Place bacon in the bottom of a pressure cooker and cook over medium-high heat, turning occasionally, until evenly browned, about 8 minutes. Add onions; saute for 1 minute. Add green beans; toss until coated in bacon grease. Add water.
Close cooker securely and place pressure regulator over vent according to manufacturer's instructions. Heat until steam escapes in a steady flow, about 5 minutes. Reduce heat to medium-low and cook for 5 minutes. Let pressure release naturally according to manufacturer's instructions, 5 to 10 minutes. Unlock lid and remove. Season with garlic powder, salt, and pepper.