PowerSwitch

Many off grid homes use a 12 volt battery system for lighting etc. with the batteries charged by PV, wind or other means.
12 volts has the merits of cheapness, safety and simplicity, and I would allways recomend this voltage for small systems.
The drawback is that exceedingly thick cables are required at considerable expense for all but the smallest loads.

Changing to 24 volts is the obvious solution, but far from ideal as the range of lamps and small appliances is far less for 24 volts.

An excellent scheme for the larger installation is to use 3 wire DC.
This gives a choice of 12 volts for lighting or small loads but 24 volts for larger loads.

A 24 volt battery is used with a center tap which is normally earthed.
This wire is the neutral.
Two other wires are used, positive and negative, connected to the battery terminals. These wires are known as the positive and negative outers.

12 volt loads are connected between the neutral and either of the outers.
Care should be taken that the loads on the 2 outers are equal, so as to ensure an equal discharge on each half of the battery.
Larger loads should be 24 volts and be connected between the 2 outers.

No fuses or switches should be in the neutral.
Fuses are required in the outer wire for 12 volt loads, and in BOTH outers for 24 volt loads.

A great saving in wire results on larger installations.
Lighting circuits can serve numerous lamps, these being equally divided between the 2 outers.

Great care must be taken in wiring 3 wire circuits so as to ensure correct voltages throughout, and for most loads correct polarity also.

The OLD standard colours for 3 wire DC were
RED=POSITIVE OUTER
BLACK=NEUTRAL
BLUE=NEGATIVE OUTER
Although officialy obsolete, these colours are still sometimes used for DC.

On 12/0/12 volt systems as described twin and earth cable is often used, with the brown core for the positive outer, the blue core for the negative outer, and the bare earth wire for the neutral.
Use of the bare earth wire as a current carying conductor is contary to regulations, but I cant say that this worries me at such low voltages.
It would be good practice to sleeve the bare earth wire in black or white in such cases, not green/yellow as used for earth.

Despite care in balancing the loads between the positive and negative outers, some imbalance in the state of charge between the 2 halves of the battery is virtually certain.
This may be relieved in a number of ways.
Connect a frequently used 12 volt lamp to a changeover switch, in order that it may connected to either side of the system. Every few days check the state of charge of each side of the battery, and ensure that that this light is used from the most charged side.
Alternatively install 2 PV modules, each with its own 12 volt charge controller, one for each half of the battery. (most of the charging may be at 24 volts from a W/T or large PV array, but a small dedicated module for each half of the battery allows for any small imbalance)

Some years ago I installed a very large 3 wire system in an off grid home, it has given every satisfaction.
Most lighting is 12 volts and some small appliances.
24 volts is used for power tools, large flourescent lights, a kettle, a small oven and a microwave, and refrigeration.

As a historical note, mains supplies used to be DC, with the mains in the street being 3 wire. Alternate houses being connected between the neutral and the positive or negative outers.
Larger premises would have both outers brought in thereby permitting of a higher voltage for power.
The voltage was originaly 120/0/120, latter doubled to 240/0/240, with 480 between the outers. This remained the standard until the end of DC

Thanks Adam, good explanation.

In a house with existing mains wiring, is it worth having a 12V battery/PV system, with 12V used close to the system for charging small batteries and devices, and a decent size inverter also close to it (short 12V wires), with a changeover switch to disconnect the house from the grid and connect it to the inverter? Obviously you'd need be be careful not to turn on things like kettles when running from the inverter...

It depends on how doomerish you are !
A central inverter and changeover switch has the advantage of utilising existing wiring and appliances with minimum disturbance of rewiring.
There are however a number of pitfalls.
1) the inverter might fail, it should therefore be duplicated which adds to the substantial costs.
2) an EMP event will kill the inverter, and possibly the spare.
3) a risk of kettles or hairdyers being used and either tripping the inverter, or if this be large enough, rapidly running down the battery.
4) The changeover switch must be rated for the maximum possible load current whilst on grid power, no matter how small is the load on inverter, it will therefore be a costly item.

Certainly much better than nothing, and fine for short term breakdowns or rota cuts, but not perhaps the best solution for the long emergency.

For TEOTWAWKI I would favour a simple 12 volt only system for up to about 500/1,000 watts load, and 3 wire DC at 12/0/12 volts for from about 500 watts up to a few KW.
Beyond a few KW, which is a very large system indeed, it might be worth considering either duplicated or even triplicated inverters, or mains voltage DC.

The large 3 wire system that I installed has a maximum load of almost 10 KW, about 400 amps at 24 volts, though the average is far less. That is about as big as is sensible.
The biggest loads are a saw bench at about 100 amps and a microwave oven at about 60 amps.

I've decided to go with your 3-wire system in the end Adam. So I'll have two 12V batteries in series, with the charge controller running in 24V mode, so I'll get the full output of my panels within it's 15A limit. I'm putting in some rocker switches to alternate the loads between the two batteries, and have also bought a couple of digital battery voltage displays, that are powered from the batteries themselves, so that will make monitoring easy.

The way I'm setting it all up, it would be quite straightforward to rewire the system for 12V operation, if equipment failure in difficult times required it.

The only job remaining, apart from getting the panels on the roof in a couple of weeks, is figuring out where to put all the DC stuff so it's out of the way but accessible!

Sounds good.
Any new equipment should, if possible be 24 volt so as to minimise the unbalance between sides of a 3 wire system.
Large loads should be 24 volt if possible.
Small loads may be 12 volt and should be as balanced as possible.

As noted, lamps or other loads may be switched from one side of the system to the other in order to balance the state of charge between the two sides of the battery.
The obvious way of doing this is to use a simple 2 way 3 terminal switch.
Connect one side of the lamp to the neutral, and the other side of the lamp to the common terminal of the switch. Connect the positive outer to one switch terminal and the negative outer to the other terminal of the switch. Note that this arrangement reverses the polarity of the lamp according to which side of the battery is used.
Fine for incandescent lamps but wont work with most flourescent or LED lamps.
If polarity matters, a special 6 terminal switch is needed.
I can supply these, or a part number for you to order if not available locally.

I've got a couple of DPDT (6-terminal) switches on the way, as all the loads I'll be switching need the right polarity.

At some point I'll probably get a 24V input inverter with a higher power output, but not for a while...

The other thing I could get as a 24v-12v converter for the DC loads, but then I habve yet another component to fail one day, and an efficiency loss as well.

Yes a DPDT 6 terminal switch is what you need.
I would agree that any additional or replacement inverter should be for 24 volt input.
24 volt to 12 volt converters are indeed another source of loss and another point of failure.
In general connect 12 volt loads between the neutral and either of the outers, not via a stepdown converter.
Existing 12 volt flourescent lights may be used thus, but additional or replacement units might be better if 24 volts.
12 volt CFLs are readily available, 24 volt ones much less so.
Some very good 12 volt LED lighting is now available, 24 volt much less so.
Another merit of a 3 wire system is that most 24 volt fans will run at reduced speed on 12 volts, and be more efficient and virtually silent, with the option of full speed operation at 24 volts when needed.

In years to come, your 3 wire system could be expanded to a sufficient size to work a 24 volt microwave oven or electric kettle.