The key parameter - the one we need to calculate to compare the two process options - is the specific energy consumption (SEC) in energy per unit volume or power divided by volume flow rate. And this has exactly the same units as pressure. However, the SI unit of energy – the Joule - is tiny.

And this can be most easily demonstrated using one of these [SHOW SMART METER]: this is my smart meter showing the electrical energy consumption in this house. Because when I pay my electricity bill, I'm charged according to the number of units I use - it's about 17 pence a unit. That unit is the kWh. That's 1000 Watts and 3600 seconds - 3.6 million Watt-seconds - or Joules: remember, energy is power multiplied by time. And so far today [LOOK AT SMART METER] we’ve used 11.62 kWh of energy [SHOW TO CAMERA]; it’s in the bottom right hand corner.

Well that’s about 40 million Joules. And if my electricity company were to charge me per SI unit of energy - the Joule - they'd be charging me 5 millionths of a penny per unit. What a bargain.

It's the same thing with pressure. This filled bottle of water is about 30 cm tall [PLACE BOTTLE ON HAND]. I can hold it quite comfortably in the palm of my hand. And in doing so, my hand is experiencing around 3,000 Pascals of pressure. And I'm not even flinching.

To avoid a huge number of noughts on my smart meter reading and on water pressure gauges everywhere, the kWh is used to represent energy, and the bar - that's 100,000 Pascals and about 1 atmosphere – used to denote pressure. 

Now let's get back to the sums.