infrastructure//data center//power density

Power density is the electrical power drawn per unit of space in a data center, quoted in kilowatts per rack or per square metre, and it is the number that decides whether a hall can host a given kind of hardware at all. Every watt delivered to a rack becomes a watt of heat in the same spot, so density limits both the electrical feed and the cooling.

Power density is the electrical power drawn per unit of space in a data center, quoted in kilowatts per rack or per square metre, and it is the number that decides whether a hall can host a given kind of hardware at all. Every watt delivered to a rack becomes a watt of heat in the same spot, so density limits both the electrical feed and the cooling.

The first trap is between power and energy. Power is a rate, measured in watts (electric circuit); energy is power over time, measured in watt-hours. A campus described as 50 MW can draw 50 megawatts at any instant; running flat out for a year it would consume about 50 MW times 8,760 hours, roughly 440 GWh (a European household uses on the order of 3,500 kWh a year, so that is the yearly use of over a hundred thousand homes), and that energy figure is what the electricity bill counts:

E=P⋅t,50 MW×8760 h≈4.4×105 MWh.E = P \cdot t, \qquad 50\ \text{MW} \times 8760\ \text{h} \approx 4.4 \times 10^{5}\ \text{MWh}.E=P⋅t,50 MW×8760 h≈4.4×105 MWh.

The left side is what the meter totals; the capacity a site is sold or permitted for is always the power. A density in kilowatts per square metre is likewise a rate per area, never consumption.

The scale has moved by an order of magnitude. An enterprise rack of the 2010s drew around 5 to 10 kW and was cooled by air; racks of H100 servers reach about 40 kW; a rack of current NVIDIA systems such as the GB200 NVL72 draws around 120 to 130 kW and needs liquid piped to the chips. A hall built for 8 kW per rack cannot simply receive the new racks: its busways, PDUs and cooling were sized for a fraction of that (power chain, rack).

Density is a trade. Packing GPUs close shortens the copper links between them (interconnect) and lets one rack act as one machine, at the price of liquid cooling, heavier floors and a larger blast radius when one rack fails.

The figure per square metre hides the plant. A hall's density says nothing about the space taken by transformers, UPS rooms, chillers and generators, which on a dense site can exceed the space for the computers.

Density is the reason colocation contracts are written in kilowatts, and the reason AI growth is now limited by power and cooling more than by chips.