Examining water numbers - Help needed

There are about 800 million people in the world that don't have direct access to potable water. With the increasing droughts, these numbers may even start to rise, rather than fall. Israel is a perfect example of a technological advanced country that has ample experience in turning Seawater into Potable water through the process of Reverse Osmosis.

The figures are somewhat conflicting, but I'm going to build the "optimum" scenario : Most water for least amount of Energy. In 2010 they desalinated 329 million cubic meters at an "energy cost" of 2.5 KWh/Cubic Meter, if we multiply these numbers and convert them to Terawatt hours we get 0.82 TWh for 329 Million Cubic Meters of Desalinated Water..

This means that 1 million Cubic Meters of desalinated water requires 0.0025 TWh.

Consider this the numbers in this link

This website states that the minimum daily water requirements per person is 50 liters. This is to prepare food, Hygiene, flushing the toilet, etc. This comes down to 18.250 liters per person per year or 18.25 Cubic Meters. IF these people already have access to 20 liters of water a day, they need another 30 liters a day or 10.950 Liters a year or 10,95 Cubic Meters annually.

Let's do the math for a Billion people : 10.95 * 1.000.000.000 = 10.950.000.000 Cubic Meters Required Annually.

10.950.000.000 Cubic Meters / 1.000.000 = 10.950 * 0.0025 TWh = 27,375 TWh

In the UN scenario we would only need 27 TWh to provide 30 liters extra a day to 1 billion people.

I'm still very sceptical about these numbers, 27 Terawatt hours is equivalent to roughly 4 1000MW installations that run at a capacity factor of 90% . If that's all, why aren't we building them?

Can anyone point me to something I've overlooked?