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I'm about to buy a house with an existing solar system, with 12V batteries and a 240V inverter driving the mains and lighting.

I will be installing new smart lights throughout. My question is, should I stick with a conventional 240V lighting circuit, or is it more efficient to go for 12V lights, bypassing the inverter? So I guess it's the payoff between voltage drop on 12V circuits vs power loss through the inverter.

The house is on 3 levels with a 80m2 footprint.

The house has no mains supply, it's solely solar with a backup generator.

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    \$\begingroup\$ Run a few numbers on cable loss at 240 V versus 12 V and see what you find. Do the same for rectifier losses. Add the results. Draw conclusions. \$\endgroup\$ – winny Nov 10 '17 at 11:10
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    \$\begingroup\$ You should also consider the lifetime of typical 240V and 12V LED lights - the 240V variants die much, much quicker because of the power supply inside them that has to convert the 240V down again. The benefit of not having to buy new lights as often might outweight the cost of the thicker wire that's needed. 24V or even 48V DC might also be an option but you'd have to do the numbers. (DC/DC converters are pretty efficient) \$\endgroup\$ – Jonathan S. Nov 10 '17 at 12:06
  • \$\begingroup\$ Also if you go with a 12V lighting system you have to consider efficiency loss of the 240V to 12V converters needed to power the lighting when you are operating off the AC mains. Solar is not effective every day!! \$\endgroup\$ – Michael Karas Nov 10 '17 at 12:36
  • \$\begingroup\$ If you decide to switch to DC, people (me included) will appreciate if you share the experience and the results. Do you consider using DC for anything else than lighting? \$\endgroup\$ – A.K. Nov 10 '17 at 17:52
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    \$\begingroup\$ the 12v setup can be a lot more efficient, but poses some challenges. many 12v strips/COBs really want closer to 14v for max efficacy. you can use a boost converter at point of use and PWM to dim from that maximum. You can also just see what kind of output/stability you get w/o DC/DC conversion, you can easily be "lucky". Voltage drops aren't a big deal if you're using "house wiring" without long runs. if you can find lights that work with 12.0 and a battery that spits out 12.7, you have plenty of room. \$\endgroup\$ – dandavis Nov 10 '17 at 20:10
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My question is, should I stick with a conventional 240 V lighting circuit, or is it more efficient to go for 12V lights, bypassing the inverter? So I guess it's the payoff between voltage drop on 12V circuits vs power loss through the inverter.

The house has no mains supply, it's solely solar with a backup generator.

Given the two items in bold above my recommendation would be to go 12 V.

  • You can then switch off the inverter which is going to waste power in standby. If you use 240 V AC you will have to leave the inverter running in case someone switches on a light. The 12 V option allows you to switch off the inverter at night, etc.
  • 12 V lamps are readily available, won't need local power supplies and are simple to wire and maintain.
  • You may require DC-rated switches if the currents become significant.
  • In the event of a battery failure you can jump-start your house lighting from a vehicle battery or 12 V charger from your generator. (Check to see if it has a 12 V output too.)
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Calculating wire loss is simple, here's a US version with AWG. I'll use metric, so 1.5mm2 wire is 11 mOhm/m.

One could say that higher voltage allows using a thinner cheaper wire, however building code and economy interferes with the calculation. For example here, 1.5mm2 wire is the standard for lights, so that would be used in the 240V case. Thus the 12V case is more about using the same 1.5mm2 versus thicker wire.

Another complication is that 230V LED lights do not have power factor correction, so the current they draw will not be a sine, they would draw high current peaks of unknown amplitude and duration. This also depends on the waveform at the output of the inverter. If it's a square or modified sine, the cable losses will be different...

However cable losses will still be rather tiny, let's consider a 10W bulb, on 230V this would draw 50mA RMS (rounding up), let's fudge this to 150mA peaks with a duty cycle of 1/3, this gives 2mW losses on a 10m wire, which is negligible.

Thus let's neglect the cable losses for 230V.

For 12V the same bulb would draw 0.83A DC which amounts to 76 mW per 10m of wire. Again not too bad, but remember losses are in RI^2 so they are proportional to the square of the number of bulbs...

Also losses are in RI^2 but R is inversely proportional to the copper cross section which determines the cost of the wire. A wire that is twice as expensive (2x the amount of copper) will not halve the losses! It will only reduce them by 30%... (square root of 2)

Anyway. Draw your installation with number of bulbs and wire lengths.

Compare the lumen/watt efficiency of 12V and 230V bulbs. Also the price and availability. Compute how many watts/bulbs you need for your target lumens. You should of course compare two installations with equal light output...

So you get the 12V current, and thus the cable losses. Most likely they will be tolerable.

Now add the inverter losses to the 240V case, looking at the inverter datasheet, and compute how much current each case will use from the 12V battery...

My guess: the winner will be determined by the efficiency of the 12V LEDs you find. Remember efficiency is not the only criteria, color rendition is also very important for comfort, and good color rendition increases cost and decreases efficiency.

Make sure you get a sample of the LEDs first before buying the whole lot, and make really sure they look good to you. Some have very high efficiency, but they look nasty! (like: greenish, bluish, make people look like aliens, make your blue veins stand out, etc). These are for the corridors only!

I put CRI95 LEDs in my kitchen... the food looks good ;) some LEDs make your fresh steak look like roadkill from a month ago...

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You need to first consider which would have more losses the 12VDC to 240VAC i.e., inverter conversion loss+ wiring loss or incase going with 12VDC lighting the line losses here.

But even if you consider using smart LEDs for lighting the cable losses at 12VDC is going to be much higher as the length increases, unless you plan to have bare minimum of one bulb per room. In short if your current draw is going to be higher at 12VDC you might want to keep the wiring short due to increased voltage drop.

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  • \$\begingroup\$ that's pretty much my question :-) \$\endgroup\$ – pinoyyid Nov 11 '17 at 11:33
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    \$\begingroup\$ I would say stick with the 12V lamps.They last a lot longer. Plus you can run directly from your batteries too. \$\endgroup\$ – The_Vintage_Collector Nov 11 '17 at 12:34

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