A slow cooker seems like it ought to be expensive, sitting switched on for six or eight hours at a stretch. In fact it is one of the cheapest ways to cook a hot meal, because what matters is not how long something runs but how much power it draws while it does, and a slow cooker draws very little.
Low power beats long hours
A slow cooker typically pulls somewhere around a hundred to three hundred watts, a tiny fraction of an oven's two or three thousand. Multiply that small draw by a long cooking time and the total energy is still modest, often less than cooking the same casserole for a couple of hours in a hot oven. The well-insulated pot holds its heat, so the element only sips to maintain a gentle simmer. Run the figures through the cost calculator and the long runtime stops looking alarming.
The meals it loves
Slow cookers excel at exactly the dishes that are otherwise time-consuming and energy-hungry: stews, casseroles, curries, soups, pulled meats and dried beans and pulses. The long, low heat turns cheaper, tougher cuts of meat tender, so it saves on the shopping as well as the energy bill. It is a natural partner to batch cooking, since a big pot can be portioned and frozen for several future meals.
Versus the oven and hob
For a long-cooked dish, the slow cooker usually beats both the oven, which heats a huge cavity, and a pan simmering on the hob, which loses heat to the room. The convenience is part of the appeal too: load it in the morning, leave it, and come home to a cooked meal with no oven to mind. For quick cooking it is the wrong tool, but for anything that wants long, gentle heat it is hard to beat on cost.
Getting the most from it
Keep the lid on, since lifting it lets out heat and steam and lengthens the cook. Cut ingredients to a similar size so they cook evenly, and do not overfill. Use the low setting where the recipe allows, as it draws even less. With a little planning, the slow cooker quietly turns out cheap, generous meals for a few pence of electricity each.