LITTLETON, Colorado, Jan 25 (Reuters) - Clean electricity generation in the United States hit new highs in 2023 but expanded by its smallest margin in more than a decade, due to below-normal wind speeds and a drop in hydro power output due to drought, data from think tank Ember shows.

U.S. clean generation grew by just 0.4% last year, the smallest annual increase since 2012, when clean output contracted due to a drop in both hydro and nuclear output.

The slow growth pace came despite additions to renewable energy supply capacity throughout the country, and resulted in the first contraction in overall electricity generation since COVID-19 lockdowns stifled total energy use in 2020.

WIND CURBS

U.S. wind supply capacity is estimated to have increased by between 7.1 gigawatts (GW) and 12 GW in 2023, according to the U.S. Department of Energy, although the exact degree of expansion is still to be confirmed.

The average annual increase in U.S. wind capacity between 2015 and 2022 was 9.5 GW, so a further steep climb in wind capacity was expected in 2023.

However, a mix of supply chain disruptions alongside increases in materials and labour costs caused a sharp slowdown in the momentum of wind project installations last year.

Nonetheless, the total footprint of U.S. electricity generation from wind sites is widely assumed to have expanded in 2023 over 2022's total, and should have resulted in a commensurate increase in total wind electricity generation.

However, unusually low wind speeds - especially during April, May, June and November - resulted in a nearly 3% drop in total wind electricity output last year, Ember data shows.

DROUGHT-HIT HYDRO

U.S. power providers were also hit by a more than 7% drop in output from hydropower sites due in 2023 to drought in key hydro generation areas, especially in western states.

In combination, hydro and wind facilities generated around 665 terawatt hours (TWh) of electricity in 2023, compared to 695 TWh in 2022, Ember data shows.

A roughly 19% rise in solar output to 243 TWh helped offset some of lower wind and hydro output, and alongside fairly steady nuclear output allowed power firms to boost clean electricity generation to a record of 1,750 TWh, up from 1,744 TWh in 2022.

FOSSIL CUTS SUSTAIN ENERGY TRANSITION MOMENTUM

The share of clean power in total generation hit a new high of 41.1% in 2023 thanks in large part to continued cuts to the use of coal in national electricity production.

Coal-fired electricity output shrank by 19% in 2023 to around 672 TWh, and the lowest total since at least 2000.

Gas-fired electricity increased by nearly 7% from 2022's total to 1,804 TWh, but total electricity output from fossil fuels contracted by 2% last year to its lowest tally since 2020.

Fossil fuels still accounted for around 59% of total U.S. electricity generation last year, but should see a larger decline in the generation mix going forward once output from wind sites pick up due to a recovery in wind speeds and from further expansions in grid-connected capacity.

Alongside expansions to solar generation capacity - which the U.S. Department of Energy estimates grew from 17 GW in 2022 to 31 GW in 2023 - higher wind power looks set help U.S. clean electricity generation regain momentum in 2024, and should keep the country's energy transition goals on track.

(Reporting by Gavin Maguire; Editing by Michael Perry)