I love my slow cooker! I've heard that the power they use is less than a 60W light bulb, so it should be a very energy efficient way to cook.
I'm a soup addict and love to make my own stock from stuff that would ordinarily go in the trash. Every time we roast meat on the bone or have whole chickens, I freeze them until I have enough to fill the pot then I add my stockpile of frozen veggie trimmings such as onion ends, broccoli peelings, soft snap peas, sprouted garlic, and potato peels. I add bay leaf, a splash of balsamic vinegar to leach all the gelatin and vitamins out of the bones, optional leftover white wine and top with water to the rim. Turn on high until it simmers, then turn to low and let it sit for 12 hours. Strain out the liquid (I use a big metal colander), place in a cool place to let the fat separate (save that too!) and keep as plain broth or spice up into a soup by adding the bits of meat and more veggies and a starch such as barley, rice or small pasta back in.
Hambones plus red lentil and chickpeas with curry or garam masala spices makes an awesome thick, rich soup. Cheap rotisserie chicken carcasses give a great roasted flavor, and beef bones can render a rich base to a winter veggie and barley soup. I've been known to leave fancy dinner parties and family holiday meals with my stock ingredients wrapped in tin foil to take home. These 'free' soups keep my weekly work lunch spending on takeout to $0.