Inverter Load Analysis [RE-wrenches]

[Doug Pratt]

Oh yeah, off-grid...I remember that! It's a guessing game. Customers usually
figure their loads too high, us techies usually figure them too low. After
all, who would leave a light on they weren't actually USING at the moment?

10 watts and 92% inverter efficiency sounds about right to me Ray. You could
bump your battery efficiency up a bit if a lot of loads are run daytime, but
frankly a bit of oversizing is probably a good thing. I've rarely
encountered an off-griddie who had too much power. If the system is too
expensive for the client you can always shave off a few PV modules and let
them lean on the backup generator for a few years till they can afford to
upgrade. Just leave 'em room for that upgrade.

Cheers,
Doug Pratt

-----Original Message-----
From: Ray Walters [mailto:ray at solarray.com] 
Sent: Wednesday, July 27, 2005 9:21 PM
To: RE-wrenches at topica.com
Subject: Inverter Load Analysis [RE-wrenches]


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