How the calculation works
PUE and DCiE. Power usage effectiveness was introduced by The Green Grid and is published as ISO/IEC 30134-2.
- PUE = total facility power ÷ IT power (1.0 would mean no overhead at all)
- DCiE = IT power ÷ total facility power = 1 ÷ PUE, as a percentage
- Overhead = total − IT: cooling, UPS and transformer losses, lighting and the rest of the building
The ratio is the same for energy over a period (kWh ÷ kWh) as for average power. Measured over a full year it captures seasonal changes in cooling; a single reading does not.
Power cost.
- Facility energy = IT kW × PUE × hours
- Cost = IT kW × PUE × $/kWh × hours, with 730.5 hours a month and 8,766 a year (365.25 days), the same month the colocation cost per kW calculator uses
- Comparing facilities: the same IT load at each facility’s PUE and rate; the difference is what the efficiency gap costs you
Limits. The calculator uses only your figures; it has no industry averages. It assumes a steady average IT load; with a load that changes over the month, use your average kW. A provider’s PUE multiplier in a contract may be fixed or measured, and it may differ from the facility’s reported PUE.
Worked example
A 40 kW colocation deployment priced on metered IT power at $0.11/kWh times a PUE multiplier. Facility A quotes a multiplier of 1.4, facility B 1.6:
| Facility A | Facility B | |
|---|---|---|
| Average power billed: 40 kW × PUE | 56 kW | 64 kW |
| Energy a year: × 8,766 h | 490,896 kWh | 561,024 kWh |
| Cost a year: × $0.11 | $53,999 | $61,713 |
| Cost a month | $4,500 | $5,143 |
At the same rate, facility B costs $7,714 a year more ($643 a month), 14% more, for the same IT load. If B’s rate were $0.09/kWh, it would cost $50,492 a year, less than A despite the higher PUE, which is why the rate and the multiplier have to be compared together.
Measuring PUE: a site drawing 1,740 kW at the utility meter for 1,200 kW of IT load has a PUE of 1,740 ÷ 1,200 = 1.45, a DCiE of 69%, and 540 kW of overhead.
PUE in colocation pricing
Providers recover cooling and other overhead in different ways: a multiplier on metered IT energy, a higher per-kWh rate, or an all-in price per kW or per circuit. Put each quote on the same basis before comparing them; the colocation cost per kW calculator does that across space, power and cross-connects. The PUE glossary entry covers where PUE is measured and what moves it.
