* Renewables met 43% of usage in Q1-Q3, down 5 points

* Power consumption up 4% yr-yr in Jan-Sept 2021

* Electricity production increased 3.8% overall

* Onshore wind production was down 16.7%

* Conventional electricity output up 13.2%

FRANKFURT, Sept 28 (Reuters) - Renewable energy's share of Germany's power consumption dropped to 43% in January-September from 48% a year earlier as wind output fell, preliminary industry data showed on Tuesday, demonstrating the impact of weather on supply security.

Onshore and offshore wind met 19% of demand in Europe's biggest economy, down from 24% a year earlier, utility industry association BDEW and the Centre for Solar Energy and Hydrogen Research (ZSW) said in preliminary figures.

The figures were calculated under European Union requirements that base market share on usage rather than production, a basis also adopted by the Berlin government for its climate target definitions.

Germany's power consumption overall in the nine months increased by 4% year-on-year to 416 billion kilowatt hours (kWh) as the economy developed more strongly after COVID-19 related lockdowns had curbed activity through much of 2020.

Domestic electricity production increased 3.8% to 426.3 billion kWh, leaving Germany a net exporter in the period, the data showed.

Renewable generation, which along with wind includes solar, hydro, biomass, waste and geothermal energy, contributed 177.7 billion kWh to the total, down 7.1% from a year earlier.

That included onshore wind at 63.2 billion kWh, down 16.7%, photovoltaic at 45.9 billion, up 2.3%, biomass at 32.5 billion, down 0.9%, and offshore wind at 16.0 billion, down 15.4%.

While wind output underperformed, a factor that has proven to be a wild card in wholesale markets, photovoltaics had an average year and hydropower output was up 9.1% in January to September.

Conventional electricity production from nuclear, coal and gas stood at 248.6 billion kWh in the nine months, 13.2% higher than in the same 2020 period.

Germany aims to cut carbon emissions by 2030 by 65% from 1990 levels under a target set in May that was raised from 55%.

BDEW said expanding renewables and increasing energy efficiency were priorities for the new government after the Social Democrats on Sunday narrowly won a national election. (Reporting by Vera Eckert, editing by David Evans)