That&#x27;s the point, if you don&#x27;t do the conversion to low voltage at the source you lose a lot in losses from the resistance in the lines.<p>Let&#x27;s say you&#x27;re using the normal 14 gauge solid core wire commonly found in houses right now for a 5V supply.  A 200ft run of that in the house (both ways) isn&#x27;t out of the question if there&#x27;s a central supply in a 2 story building.  That&#x27;s about 1 ohm [1], which doesn&#x27;t sound like much at first until we look at what it ends up meaning.<p>A phone might take an 1 Amp to charge at it&#x27;s fast rate (tablets 2 or more), so we&#x27;ve got 1 Amp, with a 1 Ohm resistance.  That means we&#x27;re going to have a 1 Volt drop to that outlet, which means the phone is likely not going to charge at the correct current since it&#x27;ll only see 4 Volts at the outlet.  It also means you&#x27;ve just had a 20% loss in the power line in order to have a larger central power supply.<p>Combine that with the fact that most Switch-Mode Power Supplies aren&#x27;t actually that efficient when not under load (part of the problem) then it might not be unreasonable to have it running at 80% efficiency to begin with.  At this point you&#x27;re down to 64% total efficiency instead of the 80-90% that the small wall warts end up getting you.<p>You could use a higher voltage, 48V isn&#x27;t uncommon because of the telephone industry, but then you need conversion at the outlet again and end up with the same problem you&#x27;ve got with wall warts.<p>The other option is to use more copper to carry the current, say 10 gauge, which brings the losses to 0.4V and about 10% losses, but you need 150% more copper than 14 gauge.<p>[1] <a href="http:&#x2F;&#x2F;www.cirris.com&#x2F;learning-center&#x2F;calculators&#x2F;133-wire-resistance-calculator-table" rel="nofollow">http:&#x2F;&#x2F;www.cirris.com&#x2F;learning-center&#x2F;calculators&#x2F;133-wire-r...</a>