Inverter Load Analysis [RE-wrenches]

[Windy Dankoff, Dankoff Solar]

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Hey Ray,

I'm glad you appreciate the PV System Design Worksheet that I shared 
with the Wrenches a couple years ago.

You're probably right about the redundancy that you describe. Then 
again, how do you determine the inverter efficiency? It's really low 
during low-load hours, but that bears less weight in the energy calc. 
Your 92% may be high for a modest remote home, for that reason. I 
believe that the best we can do is refine our guesswork, be 
conservative, and accept the nature of the game.

I entered a default battery efficiency of 88% thinking it is already 
high, to account for relatively high daytime loads with no loss. If all 
loads were at night, efficiency would be 80-85% with good batteries and 
a well-designed charging scheme. Even then, there are different ways to 
define and determine batt efficiency.

Doug's advice about customers always using more than they calculated 
points to always calc'ing on the high side anyway. I know you agree.

I won't comment about your off-grid biz woes, but to say that I hope 
the PV dealers who are drawn to strictly grid-tie will leave more and 
better off-grid biz to guys like you who really light the field.

Windy



> Date: Wed, 27 Jul 2005 22:21:26 -0600
> From: Solar Ray <ray at solarray.com>
> Subject: Inverter Load Analysis
>
>
>
> Hi All;
>
> I have possibly a dumb question: I use a spreadsheet originally 
> designed by
> Windy Dankoff, and it figures inverter average efficiency into the 
> total
> energy calculation. I added a page that lists all the appliances and I 
> list
> the inverter no load draw times the hours it will be on. Is this 
> redundant?
> I use a 92% efficiency number for the inverter overall, but should I 
> raise
> this, since I'm adding in the inverter's no load draw in the other 
> page?
> For an average load of say 500 watts and a no load draw of 10 watts,  
> that
> is about 2% of the total inverter draw. I'm considering raising my 
> inverter
> efficiency number by 2% to account for this.
> Also, I was considering raising my battery efficiency number 
> (currently I
> figure 88%) because many of the big loads (refrigeration, water 
> pumping)
> occur more in the day, and so the battery isn't be cycled as much to 
> power
> those loads. I was considering going up to 90 or92% for average battery
> energy efficiency on projects with larger daytime loads. (notice I said
> "energy efficiency" which is charging efficiency  x voltaic efficiency)
> Off grid design seems to be much more difficult and time consuming than
> grid tie stuff. A lot more call backs too. I wonder if the days are
> numbered for us old fashioned off grid load analysis freaks. 10 times 
> the
> trouble for half the pay will send even the best of us looking  for 
> rebate
> programs.
>
> Ray Walters
> ray at solarray.com
> President, SolarRay, Inc.
> NABCEP Certified Solar PV Installer
> BS Mechanical Engineering, UT Austin 88
> Returned US Peace Corps Volunteer

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