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CAISO Control Area
This graph shows data for the CAISO control area only, which covers
about 75% of electricity use in California.
It includes the service areas of the three major
utilities ( PG&E, Southern California Edison, and San Diego G&E), but not
the Sacramento Municipal Utility District (SMUD) or the Los Angeles Department of
Water and Power (LADWP).
In addition to the resources in state, transmission lines connecting
California to the rest of the Western region allow electricity to be
imported and exported.
More detail and site links can be found here.
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Maximum Capacity This number is an estimate of
the total generation capacity inside the CAISO control area.
It does not include generators in the municipal utility districts.
It is computed as the sum of the online capacity of all the generators
licensed to operate in the state.
The fact that a generator is licensed, and so included in this figure,
does not mean that it will be operating or selling electricity into the
CAISO on any given day. For more detail and links to the original data
sources click here.
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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.
Click here for a more detailed discussion.
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Current Load
"Load" is the technical term for total demand for electricity.
It is the amount of electricity that customers are pulling out
of the grid at any given moment. This amount is monitored in real
time by the CAISO, and published every 10 minutes.
The CAISO load accounts for about 75% of total electricity use in California.
Details and data links here.
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Forecast Load
While the load changes every time someone turns on a computer or switches off a light,
on the average it can be predicted, given information about the weather,
the daily habits of individuals and businesses, etc.
Every day the CAISO publishes two-day-ahead and one-day-ahead forecasts of what the
loads will be for each hour, which are used to schedule delivery
of adequate electricity. The curve shown here is from the
one-day-ahead forecast (ie. this forecast was computed yesterday).
More detail and data links can be found here.
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Load plus Reserves
To guarantee the reliability of the transmission and distribution
system, the CAISO must be able to correct any sudden imbalances between
supply and demand that may occur.
This means that a certain amount of
"reserve capacity" has to be available at all times. The current industry
standard sets reserves equal to about 7% of the system load.
For planning purposes, the reserve requirements are based on the forecast load,
so the line on the graph is today's forecast load plus an additional 7%.
The CAISO calls a Stage 1 Emergency when reserves drop below 7%, a Stage 2
Emergency when reserves are less than 5%, and a Stage 3 Emergency when
reserves drop to 1.5%. In a Stage 3 Emergency, rolling blackouts
may be initiated to reduce the system load.
A more detailed discussion is presented here.
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Forced and Planned Outages Every morning the CAISO publishes
figures for the quantity of generation that will be offline for that day.
A generator offline (ie. not operating) is called an "outage".
Planned outages are those that have been scheduled ahead of time with the
CAISO, while forced outages are those that occur with little or no advance notice.
The graph shows a separate line for each---the total generation offline is the sum of the two.
The outage figures are updated at noon. We use the previous day's figures until noon as an estimate.
Today's numbers are: Planned Outage 12894 MW; Forced Outage 3450 MW.
More information and data links here.
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Imports and Exports The electricity transmission grid
that serves California is interconnected with the rest of the western
United States and Canada, allowing the state to import and export electricity.
These imports and exports must be scheduled with the CAISO, so that they know
how much electricity is flowing through the grid at any time.
The values for imports and exports shown here are taken from the
hour-ahead market data available at the CAISO website.
More information and data links available here.
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