The company, based in the southern boomtown of Shenzhen, said in a statement Sunday that net profit came in at 81.38 million yuan ($12.92 million) last year, down from 1.38 billion yuan in 2011.
A subsidiary of Buffett's Berkshire Hathaway owns 9.56 percent of BYD.
"Under the impact of slower economic growth, the automobile industry in the PRC (People's Republic of China) remains weak," the statement said.
China is the world's biggest auto market, with total sales rising 4.3 percent year-on-year in 2012 to 19.31 million vehicles.
But slowing domestic economic growth, limits on car numbers by some cities and a political row between China and Japan that hurt sales of Japanese-brand cars weighed on the market last year.
China's economy expanded 7.8 percent in 2012, its slowest pace for 13 years, in the face of weakness at home and in key overseas markets.
BYD, which makes both conventional as well as hybrid and electric cars, sold 420,000 vehicles last year, which it said was a "slight decrease" from 2011 but gave no comparative figure.
"The solar energy business was under great pressure due to the sluggish photovoltaic industry, resulting in considerable adverse effect on the overall results of the group," it added.
BYD also produces telephone handsets, rechargeable batteries and solar products including cells and panels.
The global solar industry has been burned by the economic slowdown in Europe and the United States, together with weak prices blamed on Chinese companies which sought to build market share by undercutting competitors.
Last week, China's Suntech -- once the world's largest solar panel producer -- said its main subsidiary was facing bankruptcy.
BYD closed up 0.47 percent to 23.51 yuan in Shenzhen trading on Monday but its Hong Kong-listed shares were down 0.40 percent at HK$24.85 ($3.23) in late afternoon trade.
