The pilot had just departed in the tailwheel equipped STOL (short takeoff and landing) airplane from his private rural grass airstrip about 25 minutes before sunset. The airplane collided with a power line about 500 yards south of the runway, impacted terrain, and came to rest inverted. A postimpact fire consumed a majority of the airplane. A witness stated that the airplane was flying normally before the power line collision. Aerial marker balls were not installed on the power line, nor were they required to be. The pilot stated that he did not remember the accident sequence due to his serious injuries, but before the accident, the wind was about 4 mph from the south and the density altitude was high. Prior to the accident flight, he completed multiple flights and landings in the airplane. Since this grass airstrip was on his property, he was aware of the power lines and frequently flew over them during takeoff and landing. The reason for the collision with the power line could not be determined.