In a large pot, brown the ground beef and chopped onion over medium heat. Drain excess fat if necessary.
Add the kidney beans, pinto beans, tomato sauce, diced tomatoes, chili powder, cumin, garlic powder, salt, and black pepper. Stir well.
Bring the mixture to a boil, then reduce the heat to low, cover, and simmer for about 30 minutes, stirring occasionally.
Serve hot, garnished with shredded cheese, sour cream, or chopped green onions.