Efficient inverters for stand alone power systems? [RE-wrenches]

[Drake Chamberlin]

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Hi Troy,

The transformer-less, automotive inverters are amazing.  I'm using 
one in an off grid cabin.  It has a fraction of an amp, no load draw, 
and powers CF lighting and small NiCad chargers (not DeWalt or other 
power tool batteries) at what appears to be incredible efficiency.  I 
leave it on all the time.  There is no hesitation on startup.

I'm using a Xantrex 600, that surges up to 1200 watts.  Xantrex also 
makes a 1200 Watt model that surges up to 2400 Watts.  These units are cheap.

The general problem with these is that you can't ground the neutral, 
without destroying the device.  It violates the NEC to use 
one.  Another issue is that the wave form of the one I have is 
modified square, but works well with most things.  Also, I don't know 
if they come in higher DC voltages than 12 V.

An approach I've considered is to use one of these units for the 
loads that will be on most of the time, and have a pair of Outbacks 
stacked for bigger loads.  The Outbacks will sleep most of the time, 
and a battery bank balancer will be used to keep the battery bank happy.

Drake

At 01:34 AM 8/8/2007, you wrote:

>I'm trying to find an efficient inverter for stand alone power systems.
>
>It seems that in the grid-tied boom, that nobody (?)  has really
>addressed the fundamental problem with stand-alone inverters:
>efficiency at low power draw.  Analyzing a couple of stand alone
>residences & small office applications, the *typical* power draw is
>quite small. You still need peaking performance for the occasional
>surge requirement, but 80% of the time the base load may be 100-200
>watts or even less.
>
>Given the Outback Inverters as an example (a favorite of many off- 
>grid designers), the weighted average efficiency is only around 75%
>or so for these applications. The most insidious thing is, as you
>improve the efficiency of the base load appliances, efficiency drops
>further (approaching zero). So for example if only one 40 watt
>computer is running the efficiency is only 40%.
>
>Obviously the problem has to do with the fact you are running against
>the quiescent current requirements of the inverter controller (which
>should be only a small fraction of a watt) and switching losses (the
>dominant issue I'd guess). But this has been addressed in switched
>converters long ago with by reducing switch frequency  and/or
>transitioning to PFM at low load. We've designed power converters
>that are 95% efficient down to fractions of a watt. The point is, it
>should be feasible to design an inverter that is efficient down into
>the few watt range (and lower).
>
>I'd like to hear if there are any multi kW inverters (ie 2-5 kW)  out
>there that are efficient down to low power draws (tens of watts) .
>And I wouldn't mind hearing any commentary from inverter
>manufacturers on the subject.
>
>Thanks,
>
>Troy Harvey
>-------------------
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