When Is Electricity Cleanest?
Pick a grid zone to see, for each hour of the local day, how much of its generation typically came from low carbon sources (nuclear, hydro, wind, solar and other renewables) over the most recent complete days, up to 18 September 2026. The best and worst hours are marked. This is a pattern from the past, not a forecast, and it is a share of generation, not a carbon intensity.
A historical pattern, not a forecast. These bars show what happened on average over the most recent complete days. Tomorrow's weather, demand and outages can make any single day quite different.
Electricity in California Independent System Operator is typically lowest carbon between 08:00 and 17:00, when low carbon sources supplied 78% of generation on average, and least clean between 18:00 and 02:00 (40%), over the 29 complete local days from 2026-08-21 to 2026-09-18. This is a historical pattern, not a forecast.
Show the hourly values as a table
| Local hour | Low carbon share |
|---|---|
| 00:00 | 43.0% |
| 01:00 | 45.5% |
| 02:00 | 46.7% |
| 03:00 | 47.0% |
| 04:00 | 46.8% |
| 05:00 | 45.2% |
| 06:00 | 43.1% |
| 07:00 | 62.6% |
| 08:00 | 75.8% |
| 09:00 | 78.7% |
| 10:00 | 80.1% |
| 11:00 | 80.2% |
| 12:00 | 80.0% |
| 13:00 | 79.6% |
| 14:00 | 78.5% |
| 15:00 | 76.9% |
| 16:00 | 74.3% |
| 17:00 | 66.3% |
| 18:00 | 43.6% |
| 19:00 | 35.0% |
| 20:00 | 35.7% |
| 21:00 | 36.3% |
| 22:00 | 39.8% |
| 23:00 | 42.7% |
Source: U.S. Energy Information Administration (EIA). (US Government work, public domain) Data as of 18 September 2026. Hourly generation by fuel from the grid operator data, summarised by World Power Monitor. Each zone's own window is shown under the chart.
Source: ENTSO-E Transparency Platform (transparency.entsoe.eu), CC BY 4.0. Aggregated and converted by World Power Monitor; ENTSO-E does not endorse this site. (ENTSO-E Terms of Use plus CC BY 4.0 for listed data items) Data as of 18 September 2026. Hourly generation by fuel from the grid operator data, summarised by World Power Monitor. Each zone's own window is shown under the chart.
Key Facts
- The tool covers 104 grid zones (57 US grid operators from EIA and 47 European bidding zones from ENTSO-E) with hourly generation by source, using the most recent complete days up to 18 September 2026.
- Low carbon means nuclear, hydro, wind, solar, bioenergy, geothermal and other renewables. Coal, gas, oil, other fossil fuels and unclassified generation are not counted as low carbon.
- Hours are local clock hours in each zone's own time zone, so the pattern follows daylight saving time.
- The bars are a historical average of recent days. They are not a forecast and not a carbon intensity.
Methodology and caveats
- What is measured. For each hour, low carbon generation divided by all generation reported for that hour by the grid operator, in percent. Storage output counts in the total but not as low carbon. Imports and exports are not part of the calculation, so this describes power generated inside the zone.
- The window. Up to the latest 30 complete local days in the zone's hourly data. A day is used when its whole span falls inside the window and at least 22 of its hours have complete reports; an hour is skipped when a fuel that the operator normally reports is missing. A zone appears only with at least 7 usable days.
- Averaging. Each hour is the plain average of that clock hour across the days used. The best hours are the run of neighbouring hours around the highest average that stays within a quarter of the day's range of it; the worst hours work the same way around the lowest. When the whole day varies by less than 2 percentage points no hours are singled out.
- Local time. Hours are grouped by the wall clock in the zone's registry time zone, not by UTC, and daylight saving days are handled by the clock: on a change day the missing or repeated hour has one sample fewer or more.
- Not a forecast. Wind, cloud, demand, outages and prices vary day to day. A window that was clean on average can be dirty tomorrow.
- Share, not intensity. The tool does not convert the mix to grams of CO2. A high low carbon share is a good sign but not the same as low emissions.
- Zones left out. Zones without hourly generation, without enough complete days, or whose source is not cleared for reuse are omitted rather than shown with gaps.
- Data layer. Grid operator data only. It is never combined with Ember annual statistics, and each zone shows its own source and data window.