There are a few other things that add to pressure cooking's efficiency. This is from a person who uses a pressure cooker (me), not an expert in heat transfer nor any other discipline of physics.
1. Most pressure cooker applications other than soup are steaming applications, and since the pressure cooker traps the steam in, you don't use nearly as much liquid as you would in traditional cooking. Less liquid = less energy to heat it up.
2. Less loss of heat through the top. After a pressure cooker reaches the desired pressure, you actually turn the burner down to its lowest setting (or whatever the lowest setting is on your stove to maintain pressure--on my stove, it's the lowest).
That's all I can think of for now, but I will say that a pressure cooker is a neat cooking tool. Especially if you live in a hot climate, like I do. Most of my summer cooking is on the grill, naturally, but I use the pressure cooker a bit, too, and it doesn't heat my house up too badly.
However, ever since the Boston marathon bombing manhunt, the authorities don't like people buying pressure cookers.
That's just not true. I bought my pressure cooker after the Boston Marathon bombings, and nobody gave me a second look. Anyway, modern pressure cookers have multiple safety mechanisms so they don't go boom like your grandma's pressure cooker did.