deciding the voltage and the plugs to use. This was not clear a short couple of years ago when we were building the house, but if we were building a house now they would be USB sockets. The eponymous "universal" has become a self-fulfilling prophecy, and the U could also stand for "ubiquitous". Not only do USB cables charge phones, tablets and cameras, there is also a range of appliances that can plug into computers, such as fans and lights.

Although a USB socket would be the way to do it, I have to wonder whether it would be worth it. Intuitively it seems that sending low voltage electricity around the house would save power lost in the inverter converting DC to AC and in all those inefficient chargers converting it back again.

But, power losses over wires are proportional to the square of the current, so 5 volt USB voltage is going to lose 400 times more power over the house wiring than 100 volt AC mains.

That sounds terrible, but is it a lot? Is it more than what you'd lose in the devices?

It depends on how much power the devices draw. If it's a 500mA camera charger, ten metres of wire away from the power source, then it's only going to lose half a percent of the power. Some chargers go up to 5 amps, in which case you'd be losing 5% of the power. This is comparable to the inefficiencies of converting at each end, so it's probably not worth the extra cost of putting the wiring in.

I've been assuming the same gauge of house wiring used for AC and DC, at something like 5 milliohms per metre, although obviously not the same actual wires.

Of course if the DC wiring were thicker, the line losses would be less. Perhaps you could turn your house into some kind of a battery where you had one strip on one side of the walls and another on the other side, holding charge between them. This seems like a recipe for disaster since all you'd need would be a nail through it and it would short out. With increasing numbers of houses with solar panels, some kind of embodied electrical capacity in the building materials may be worth investigating though.