99 Islands — United States of America

99 Islands is a 12 MW hydro power plant in South Carolina, United States of America. It is operated by Duke Energy Carolinas LLC. Based on reported annual generation of 68 GWh, it can supply roughly 19k homes. It ranks #5365 of 10,884 United States of America power plants by installed capacity. Commissioned in 1910, it is around 116 years old — an older, legacy facility. As a non-combustion source, it has no direct CO₂ emissions from generation. In context, hydro supplies about 5.3% of United States of America's electricity; the national grid averages 384 gCO₂/kWh (43.0% low-carbon) (2025).

Plant data: WRI Global Power Plant Database (CC BY 4.0), id USA0003272.

Data provenance

Source of the capacity and fuel values:

capacity: Wikidata P2109; fuel: WRI GPPD

In context: how this plant compares

At 12 MW, 99 Islands is well above the median hydro plant in United States of America (8 MW). Hydropower converts the energy of falling or flowing water into electricity; output depends on rainfall and reservoir level, and large dams also provide grid balancing and storage.

Capacity comparison computed from the WRI Global Power Plant Database; fuel-type context is general engineering background.

Key facts

Load factor

64%, from 68 GWh of reported generation; the median for 1342 hydro plants in United States of America that report generation is 42%

Generation trend

92 GWh in 2013 and 68 GWh in 2019 (−26%)

Highest and lowest year

2013: 92 GWh; 2016: 45 GWh

Share of United States of America hydro capacity

less than 0.1% of 102,513 MW across 1,449 plants in this dataset

Age among peers

40 of 1,444 dated hydro plants in United States of America are older

Calculated from the capacity and generation records in this dataset (WRI Global Power Plant Database (CC BY 4.0)).

Reported generation trend

Annual generation (GWh), WRI Global Power Plant Database (CC BY 4.0).

Owner

Operated by Duke Energy Carolinas LLC. All plants by this company →

Local climate & thermal context

This hydro plant converts the energy of falling or flowing water through hydro turbines. It sits in a humid subtropical climate (Köppen Cfa) — Northern Hemisphere, latitude 35.0°N — which shapes how much energy it can produce and how its output varies through the year.

Monthly mean temperature

Heating degree-days here run 31% below the median power plant in this dataset — a proxy for how much extra energy heated equipment must replace through its surfaces in winter.

Climate heat-demand index: 38/100 — this site sits in the mid third of the power plants we cover by heating degree-days.

Climate normals: WorldClim 2.1 (1970–2000 monthly normals, 10 arc-min, CC BY 4.0); zone: Köppen-Geiger world climate classification (Kottek et al. 2006, 0.5° grid). Degree-days & heat-demand index computed by PowerAtlas — a modelled heat-demand proxy, not a measured site figure.

Site climate & environmental severity

This site sits in a moderately corrosive environment (estimated ISO 9223 class C3 — Medium), with humidity / wetness the leading environmental stress.

Indicative estimate via the ISO 9223:2012 informative method (atmospheric corrosivity from temperature, time-of-wetness and airborne salinity), using WorldClim climate normals, the Köppen-Geiger class and coast distance. Indicative, not a measured corrosion rate.

How it compares & nearby plants

The #593 largest hydro power plant of 1449 in United States of America by capacity.

United States of America has 1449 hydro power plants in this dataset, together about 102,513 MW of capacity.

Nearby power plants

• William States Lee III nuclear power plant · Nuclear · 2,234 MW · South Carolina
• Cherokee nuclear power plant · Nuclear · 3,840 MW · South Carolina
• Cherokee Falls · Hydro · 4.3 MW · South Carolina
• Broad River Energy Center · Gas · 985 MW · South Carolina
• Cherokee County Cogen · Gas · 101.2 MW · South Carolina
• Gaston Shoals · Hydro · 6.7 MW · South Carolina

Location

Coordinates 35.0314, -81.4936 from WRI Global Power Plant Database (CC BY 4.0). View on OpenStreetMap.

Frequently asked questions