Your kitchen accounts for a third of your home's energy use, and much of that comes from cooking. Though it seems counterintuitive, a slow cooker is the most energy-efficient vessel for cooking dinner because it heats a small pot filled with food, not all the air around your food, the way an oven does. Plus, they're great for one-pot meals—less energy used, less cleanup for you! If you're cooking a meal for seven hours in a slow cooker, it'll cost you less than half of the energy cost it would take to cook the same meal in an oven for an hour (as long as you follow these common slow cooker mistakes to avoid).
If you are in the market for a new oven, consider a convection oven. According to the Consumer Energy Center, these ovens are more efficient in distributing heat, reducing energy use by about a third of what standard electric or gas ovens use.