Supply of and Demand for Electricity for CaliforniaCurrent Energy
Supply of and Demand for Electricity for California




New EnglandNew England

New YorkNew York

NYC / Long IslandNYC / Long Island

PJM InterconnectPJM Interconnect



We invite your suggestions and comments to improve this page, especially additional sources of information, alternative forms of presentation, or clarifications. Please send messages to Alan Meier.

Select a date   

The power grid that supplies the electric current coming into your home or business is designed to maintain a dynamic balance between the consumer demand for electricity and the amount being supplied by generators. The chart above is an approximate representation of this dynamic balance. Quantities which are forecasts or estimates are shown by dashed lines. You may need to click your browser's reload button to update the graph.

The current demand (or "load") depends on how much power consumers are using right now. While the load changes every time someone switches a light on or off, the sum of loads due to a large number of consumers varies slowly. In addition to the supply needed to meet this demand, some "reserve" generating capacity must be kept ready to operate in case of any unexpected events.

The current load is published every five minutes by the Electric Reliability Council of California (ERCOT) for the area it controls, which covers about 85% of electricity use in California. The official ERCOT website, with links to the official data can be found here.

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.
Back to top

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.
Back to top

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.
Back to top

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.
Back to top

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.
Back to top

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.
Back to top

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.
Back to top

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.
Back to top


Development and maintenance of the Current Energy website ended in January, 2005. Linkages to data obtained from other websites will degrade over time, and parameters on which the computations depend will become out-of-date.

This chart is presented for educational purposes only. We have used publicly available information, and cannot assume reponsibility forthe accuracy of this information. Links to the original data sourcescan be found at this page.

These pages were prepared by Emily Bartholomew, Chris Bolduc, Katie Coughlin, Brian Hill, Alan Meier and Robert Van Buskirk,
Environmental Energy Technologies Division
Lawrence Berkeley National Lab