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.parr...@calsolareng.com
Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885
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