Interesting question. Here is some information:The most energy-efficient way to boil water depends on the available resources and equipment. However, here are some methods, listed in order of their efficiency:

1. **Electric kettle**: Electric kettles are generally the most energy-efficient way to boil water, as they directly heat the water with electricity and have minimal heat loss. They use around 0.12-0.15 kWh of electricity to boil 1 liter of water.

2. **Microwave**: Boiling water in a microwave is another efficient method, using around 0.15-0.20 kWh of electricity to boil 1 liter of water. However, this method can be less efficient if you're boiling small amounts of water.

3. **Induction cooktop**: Induction cooktops are highly efficient and can boil water quickly, using around 0.20-0.25 kWh of electricity to boil 1 liter of water.

4. **Gas stove**: Boiling water on a gas stove is less efficient, using around 0.30-0.40 kWh of energy (in the form of gas) to boil 1 liter of water.

5. **Electric stove**: Electric stoves are the least efficient method, using around 0.40-0.50 kWh of electricity to boil 1 liter of water.

To further optimize energy efficiency, consider the following tips:

* Use a lid to cover the pot, as this can reduce heat loss and boiling time.

* Use a pot that's the right size for the amount of water you're boiling, as this can help minimize heat loss.

* Use cold water, as this can help reduce the energy required to boil the water.

* Avoid overheating the water, as this can waste energy.

Keep in mind that these values are approximate and can vary depending on the specific equipment and usage patterns.