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

[Bob Ellison]

I have wondered about this and think the Morningstar 300 watt sinewave
will handle some of the small, nagging, constant loads. Lights and
such on a seperate circuit. It is something like 90% effecient at 100
watts but I do not know if it is NEC legal.
This would leave the main inverter asleep untill it is needed for the
bigger loads.


Bob


On 8/8/07, Drake Chamberlin <Drake.Chamberlin at redwoodalliance.org> wrote:
>
> 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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