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This page contains more information about the different lines on the Electricity
Supply and Demand chart for California, as well as links to the original data sources.
Use these links to jump to the terms that interest you:
CAISO Control Area
The CAISO defines a control area as a "geographic area which
regulates its generation in order to balance load and maintain planned interchange schedules with
other control areas" (
CAISO Summer Assessment, p. 40). Loosely, it is a portion of the grid over which a single
entity has responsiblity for maintaining the balance of supply and demand, and ensuring reliability.
The CAISO control area consists of the former control areas of the three Investor Owned Utilities
(PG&E, SDG&E, and Southern California Edison), and the service areas of some of the Municipal Utility Districts.
It does not include Sacramento (SMUD) or Los Angeles (LADWP).
Within it's control area, the CAISO is responsible for scheduling generation and
load, contracting for all the services necessary to maintain grid reliability
("ancillary services", in the technical jargon), and dealing with any and all
contingencies. To account for all the power that flows through
the wires, they also need to keep tabs on how much electricity is entering and
leaving through the borders of their control area.
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Maximum Capacity
As outlined in the Primer above, we need to make a distinction between the
market supply of electricity (the amount available to be sold) and the physical
capacity of generation resources. The Maximum Capacity defined here corresponds
to the latter concept - it represents the total amount of electricity that
could conceivably be produced by all the generators in the CAISO control area.
Anyone who wants to operate a generator inside California has
to obtain a license from the state government to do so.
The California Energy Commission maintains
lists of all the generators licensed to operate in the state (including the Muni's)
at their Siting Information Page ,
under Historical and Statistical Data. From this list, the total on-line capacity
in state is about 54,000 MW. There are about 9,000 MW in the Municipal districts, and
about 45,000 Mw in the CAISO control area. The latter figure is used on the chart.
This is a maximum because at any given moment some generators may not be operating, or
not operating at full capacity, for a wide variety of reasons.
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On-Line Capacity
For system reliability purposes, it isn't enough to know if a generator is hooked
up to the grid -- you also need to know if it's going to be operating.
For this reason, every day the generators must notify the CAISO if they plan to take
capacity off-line.
A generator that is off-line is called an outage or a curtailment.
Outages can be scheduled in advance (usually for maintenance purposes), in which
case they're known as planned outages. Or they may occur with little or no
advance notice, in which case they're called forced outages.
Each day the CAISO publishes a detailed list of these planned and forced outages.
Our definition is On-Line Capacity = Maximum Capacity - (Forced + Planned Outages) .
It's important to realize that plants can go off-line or come on-line at any time,
so these daily numbers are necessarily approximate. As the CAISO states, "[b]ecause
outages may overlap or may not all be simultaneously in effect, this report may indicate
a different total outage/curtailment than a real-time report using only data from
curtailments/outages in effect at that time."
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Imports and Exports
Keeping track of the electricity flowing through the grid entails monitoring how
much enters and leaves the CAISO control area. Electricity going out is called an
"export" while electricity coming in is (not surprisingly) referred to as an "import".
Exports and imports don't necessarily leave the state -- they may also be due to exchanges
with the Municipal Utility Districts inside California. So, for example, if a generator
inside the district controlled by LADWP sells power to SDG&E, this will be counted as
an import to the CAISO area. A Muni buying electricity from a generator inside
the CAISO area would count as an export.
The information on our chart is taken from the
CAISO OASIS Hour Ahead Market Information.
This tells you the volume of imports and exports that have been scheduled about
two hours in advance of the current (or operating) hour.
The CAISO also operates real-time markets, and buys electricity
out-of-market in real time, so the real-time values of imports/exports may be somewhat different.
On the graph, we define Net Imports/Exports = Imports - Exports , with both imports and exports
represented as positive numbers. The net is positive if there is a net gain within the ISO control area.
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Potential Capacity
The potential capacity represents, at the current time, the maximum amount of generation
that could be available to serve load. On the graph we define
Potential Capacity = On-Line Capacity + Net Import/Export - Reserves.
Reserves are defined here as 7% of the current load.
The current load on the system is determined by what everyone who uses electricity
is actually doing at the moment. As such, it fluctuates constantly. The system is
designed to handle these fluctuations within limits, but very large fluctuations
(due for example to an unexpected generator failure) can cause major problems, including
equipment failures that are expensive and time-consuming to fix. To avoid these
problems, all grid operators are required to make sure that a
sufficient amount of reserve capacity is available. There are different types of reserves
(spinning, non-spinning), as well as other forms of so-called ancillary services
that the CAISO must acquire to be able to dynamically maintain the balance between supply and demand.
The Western Systems Coordinating
Council sets the standards for grid operators in the western part of
the continent (they have to use the same standards, because all the grids are interconnected).
These are known as Minimum Operating Reliability Criteria, and are discussed in
great detail here,
The actual formula is somewhat complicated, but results in the
requirement that capacity equal to about 7% of the load be set aside as a reserve.
The CAISO calls emergencies (Stage 1,2 and 3) when reserves are in short supply -- physics
requires that supply in the grid equal demand in the grid, irrespective of what supply in the
market is doing, so a potential shortfall shows up first in the reserves.
The quanitity of supply represented by "Potential capacity", compared with
the current load, is meant to give a rough sense of how close we are to
over-taxing the available resources, but can't be used as an exact measure of
how much electricity is really available.
One reason for this is that the CAISO also operates real-time markets, but data
on the volume of electricity traded in these markets is not available to be
posted on our chart.
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Current Load
The current load is the total demand for electricity in the CAISO control area at the
current time. Since it's impossible to track changes in demand that occur every second,
the current load really represents an average over a time period of about ten minutes.
Although the load varies everytime anyone flicks a switch, on the average it's fairly
predictable, with variations due to daily, weekly and seasonal patterns.
Load that is fairly constant over the entire day is referred to as the "baseload"--
for residences, it includes such things as refrigerators, water heaters, and lights
that are never turned off. Baseload for residential and commercial buildings can be
surprisingly large, considering that there is little activity at night.
Seasonal variations in load are due mainly to changes in the need for heating
and cooling, and can be quite substantial. This is somewhat inconvenient in terms
of managing supply, since generation capacity has to be built to satisfy the peak
load, but for much of the year the actual loads may be much less.
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Forecast Load
The demand for electricity is fairly predictable, given information
about weather, the habits of individuals and businesses, economic activity, etc.
To facilitate the operation of the grid the CAISO publishes various forecasts of
the daily system load. We use their one-day-ahead forecast on our chart, which is
available through the
CAISO OASIS web site.
The ISO makes adjustments to this forecast two hours ahead of the operating hour, and uses this
value in the chart at their website, so the forecast on their chart may differ somewhat from ours.
The forecast load is an important quanitity because it is used to estimate reserve
requirements, which need to be booked in advance as much as possible.
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