In case anyone is interested, here is the reply from the EST.
Most interesting point is that a slow cooker is probably no more energy efficient than an oven.
And, that, as Takver said, the best answer is a hay box!
Thank you for your recent enquiry on cooking methods.
Cooking is an area where there are very few hard and fast rules, sadly, but based on the information we have about efficiencies of different cooking methods, and the prices and carbon dioxide emissions of mains gas and electricity, I would think that the most economical way (and most environmentally friendly way) to cook the meal will probably be the gas hob. This is because one unit of gas is about three times cheaper than one unit of electricity. Electricity also produces about two and half times as much carbon dioxide per unit used than gas, because of inefficiencies in the generation and transmission of electricity.
Using the electric oven, or an electric-powered slow cooker, may well be more efficient, as electric appliances are generally more efficient that gas ones. So the electric appliances will probably use fewer ?units? of energy to cook the meal. However, because of the far higher price and higher carbon dioxide levels produced from each unit of electricity compared with each unit of gas, the cheaper and more environmentally friendly option will be to use the gas hob.
If you are using a green electricity tariff (where the electricity is produced from renewable sources and produce no carbon dioxide), then using the electric oven or slow cooker will produce less carbon dioxide than using gas; but it will still cost you more!
I do not imagine that a slow cooker would use any less energy to cook a stew than the electric oven. The total energy needed (in kWh, kilowatt hours) is a product of the power of the appliance (in kW) and the time it is on for (hours). If the slow cooker has only a third of the power of the over, but is on for three times as long, it will still use the same amount of energy overall to cook the meal.
Unfortunately, I do not think there is a way for you to test this out yourself at home, as measuring the flow of gas to your hob would probably be impossible to do. Also, short of buying the slow cooker and testing it while cooking a meal, and comparing that to the result from your oven, there is no easy way to compare the total energy consumption of the two.
Alternatives which will help to reduce the energy used to cook the stew are to use a pressure cooker on your gas hob, or to use a ?hay box? (or hay cooker). Pressure cookers will certainly help to reduce the energy needed to cook the stew, and also tend to cook the food more quickly. A hay box is a very well insulated box, without a heat supply, into which you place a hot casserole dish or pan which you would otherwise leave to simmer. The food then continues to cook in its own heat. Because the box is very well insulated, very little heat escapes. This method takes a little longer than simmering the food on the hob ? about one and a half times as long as you would leave the food simmering on the hob. However, it will save you gas because you are not using the hob during the ?simmer? phase, which has been replaced by the hay box.