Matthew Hunsaker
Renewable Integration Manager
801-819-7670
mhunsaker@wecc.biz
TO:
Joint Guidance Committee (JGC)
FROM:
Variable Generation Subcommittee (VGS)
DATE:
March 26, 2012
SUBJECT:
Variable Generation Capacity Assignment
There have been many studies performed to estimate a capacity assignment of wind and solar
power for adequacy planning. The capacity value attributed to variable generation (VG) is
dependent on several factors, including weather patterns, local-grid topography, resource mix,
and overall VG penetration. Because of these factors, it is difficult to provide a capacity-credit
value that can be used for all balancing authorities (BAs) in the Western Interconnection. To
estimate the capacity credit attributable to wind and solar power, each BA would need to
perform studies based on their own specific system.
The VGS has reviewed three reports that summarize different methods of determining capacity
value of wind and solar power for resource planning purposes.



Methods to Model and Calculate Capacity Contributions of Variable Generation for
Resource Adequacy Planning1 (produced by the National Renewable Energy Laboratory
(NREL))
Summary of Time Period-Based and Other Approximation Methods for Determining the
Capacity Value of Wind and Solar in the United States2 (produced by NREL)
Determining the Capacity Value of Wind: An Updated Survey of Methods and
Implementation3 (produced by the North American Electric Reliability Corporation (NERC)
Integrating Variable Generation Task Force)
The table below lists some of the capacity assignments used by different North American
Entities. These are summarized based on information in the two NREL reports listed above. The
table is provided to help readers visualize the range of capacity-credit values that different North
American Entities have given to wind and solar power plants. It is possible that some of the
Entities in the table have updated their capacity-credit calculations since the publication of the
NREL reports.
1
M. Milligan, K. Porter “Determining the Capacity Value of Wind: An Updated Survey of Methods and
Implementation.” National Renewable Energy Laboratory, June 2008.
2
J. Rogers, K. Porter “Summary of Time Period-Based and Other Approximation Methods for Determining the
Capacity Value of Wind and Solar in the United States.” National Renewable Energy Laboratory, March 2012.
3
“Methods to Model and Calculate Capacity Contributions of Variable Generation for Resource Adequacy and
Planning.” NERC, May 2011.
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Entity
Wind Capacity Credit
Solar Capacity
Credit
Notes
Arizona Public
Service
N/A
45.2% - residential
PV, 18.4% tilt, southfacing
Average between 2003
and 2007.
47.4% - commercial
PV,10% tilt southfacing
70.2% - commercial
PV, north-south single
axis tracking
BC Hydro
24%
N/A
Applies to onshore
wind, offshore wind,
and solar power.
Based on effective
load carrying
capability.
Bonneville Power
Administration
0%
N/A
Hydro-Quebec
30%
N/A
Determined through
the FEPMC model.
PJM
13% for new projects,
rolling three-year
average capacity value
for existing projects.
38% for new projects,
rolling one-year
average capacity
value for existing
projects.
Based on average
capacity factor from 26 p.m. between June
and August.
Portland General
Electric
5%
5%
From 2009 Integrated
Resource Plan.
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Entity
Wind Capacity Credit
Solar Capacity
Credit
Notes
New York ISO
New onshore projects:
10% (summer); 30%
(winter).
51%-90% at two
percent solar
penetration (using
ELCC method).
Solar capacity credits
come from Energy and
Capacity Valuation of
Photovoltaic Power
Generation in New
York.
New offshore projects:
38% for summer and
winter.
Existing projects:
Capacity factor from 2-6
p.m. between June and
August (summer credit)
and from 4-8 p.m.
between December and
February (winter credit)
of the prior year.
ISO New England
Five-year rolling average
from 1-6 p.m. between
June and August of the
previous year (summer)
or from 5-7 p.m. between
October and May
(winter).
N/A
The report New
England Wind
Integration Study
suggested capacity
values between 28%
and 36%.
MISO
12.9% for 2011 planning
and 14.7% for 2012
planning
N/A
MISO reviews capacity
credits annually
Southwest Power
Pool
Typically around 10%
N/A
SPP uses a monthly
method for 12 capacity
values for wind plants,
examining the highest
10% of load hours in
the month.
Minnesota
Department of
Commerce/Xcel
5% - 20%
N/A
Based on a study
performed in 2006.
Capacity values
depend on penetration
level of wind.
W E S T E R N E L E C T R I C I T Y C O O R D I N A T I N G C O U N C I L • W W W . W E C C . B I Z
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Entity
Wind Capacity Credit
Solar Capacity
Credit
Notes
PacifiCorp
8.53%
N/A
From 2011 Integrated
Resource Plan, using
ELCC.
ERCOT
8.7%
N/A
ERCOT is performing
a new LOLP study, to
be completed Q3
2012, which may
provide a new value.
Mid-Continent
Area Power Pool
Median value of 4-hour
peak period (using up to
10 years of historic
data).
N/A
Any contiguous 4-hour
period is fine as long
as the peak is within
the period.
Nebraska Public
Power District
0% based on Southwest
Power Pool criteria, 8%
based on Midwest
Reliability Organization
long-term reliability
assessments
N/A
NorthWestern
Energy
0%
N/A
Ontario
Independent
Electric System
Operator
Estimate of peak
demand capacity
contribution, based on
historic and simulated
wind data.
N/A
Idaho Power
5%
N/A
From 2011 Integrated
Resource Plan, based
on peak hour planning.
Pacific Northwest
(Northwest
Resource
Adequacy Forum)
5%
N/A
Derived anecdotally
from historic wind
generation data in the
BPA wind fleet.
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155 NORTH 400 WEST • SUITE 200 • SALT LAKE CITY • UTAH • 84103 -1114 • PH 801.582.0353 • FX 801.582.3918
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Entity
Wind Capacity Credit
Solar Capacity
Credit
Notes
California PUC
Set at the minimum
output achieved
historically in 70% or
more of the hours for
each month. Based on
three years of historic
data from 1-6 p.m.
between April and
October and from 4-9
p.m. between November
and March.
N/A
This methodology was
implemented in June
2009.
Public Service of
New Mexico
5%
55%
From 2011 integrated
resource plan.
Tri-State
Generation and
Transmission
Less than 2%
20% - 57%
From 2010 integrated
resource plan.
Public Service
Company of
Colorado
12.5%
55%
Wind capacity credit
used in the 2007 IRP
plan. Solar capacity
credit used in the 2011
electric resource plan.
State legislation
requires the CPUC to
revise wind and solar
capacity values based
on effective load
carrying capability.
W E S T E R N E L E C T R I C I T Y C O O R D I N A T I N G C O U N C I L • W W W . W E C C . B I Z
155 NORTH 400 WEST • SUITE 200 • SALT LAKE CITY • UTAH • 84103 -1114 • PH 801.582.0353 • FX 801.582.3918