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Maximum Capacity This number is an estimate of
the total generation capacity serving the PJM control area
as indicated in PJM
planning documents.
The fact that a generator can serve PJM load, and so is included in this
figure, does not mean that it will be operating or selling electricity
into the PJM on any given day. In this plot, maximum capacity has been
offset by 300MW for visibility. This is about one line width.
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On-Line Capacity
On-Line Capacity is equal to the Maximum Capacity minus Generation Outages.
It represents the total generation capacity in the PJM control area that can
feasibly operate that day. See Generation Outages for more detail.
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Potential Capacity The generation capacity potentially available to
supply electricity is computed here as the sum of
On-Line Capacity + Net Imports/Exports - Reserves .
We use a reserve margin equal to 7% of the load.
The generation capacity actually available in any given
hour will differ from our estimate of the potential capacity for a variety of reasons.
Hydroelectric capacity may be reduced due to water shortages.
Some plants are only available a certain number of hours per month,
or may have restrictions on their operation due to air quality concerns.
There may also be resources available to the ISO in the form of voluntary
load reductions ("negawatts"), which are not included here.
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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 PJM Interconnect, and published every 5 minutes.
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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.
Currently the forecast is unavailable, but will be made available
shortly.
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Outages Every morning the PJM publishes a
forecast
for the quantity of generation that will be offline each
day for the next six months.
The outage figures are updated at 4AM Eastern time.
We use the previous day's figures until 4AM Eastern time as an estimate.
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Transmission Capacity Published PJM
Operational
Data
contains transfer limits for each of five
transfer interfaces.
The transfer limits are summed to produce the transmission capacity data.
Additionally, it should be noted that PJM posts
Conditions Impacting
Transmission Capability.
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Transmission Also published in PJM
Operational
Data are actual flow for each of the five transfer interfaces.
The actual flows are summed to produce the transmission data.
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