[RE-wrenches] Demand Charge Reduction by PV

[Joel Davidson]

Hello Peter,

I have seen 40% to 70% monthly demand charge reduction for some southern California PV projects for some months, but it is still a crap shoot. 15 minutes and 1 second of clouds during the peak demand period will trump a client's energy management efforts unless they are willing and able to shed loads during cloudy periods. I tell clients that they cannot rely on the weather to cooperate, to monitor and control their demand, and to think of any PV savings on their monthly demand charge as a windfall.

Joel Davidson

----- Original Message ----- 
  From: Peter Parrish 
  To: 'RE-wrenches' 
  Sent: Thursday, March 18, 2010 9:38 AM
  Subject: [RE-wrenches] Demand Charge Reduction by PV


  I failed to clean up the subject line on this post a few minutes ago. Please respond to this post so that we can keep track of the topic properly.

   

  Esteemed wrenches,

   

  I have been wrestling with this concept about as long as we have been in business. How to estimate how much a pv system will reduce the demand charge for a customer.

   

  I know the "worst case" goes as follows: 

   

  (1)     Demand is based on measuring the consumption every 15 minutes and keeping track of those numbers for the entire billing period.

  (2)     The customer gets socked with a demand charge that is based on the highest 15 minute consumption for the entire billing period.

  (3)     The customer also gets soaked with a "facilities charge" that is equal to the greatest monthly demand number for the trailing 12 months.

  (4)     Now you have a solar system pumping out Wac varying over the familiar bell-shaped curve during the day.

  (5)     In the southwest US, peak demand typically occurs early in the afternoon in the summer, during the week. Our LADWP has a mantra that goes something like this, "Peak demand occurs at 3pm PDT on the third Thursday in August!" I believe them.

  (6)     So one would expect something like 40% of the peak Wac to offset the peak demand, but what happened if the sun goes behind a cloud for those 15 minutes? Answer, "Bad luck. Your demand is back to what it was before you bought your solar system.

  (7)     It is actually worse than that. Peak demand recurs with approximately with the same value with some regularity for an extended period of time, so the sun will have to shine with full intensity every day when peak demand is expected to occur, which in LA could be every day (M-F) of the 30 day billing period.

   

  I have always taken the position that we can't guarantee that any of the demand charge will be reduced with a solar system. But what do other PV integrators tell there customers? Better yet is there any actual data on demand reduction with PV systems? It seems to me that occasionally the monthly peak demand will in fact be shaved by PV production, the question is how often in practice?

   

  I once thought of taking actual insolation data and comparing it with actual demand data and doing a Monte Carlo simulation (throwing the dice = randomly matching up demand data with solar production data) - but I haven't retired yet.

   

  I would love to hear what others are doing about this.

   

  - Peter

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

   



------------------------------------------------------------------------------


  _______________________________________________
  List sponsored by Home Power magazine

  List Address: RE-wrenches at lists.re-wrenches.org

  Options & settings:
  http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org

  List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org

  List rules & etiquette:
  www.re-wrenches.org/etiquette.htm

  Check out participant bios:
  www.members.re-wrenches.org

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20100318/bb83b55e/attachment-0003.html>