This is probably the most common question I get asked. It is difficult to explain why it is wrong but here goes:
The problem is you are not thinking of ‘heat’ or more accurately ‘energy’ as a quantative thing. Energy is measured in Watts or joules or, when talking about food, calories. They are all the same, just different units like miles or kilometers.
1,200 Calories, half a person’s normal food intake is about 1,000 Watts or a kW (kWh for the pedantic) and this is the unit you buy your electricity in. A kW of electricity costs about 12p, oil 6p, Natural Gas 4p, LPG 8p, coal 8p and pellets 5p.
There are about 10 kWs of energy in a litre of oil, so if you top up your tank with 1000 litres of oil, you’ve bought 10,000 kWs of energy. This would cost you £1,200 if you bought the same amount of electricity.
Now the answer to the question: if you are using 4kWs of energy to heat the cooker and cook your dinner, you are buying 4 kWs of energy. If you want to heat water and / or some radiators, say 8 kWs of energy, you would have to put a further 8 kWs into the cooker, the same amount of energy you would put into a separate boiler or electric heater.
You cannot take 12 kWs of energy out unless you put 12 kWs of fuel in.
The water boiler draws the heat out the cooker so you have to add more fuel to keep the temperature up.
This is uncontrolled, so you have lots of hot water, especially when cooking, and you have to dump it. Much better to use a proper system that heats a cylinder up when you want, it to the temperature you want it.
That’s a long explanation, but I have been struggling to explain this to hundreds of customers for twenty years now.
If anyone can help with a more succinct explanation I would be most grateful!