Inverter Load Analysis [RE-wrenches]

Ezra Auerbach, DragonSun Consulting ezra at lasqueti.net
Fri Jul 29 06:38:34 PDT 2005


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Hi,

Years ago I did very careful load analysis of every off grid job I was
involved in. What I learned was that folks didn't have a clue how much
power they'd really use, and to top it off they wouldn't admit to stuff
like watching a few hours of TV a day. (The old "only the news" story).

I learned to ask, a few questions like home size, number of occupants, how
many were teenagers, and any nasty habits like welding or woodwork. This
information allowed me to make an educated guess as to how many kW/h per
day the house would demand. (I tend to figure a base load of 2 kWh for a
full time home with an additional kWh for each additional occupant, with
about an additional 500 wh for each teenager.

Seems like the approach turned out as accurate as all the careful
calcuations and was a lot quicker and easier.

Regards,

Ezra Auerbach

----- Original Message -----
From: Allan Sindelar <allan at positiveenergysolar.com>
Sent: Thursday, July 28, 2005 3:33:23 PM
To: <RE-wrenches at topica.com>
Subject: Re: Inverter Load Analysis [RE-wrenches]

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> 
> Ray,
> You said a mouthful. We are seeing the future, too, and it is grid-tied.
We
> have installed somewhere around 40% of the GT systems in the state, and
it's
> way easier to make a living, even without incentives (or maybe because of
> lack of incentives: our typical customer, like yours, is the early adopter
> who wants to live their green values and isn't asking about payback
first.)
> The only thing about GT is the loss of the relationship aspect, as there's
> no maintenance, no followup, no ongoing education, so once the
installation
> is done, we may not see the customer again. I have developed some valued
> friendships over the years with off-grid folks I've met through installs.
> 
> We do it all, of course, like you: GT, off-grid, water pumping, upgrades
and
> service on systems up to 20 years old, movie stars to hippies in tipis.
> There's a body of knowledge through experience that is becoming
increasingly
> rare. At the first Energy Outfitters Las Vegas dealer event, in one
workshop
> a module manufacturer asked how many in the room had been in business for
> less than two years, and about 80% of the hands went up. Then the speaker
> asked how many only did GT systems, and again about 80% of the hands went
up
> (I didn't notice if it was the same 80%). Suddenly I realized why I get
> tired at the end of the week...
> 
> We use Windy's LA spreadsheet, too, and it's a remarkable piece of work. I
> use 15-16 watts for inverter tare (10 is for DRs and old Us), and nowadays
> it's usually for 24/7, as most of our clients will have several phantom
> loads and the inverter is always up. Keep in mind that much of the time
the
> inverter is running very small loads, and its conversion efficiency is in
> the 50-80% range, so 92% is not too optimistic. I won't say more, as I see
> Windy has already responded to this question. Just don't try to get too
> precise--it's a design guideline you're looking for, that is reasonable,
> explanable, and acceptable to the homeowner.
> 
> Allan at PosE
> 
> -----Original Message-----
> From: Ray Walters [mailto:ray at solarray.com]
> 
> 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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