@Ohmother electric slow cookers use about the same amount of electricity as a standard electric lightbulb, approximately 1.44 kilowatt hours for 8 hours on low. It's quite cheap (you can check the current KW/h costs from your electricity bill and multiply by 1.44 to find the actual cost, to you). 4 hours on high costs about the same as 8 hours on low.
If you don't already have a slowcooker (or an AGA / Reburn with the slow oven ) the wonderbag might be an idea, or you can use the old-fashioned hay box as a pp mentioned.
The use of wonderbags (and similar cooking methods) in Africa is common. It enables them to slow cook using the pots they already have rather than buying the expensive western technology and using what may be an unreliable electricity supply..