What's the best way to get hot water with a minimum of energy? -Green LivinCar:the to of water up boiling at and gas flat for with

What%26#039;s the best way to get hot water with a minimum of energy? while doing the washing up i wondered which of the following would use the least amount:
- boiling the kettle (electric), and adding that to the washing-up bowl
- running the tap until the water heated up (up to a few minutes - gas boiler at the other end of the flat)
- boiling water on the gas hob

anyone know for sure?

Additional Details

3 days ago

(for those of you finding a different answer to the one asked - i%26#039;m in a rented flat, and can do nothing at all to the outside, or interior heating system - so only have the 3 options given to work with)