[RE-wrenches] offgrid system question

Wed Aug 20 19:07:50 PDT 2008

I don't think either one of us is "striding away from this issue".  As Jeff 
pointed out, and I haven't seen included very much in this discussion, it is 
about far more than surge loads and surge responses, it is about time of 
operation.  I don't care if it is a toaster or a whole-house AC or a window 
unit the most important aspect, after determining if the inverter can even 
handle the load, is how much time said unit will operate and if the system 
is designed to even manage a unit (toaster or AC) for the normal operating 
time without seriously degrading battery life and health.

Starting surges and instantaneous loads are important considerations but 
operating hours are equally important for good system design.  I remember 
years ago Richard Perez describing the AC unit they installed in the Home 
Power offices and how important it was to know not only load but how long 
the unit would operate in relation to system sizing.

And yes, I still think an AC of that size on a system of that size is nutty 
from a practicality standpoint unless they want their generator to run most 
of the time.  Well reasoned guidance is absolutely important, as you point 
out, but the comments made were made about what was not being discussed in 
this particular case, and I still believe Jeff is right to point that out.

T.
  ----- Original Message ----- 
  From: Peter Parrish
  To: 'RE-wrenches'
  Sent: Wednesday, August 20, 2008 3:21 PM
  Subject: Re: [RE-wrenches] offgrid system question

  As the two of you stride away from this issue with self-congratulatory 
back slaps and dismissive comments about nutty clients, you might also find 
time to arm yourself with a modicum of facts about current surges in motors 
and surge response of inverters. A 3.5 ton A/C may be excessive, but is a 
3/4 ton A/C, too? What about a swamp cooler, a whole house fan or a 1 hp 
water pump? Each client (in my book) deserves well reasoned guidance based 
on facts (regardless of how clients we need to provide it to).

  BTW, I agree 100% that we need to preach energy efficiency and heating 
with gas vs. electricity.

  - Peter Parrish

  Peter T. Parrish, Ph.D., President
  California Solar Engineering, Inc.
  820 Cynthia Ave., Los Angeles, CA 90065
  Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885
  CA Lic. 854779, NABCEP Cert. 031806-26
  peter.parrish at calsolareng.com

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  From: re-wrenches-bounces at lists.re-wrenches.org 
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Tom Elliot
  Sent: Wednesday, August 20, 2008 2:57 PM
  To: RE-wrenches
  Subject: Re: [RE-wrenches] offgrid system question

  Thank you Jeff, for asking the obvious.  I've been agog that no one seemed 
to think this was a nutty situation for an off-grid home.

  T.

    ----- Original Message ----- 

    From: Jeff Yago

    To: RE-wrenches

    Sent: Wednesday, August 20, 2008 10:57 AM

    Subject: Re: [RE-wrenches] offgrid system question

    I may be out of line here, but most of these comments have been related 
to making the right program settings and startup surge current issues.  I 
did not read any comments that this client is nuts to try and run a 3.5 ton 
air conditioner from a solar off-grid system, let alone a system with such a 
very small capacity.

    I would be surprised if this small existing solar array and batteries 
can keep up with the regular loads like lights, kitchen appliances, 
refrigerators, and entertainment equipment, let alone also running a central 
air conditioner.  .

    I would not  just debate the problems of surge current, I would like to 
know how this little solar system is going to keep up with the constant load 
of approx. 4,000 watts for the AC unit plus another 500 watts for the air 
handling unit which he did not mention.  With a constant drain of 4,500 
watts, just how long do you think this battery bank will last?   For most 
locations, during peak summer days a typical AC unit needs to run constantly 
12 to 16 hours per day.

    I would be interested in hearing from others on this, but here in the 
humid and hot east it is not possible to provide air conditioning for an 
off-grid home unless we are just cooling one or two rooms with a low energy 
Sanyo type split system in the 1/2 to 3/4 ton range.  Of course if you have 
enough solar modules and batteries you could run anything, but can you 
afford it?

    We do lots of off-grid systems and the first thing we tell clients is to 
not even think about having air conditioning, electric hot water heaters, or 
electric clothes dryers unless they have a really big bag of money.

    Jeff Yago

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