On average, I doubt it would take more than 300W per system - with laptops probably not more than 150W.
You'll have to look on your electricity bill for how much a Unit (1 Unit = 1 KwHr = 1000W per hour) is - I'm not in the UK.
Samsung Galaxy S8 review
1995-2015: How technology has changed the world in 20 years
This poetic animation about monsters is illustrated in a Van Gogh painterly style
How to get WWDC 2017 tickets
Synology invites you to visit them at Insomnia on 26th – 29th August. Book your tickets now and visit Synology.