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

[troy]

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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
-------------------
Heliocentric, LLC


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