[RE-wrenches] Demand Charge Reduction by PV

Thu Mar 18 11:55:10 PDT 2010

Hi Peter 
I have posted on same topic, not well written however.  I have a customer now that I am working with his load factor is 10% to 20%, I am hoping to do a demand reduction.  Also I have some data on past experiences dating back to 2003, I was trying to get Outback to help and they were too busy and I was on to tight a budget, so the attempt was dropped.  However Christopher F made many suggestions.   

I did record some data, and I have checked the data logger, it appears over the years the 25+kW solar has saved no less than about 5 kW in demand.  As you correctly point out the max demand occurs and most of the time the solar is contributing, however on days that would not have been the monthly peak have not had solar and the savings were reduced.  I have only a few business data records, and I can not make any generalizations.  

Darryl Thayer

--- On Thu, 3/18/10, Peter Parrish <peter.parrish at calsolareng.com> wrote:

From: Peter Parrish <peter.parrish at calsolareng.com>
Subject: [RE-wrenches] Demand Charge Reduction by PV
To: "'RE-wrenches'" <re-wrenches at lists.re-wrenches.org>
Date: Thursday, March 18, 2010, 11:38 AM

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                                 

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