Inverter Load Analysis [RE-wrenches]

Allan Sindelar allan at positiveenergysolar.com
Thu Jul 28 15:33:23 PDT 2005


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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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