what voltage?

May 27, 2009 11:24 PM

12v versus 240v set-up in a new off-grid (solar PV) residence.

I'm trying to work out the benefits in a new house of wiring for either 12 volt (the way the juice is coming in from the solar panels and batteries), or 240 volt (Australian standard mains power). What are the pros and cons of one way or the other?

You lose a lot in the inverter (do you??), but kitting out a house with 12 volt appliances is difficult.

Would you pay less for flex for 12v? Would it really be safer (some claim)? You couldn't connect to the grid without an inverter.

posted by wilful to home & garden (11 comments total)

Is this an actual house-house or a cottage or something similar?

Building codes may dictate what you can and can't do with low-voltage wiring. Cottages can usually get away with things you wouldn't do in a normal house, liek not having indoor plumbing and such.

You are correct that without an inverter you can't connect to the grid either to supplement solar generation or participate in whatever net billing or buyback scheme your local utility may have in place.

Finally, as you say, it's a pain to get 12V (or 24 or 48 or whatever) DC appliances, lighting, etc. But certainly there are suppliers of this stuff and the cost may not be much compared to the rest of the solar system depending on the size - people around here have $40K solar installations.

I know someone with a DC wired house, but it's completely off-grid and they use a combination of wind, solar, batteries and a diesel generator and they simply don't use much electricity.
posted by GuyZero at 11:50 PM on May 27

You can get (or modify) most appliances in 12v versions, and I assume that those will also be designed for lower energy use.

One disadvantage of lower voltage is that for a given amount of power, you'll need much heavier wire to carry the current at 12v (since there'll be 10× the current to make up for having 1/10 the voltage).
posted by hattifattener at 12:10 AM on May 28

A house, built to code. Which does allow 12v wiring.

heavier wire? I thought the opposite. But I'm ignorant. (1/20th the voltage, BTW).
posted by wilful at 12:13 AM on May 28

A good inverter will have efficiency at around 90-95%, so you won't lose much. It's up to you to work out how much power you're willing to lose (or conversely the extra number of panels you're willing to add) in order to have access to a greater range of appliances.

An added advantage of an inverter is that the really good off-grid ones can act as management units for the whole system, for example switching the generator on and off automatically, load shifting and battery management. Have a look at the SME Island for examples of what can be done (not a plug for that particular brand, but it's just one I know of that does all those things).

You will need the same amount of flex to wire the house and the system either way.

What sort of appliances are you likely to want? 12 or 24V fridges are fairly common (for the caravan market) but you won't find a 12V dishwasher, 12V microwaves are not common, as for vaccuum cleaners, power tools, blenders, coffee grinders, hairdryers and all the other things that electricity can run - forget it. Anything that you currently have a charger for, such as a mobile phone, you can charge on 12V or 24V if you get a car charger that would plug into a cigarette lighter socket in a car.

I assume you're using tank water if you're off-grid? Whether you go for 12V or 240V, if you want to run a pump for your water, consider getting a separate solar pump with its own battery bank and back-up fuel pump.

I grew up in an off-grid house in Australia in the 1980s that was wired (and still is wired) for 12V. Getting appliances beyond basic ones was a royal pain. My parents are also having trouble selling the house now because the 12V wiring reduces the potential buyers to those who are willing to replace all their appliances before moving in and live with a restricted range of appliances.

In short, I recommend getting an inverter.
posted by girlgenius at 12:21 AM on May 28

My parents have a house (which I grew up in) with 12-volt wiring.

Our problem, right off the bat, was that we had to make custom cables for all our devices, and that we were limited to devices that took under 12VDC and didn't draw much (because our system was so dinky.) The car cigarette-lighter plugs really couldn't take the daily use and we ended up having to use 240VAC plugs (nonstandard in America) and hope nobody would plug in an actual 240V device.

I highly recommend just wiring for 240VAC because there aren't any good standards (plugs, etc.) for wiring a 12V house that work with the devices you will be using. Besides, you need to use some seriously heavy cable if you want to use anything that draws more than, say, 25 watts. Re-wiring a house is a real bitch: Half-assing it the first time around like we did is a bum idea.
In terms of appliances, we use a propane-powered Danby refrigerator as well as a propane range, so those aren't such a concern. If you want to be running a washing maschine, be sure that your inverter throws a proper filtered sine wave and not a square wave which will burn a motor out.
posted by dunkadunc at 12:56 AM on May 28

heavier wire? I thought the opposite. But I'm ignorant. (1/20th the voltage, BTW).

For the same amount of power transmission, a 12V circuit passes about 20 times the current (neglecting the ac/dc issue and potential phase factors). The ohmic losses (heating) in a given piece of copper wire therefore go up by a factor of 20 squared... 400! So, yes, heavier wire.

One way to remember this: there's a darn good reason high power transmission lines are at thousands of volts.
posted by fatllama at 2:12 AM on May 28

heavier wire? I thought the opposite. But I'm ignorant. (1/20th the voltage, BTW).

Most appliances need a certain amount of power, which is measured in watts. For example, when you boil a kettle, it might use 2500 watts while it's boiling.

