The amount of energy to boil water depends on two things: the amount of water and the temperature of the water when you start.
The more water water, the more energy it takes. The colder the water, the more energy it takes.
Let say the water is at room temperature, or 20 °C. First, the water must be heated to 100 °C, which takes energy. The amount of energy is given by the specific heat of water, which is 4.186 Joule/gram °C. That means that requires 4.186 Joules of energy to heat 1 kilogram of water by 1 °C. So if you have 1 kilo grams of water at 20 °C, you have to add this much energy:
= (4.18Joule/gram °C) (100g) (100 °C - 20 °C)
= 334400 joules/°C
Of course, if you had more or less water, or it was colder or warmer, you would adjust this equation accordingly..