# PUE at Altitude — ASHRAE Derate PUE calculations assume sea-level air density by default. At altitude, air is thinner — less mass per cubic metre — which reduces the heat-carrying capacity of air passing through condensers and heat exchangers. **Nairobi sits at ~1,700m elevation.** This matters. ASHRAE publishes derate tables that map altitude to condenser performance loss. At 1,700m, air density is roughly 80-82% of sea-level density. Condensers sized for sea-level performance will underperform — they reject less heat per unit of airflow. **Practical impact:** - Condensers need to be oversized (or fans run faster) to compensate - PUE worsens compared to the same hardware at sea level - Cooling system design must use site-specific ASHRAE data, not vendor defaults - CFD thermal modelling during factory pre-testing should simulate altitude conditions This is not a minor adjustment. A condenser rated for 20kW rejection at sea level might only handle 16-17kW at 1,700m. If you don't account for this, you'll hit thermal limits before you hit power limits. **Rule of thumb:** Always derate condenser capacity for altitude using ASHRAE data before finalising cooling architecture. Don't trust vendor spec sheets — they assume sea level unless stated otherwise. --- See also: [[Power Usage Effectiveness - PUE]] | [[Modular Data Centers MoC]]