Power (in watts) is equal to voltage (in volts) multiplied by current (in amps). So if your kettle is connected to a 240 volt power supply, and draws 10 amps, your kettle consumes 2,400 watts.

If your power supply is at 12 volts instead of 240 volts, to draw 2,400 watts you would have to draw a current of 200 amps. In other words, 1/20th the voltage means 20 times the current.

Carrying more current means you need thicker wires. This is because all wires have some resistance, which means they heat up when current passes through them, with more current meaning more heating. The amount of power lost in the cable, in watts, is equal to the current (in amps) squared, multiplied by the resistance. If you put 10 amps through a cable with a resistance of 0.01 ohms, 0.1 watts of power will be lost in the cable. If you put 200 amps through the same cable, 2 watts of power will be lost in the cable. You don't want to lose too much power in your cables because (a) it's inefficient and (b) it heats your cables up, and if the cables get too hot things start melting and it' dangerous. So if you want to run 200 amps down a cable, you use a bigger cable which has a lower resistance. For example, with a resistance of 0.0005 ohms, you will only lose 0.1 watts.

In summary: Same amount of power at a lower voltage means a higher current, which means thicker cables.

What are the pros and cons of one way or the other?

If you're going for a small solar system, to do things like charge a laptop or phone, run a radio, and small things like that, doing it all at 12 volts means you don't have to pay for an inverter.

On the other hand if you're installing a large system, and you want the ability to switch between solar and mains power, and you want the ability to use most appliances 'out of the box' instead of having to find 12 volt appliances, an inverter is a better choice.
posted by Mike1024 at 2:16 AM on May 28

I'm not sure 12V is safer, since as posts above have stated, it will be much much easier to heat up a wire with overcurrent and cause a fire. Also, where will you find a 12V tv, 12v power for computers (laptops run 12-19v), power tools, amplifiers, etc etc. I think the hassle and extra expense (gut feel only) of 12V power will cause uncountable headaches.
posted by defcom1 at 3:45 AM on May 28

You can find 12v appliances and other items in RV stores.
To understand better the advantages of AC/DC you might want to read the history that went on between Edison and Westinghouse. Edison was a proponent of DC and made the first commercial generator in New York City. Problem was, it could only send electricity a few miles and the lines farthest from the power station received the least amount of power. Why? DC is shoving electrons from the power station, through the wires, to the load, back through the wires and back into the power station. AC is different. Alternating current is just that, each line is positive at one point and negative at another. So the electrons move one way, then the other in the same wire...no shoving down the wire. You don't pay the electric company for electricity...you pay them to constantly shake the electrons you already have.

So its easy to see why the electric company uses high voltage/low amps to transmit power great distances then uses a transformer to change it to lower voltage/higher amps where desired. Transformer? It is a collection of wires that don't touch each other but are nearby. The electromagnetic field that surrounds a wire will influence the other coil and the voltage is stepped up or stepped down by the fact that one coil will have less wire winds in it.

Back to Edison and Westinghouse. Edison had imagined that each house would have their own power station the way they have their own coal furnace while Westinghouse imagined the selling electricity model we use today. Edison wanted to convince the public that AC was dangerous (true enough, but so is DC) and to demonstrate he electrocuted an elephant (quickly and completely dead) and managed to convince the government that a more humane way of execution was the electric chair. Fortunately for the world, Edison lost (can you imagine the carbon emissions that would be out there if everyone burned whatever to make DC)....or did he. It could be that NOW that there is solar/wind generation (and, interestingly enough, a small self-regulating nuclear generator that would power a neighborhood) we need to bring back Edison.
posted by CodeMonkey at 6:19 AM on May 28

Why not run both?
posted by delmoi at 6:28 AM on May 28

Although I haven't built a PV system, I *have* built a battery-based backup power system for my house. As others have pointed out, wiring for 12 volts is going to have to handle 20 times the current as wiring for 240 volts (given the same loads). If you try to run high power loads on 12 volts, you are going to spend a fortune on heavy wire and still have significant loss in the wire itself. You will also end up having to deal with making lots of connections to heavy wire, which is hard work. Can't just wrap it around a terminal screw, you need to crimp on a big cable lug every time.

What you want to do is keep the inverter(s) as close as you can to the batteries, connected with short lengths of the heaviest wire you can fit. Inverter efficiency is pretty good these days as long as you are running the inverter at significant load. They tend to waste some power at low loads, and larger inverters waste more.

If you've got some high-power stuff (like a microwave oven) that doesn't have to run all the time, and some low-power stuff that does, consider a separate small inverter for the small stuff. Then you can shut off the big inverter when not in use (many have remote controls to make this really easy) and avoid wasting energy running a big inverter at low load.

A grid-tie inverter, if you want to go that route, is another story from the point of view of efficiency (and I don't know about about them to comment further on their efficiency), but even with that you'll still want to keep the low-voltage wiring as short as you can to minimize cost and power loss in said wiring.
posted by FishBike at 6:57 AM on May 28