Server Power, UPS & Cooling Calculator
List the servers, storage and network gear in a rack to get the total load, the UPS size and runtime energy it needs, the current on each PDU feed, the heat it produces in BTU/hr and tons of cooling, and the energy it uses per month. Size power and cooling before the equipment arrives.
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Equipment
Starting watts are typical running draws for each class of device, not figures for any particular model. Replace them with the nameplate rating or a measured reading from the PDU or iDRAC/iLO for your own equipment.
Supply and protection
Results
Standard UPS sizes compared
| Rating | Usable watts | Your load | Verdict | Typical form | Select |
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Usable watts = kVA × 1,000 × the UPS output power factor you chose. A UPS running between 40% and 80% of its capacity has room for growth and inrush without being oversized; runtime at a given load depends on the battery fitted, so confirm it against the manufacturer's runtime chart.
Power and UPS calculator FAQ
How do I size a UPS for a server rack?
Add up the running watts of everything the UPS will protect, add headroom of 20 to 25 percent for growth and start-up surges, convert to VA by dividing by the power factor, and pick the next standard UPS rating above that figure. A rack drawing 2,000 W at power factor 0.9 needs about 2,700 VA with headroom, so a 3 kVA unit at roughly 70 percent load.
What is the difference between VA and watts?
Watts are the real power that becomes work and heat; volt-amperes are the apparent power the supply has to deliver. The ratio between them is the power factor. Modern server power supplies run close to 0.95, so VA and watts are nearly equal; older or smaller equipment can be 0.8 or lower. A UPS is rated in both, and the lower of the two limits is the one you must stay under.
How much cooling does a server room need?
All electrical power drawn by IT equipment ends up as heat. Multiply watts by 3.412 to get BTU per hour, and divide BTU per hour by 12,000 to get tons of cooling. A 5 kW rack produces about 17,000 BTU/hr, or 1.4 tons. Add the room's own heat gain through walls, lighting and people, and size the air conditioning above the total.
Why add headroom?
Servers draw more when they boot, when fans ramp up in a hot room and when a workload peaks, and most racks gain equipment over their life. Running a UPS at 100 percent also shortens runtime sharply and leaves nothing for a failed phase or feed. Twenty percent is the usual minimum; use more if the rack is expected to grow.
Should I use single or dual power feeds?
Dual feeds, A and B, let each server with two power supplies survive the loss of one PDU, one UPS or one circuit. Each feed must be able to carry the whole load on its own, so size every PDU and UPS for the full design load, not half of it. The calculator does this when Dual A + B is selected.
How long will a UPS run on battery?
Runtime depends on the load and the battery capacity, and it falls faster than you might expect as load rises. The calculator shows the battery energy in watt-hours needed for your runtime target; compare it with the UPS runtime chart or add external battery packs. For anything beyond 15 to 20 minutes a generator is usually cheaper than batteries.