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lower voltages mean much higher wiring losses or much more expensive wiring (or likely a combination of both). DC at a given voltage is substantially more dangerous than AC because it is prone to arcing.

20% conversion efficiency is pretty shit by modern standards

http://www.apcmedia.com/salest... is an interesting read, it's aimed at datacenter UPS systems but many of the arguments would apply equally to a house battery system.

As for the posters mention of living in a caravan a house is much bigger than a caravan (though admittedly smaller than a datacenter). So the wiring losses are less of an issue.

In what should come as little surprise to slashdotters anywhere, the telecom equipment switch to -48VDC was driven by a myth propagated by PHB's who were measuring the wrong thing http://www.pipelinepub.com/0407/pdf/Article%204_Carrier%20Grade_LTC.pdf

Take some advice http://www.apcmedia.com/salestools/SADE-5TNRLG_R6_EN.pdf from an engineer (not me) with 19 patents related to high-efficiency and high-density data center power and cooling infrastructure and read up on why so much of the non-US data centers use 400/230V AC because they are (a) nearly 6% more efficient than 48V DC systems and around 1% less efficient than 380V DC. Which helps explain why much more equipment is available for 230V AC than for 380V DC.

the 230 VAC , which has a peak to peak voltage of 325 volts, just a stones throw from 380 VDC.

I have a much better idea... let's just run datacenters at 230VAC. 90%+ efficient PSUs these days means you're throwing away standard components to save a couple percentage points on efficiency... not a good trade off.

The DC datacenter idea is a holdover from the days of 60% efficient PSUs, which is long past.

BTW, APC agrees with me:
  http://www.apcmedia.com/salestools/NRAN-76TTJY_R2_EN.pdf

The hum is caused by these giant transformers that step the power from DC to AC and create 110 volts of AC current at whatever amperage is required for normal devices. But there is a lot of wasted energy in doing that.

That may be true in your case, however, there's no reason for AC/DC conversion to be inefficient. Shop around for AC inverters for solar installations and you'll find ~97% efficient units.

APC did a study on the efficiency of AC vs. DC as well, which shows a bare minimum of extra efficiency to be had:
http://www.apcmedia.com/salestools/SADE-5TNRLG_R5_EN.pdf

The reason this works with large companies has NOTHING to do with efficiency. It's the flexibility to add new servers, one-by-one, without having to upgrade a central UPS, or taking a number of systems offline when a UPS fails. It's the compartmentalization, combined with the unimportance of any one server in a distributed environment, that makes it work.

Here's a decent guide to raised floors from APC. The conclusion is basically that they aren't strictly necessary or efficient any more, but that there are still lots of old dogs around who know what do do with a raised floor.

Big DC systems have thier problems too, if you run at telco standard 48V then the distribution losses are a killer. If you run at higher DC voltages you need all special stuff to safely handle the DC (DC is a LOT more prone to arcing at the same voltage, so you can't just take a switch or outlet rated for 500V AC and use it for 500V DC. They also only tend to save one conversion since you still need to convert from the distribution DC to the DC the server wants.

I read an APC paper not so long ago on this ( http://www.apcmedia.com/salestools/SADE-5TNRLG_R4_EN.pdf ) that conculded that a hypothetical 500V DC system was marginally more efficiant than the 230/400 three phase AC system which is commonly used in europe now but the difference was far less than that between the american AC system and the european AC system.

Viewed in this light putting the battery IN the server seems to be the masterstroke, it brings the advantages of reduced conversion steps without the downsides of trying to distribute DC arround a datacenter.

there was a UPS study on DC power ... That concluded that using the european system of 230/400 3 phase AC for distribution splitting out to 230V single phase AC near the point of use was almost as efficiant as a 400V DC system and far cheaper and easier to deploy.

http://www.apcmedia.com/salestools/SADE-5TNRLG_R5_EN.pdf

Every little bit helps

And every little bit of extra cost adds up... And make no mistake, a DC datacenter costs a LOT.

and point of load DC-DC converters are quite efficient, thus do not generate much heat.

No they aren't. There's nothing magic about DC that makes it more efficient to convert. Why do so many people seem to think that when the input and output voltages are a bit closer together, it's more efficient? In fact the opposite is closer to accurate.

AC can be converted to DC trivially, with bare minimum losses. The same is true for the inverse. You're not going to possibly see more than 3% efficiency improvements along the whole chain, and yet, you'll need a huge investment to convert to all-DC.

Here we go... The cure for your ignorance: http://www.apcmedia.com/salestools/SADE-5TNRLG_R5_EN.pdf Enjoy.

Sorry for the bad URL... try:

http://www.apcmedia.com/salestools/VAVR-5WGTSB_R0_EN.pdf

All of this is WRONG.

All this is just confusion, admittedly happily encouraged by the hardware manufactures.

MTBF is NOT and has NOTHING to do with the expected time before a drive fails.

MTBF is the expected time between failures in a SYSTEM which is REGULARLY MAINTAINED.

What does regularly maintained mean? It means that when a component reaches the end of its SERVICE LIFE that component is REPLACED.

TO WHIT: If at the end of the warranty period of your drive you replace said drive with a new burned-in hard drive, copying the data from old drive to new drive, and you keep doing this over and over again, on average it will take the MTBF before you encounter a failure.

Also, MTBF figures are notoriously inaccurate as they are arrived at using a formula which takes into account the MTBF of each component that goes into a system -- components which often have incorrect MTBF times.

Example: An electrolytic capacitor might have an MTBF of 100,000 years, assuming you replace it with a new tested electrolytic capacitor of the same type every year before all the electrolyte evaporates!

Knowing the MTBF without knowing the service life of the component or the burn-in procedures for a component is meaningless.

For more info see: http://www.apcmedia.com/salestools/VAVR-5WGTSB_R0_EN.pdf&revid=607475614&sa=X&oi=revisions_inline&resnum=0&ct=result&cd=3&usg=AFQjCNFpbPO04_wdZ8-aD-sN5yDKUViCsQ

This has been hashed and rehashed over and over.

See this document, Understanding EPO, for more information: Understanding Emergency Power Off