Why isn't the grid connection the capacity you get?
Because the grid connection feeds the whole facility, not just the servers. Cooling, power conversion, lighting and offices all draw from it, and redundancy means part of the installed equipment is held in reserve. What you can actually use for servers, storage and network is the IT capacity, and that is the number to compare between offers.
What are the three numbers?
- Grid connection: what the grid operator has agreed to deliver to the site, often in MW, and sometimes only partly secured.
- Facility power: what the site draws in total when it runs, including cooling and losses.
- IT capacity: the power available to IT equipment, usually in kW per hall, suite or rack.
How does PUE connect them?
Power usage effectiveness (PUE) is total facility power divided by the power reaching IT equipment. It is defined in ISO/IEC 30134-2. A site running at a PUE of 1.25 uses 1.25 kW in total for every kW of IT load.
As a worked example: 10 MW of facility power at a PUE of 1.25 leaves 8 MW for IT. A design-point PUE and an annual average can differ, because cooling needs change with the weather and with how full the halls are, so ask which one an offer quotes.
Where does redundancy come in?
Redundancy takes another share out of what can be sold. A site with 2N power keeps two complete paths, each able to carry the full IT load, so the installed equipment is about twice what the IT load needs. Two sites with the same grid connection can offer quite different IT capacity depending on their redundancy level. Our article on N+1 and 2N goes into the levels.
Which number should you compare?
Compare IT capacity, in kW, together with the power status behind it and the PUE. Ask whether the figure is for the whole site or available to you, whether it arrives in phases, and whether part of it depends on power that is applied for rather than secured. Requests and listings on the marketplace use IT capacity for exactly this reason.