Learn more about the data collection
Who reports, and how often
Power plant operators file Form EIA-923 with EIA. By EIA's own count about 3,000 plants across the
U.S. report every month and about 9,500 more report once a year. On Oʻahu the large plants report
monthly, but about a dozen small plants (mostly solar farms) report only annually.
What is reported
Each plant reports the electricity it generated, split by fuel and by type of engine, in
megawatt-hours (net of the plant's own use), along with the fuel it burned. This chart uses the
generation numbers for the 38 plants that EIA places on Oʻahu's grid. Batteries appear too: when
a battery charges it shows up as negative generation, so it is left out of the totals.
When the data arrives, and why recent months are provisional
Monthly figures appear about two to three months after the month ends. Final figures for a whole
year appear roughly a year later, after EIA checks them and adds the plants that report annually.
Until then the newest months are missing those small plants, so they read low; that is why the
latest months are shaded and drawn fainter. Each time the pipeline is run it fetches everything
again, so the chart corrects itself once EIA publishes the missing year.
What EIA does not count directly: rooftop solar
EIA-923 covers only plants of 1 MW or larger. Smaller solar systems, mostly rooftop panels, are
not reported by plant. Instead EIA estimates their generation for each state from
net-metering reports by utilities and third-party owners, and has published that estimate since
2014. It is for the whole state of Hawaiʻi, not for Oʻahu alone, so it has to be brought down to the
island. The dotted line does that with Hawaiian Electric's own reported Oʻahu rooftop total for each
quarter: EIA's monthly figures are scaled so that every quarter adds up to Hawaiian Electric's number
(Oʻahu's share of the state has ranged from about 65% to 83%, so a single fixed share would be off by
up to roughly 18% in some years). EIA's statewide monthly pattern only decides how each quarter's
total is split across its three months. It is still an estimate: Hawaiian Electric's figure counts
all customer-owned renewable generation, which is almost all rooftop solar. Where Hawaiian Electric
has not reported a quarter, a fixed 77% share is used instead. The dotted line is also a different
measure: rooftop energy counts on both the top and bottom of the fraction, so it shows the share of
all electricity used.
How this shapes what you see
- The main line understates Oʻahu's true renewable share, because rooftop solar is missing from it.
- Steps in the line usually mark a plant opening or closing. Kahuku Wind Power starting in 2011 and
the group of solar farms in 2020 are the biggest.
- The latest months can dip and later be revised upward.
- Oʻahu here means the plants EIA places in Honolulu County, minus two refinery cogeneration
plants that power the refinery rather than the grid.
- EIA's plant data starts in 2001, so anything earlier, such as Oʻahu's first wind turbines,
is not in it.