The conversion from megawatts to tons is a constant. The installed plant is not: it depends on which heat counts, the design-day derate and the redundancy the tenant buys.
One megawatt of IT load turns into one megawatt of heat, and one ton of refrigeration removes 12,000 BTU an hour, or 3.517 kW, so 1 MW of IT needs 284.3 tons of cooling before any overhead. Add the electrical losses and other heat inside the cooled space and an illustrative 20 MW hall needs 6,056.5 tons, 302.8 per MW. After derating chillers for a hot design day and adding a spare, the installed plant is 9,000 tons, or 450 tons per MW.
Worked in full in The Data Center Development Handbook by Julian R. Sterling, with every figure reproduced in a free workbook.See the book on Amazon →
The conversion is a constant; the plant size is not. The gap between 284.3 and 450 tons per MW is made of three decisions an engineer takes and a developer pays for: which heat counts, how much the chillers lose on the hottest day, and how much redundancy the tenant wants.
| Input | Value |
|---|---|
| Critical IT load at full build | 20,000 kW |
| UPS and distribution losses inside the cooled space, % of IT | 5.0% |
| Lighting, people, envelope gains, % of IT | 1.5% |
| BTU per hour in one ton of refrigeration | 12,000 |
| BTU per hour in one kW | 3,412 |
| Chiller nominal capacity | 1,000 tons |
| Capacity derate on the design-day ambient | 15% |
| Redundancy | N+1 |
kW per ton = 12,000 ÷ 3,412 = 3.517
Tons required = total heat load (kW) ÷ 3.517
Chillers needed (N) = ROUNDUP(tons required ÷ derated chiller tons, 0); installed = N + spares
In Excel, with the heat load in B1: =B1/3.517 for tons, and =ROUNDUP(B2/(1000*(1-15%)),0)+1 for the N+1 chiller count.
Step 1, the heat load. Every watt the servers draw ends up as heat in the room: 20,000 kW. The UPS and distribution losses add 5.0 per cent, 1,000 kW, when that equipment sits in space the same plant cools. Lighting, people and gains through the building envelope add 1.5 per cent, 300 kW. Total: 21,300 kW, or about 72.7 million BTU an hour.
Step 2, convert to tons. The IT load alone is 20,000 ÷ 3.517 = 5,686.9 tons. The full heat load is 21,300 ÷ 3.517 = 6,056.5 tons, or 302.8 tons per MW of IT.
Step 3, derate the chillers. A chiller's nameplate is measured at standard rating conditions. On the design day, with hot air entering the condensers, an air-cooled machine delivers less. At a 15 per cent derate a 1,000-ton chiller gives 850 tons. The load needs 6,056.5 ÷ 850 = 7.13 machines, so N is 8, giving 6,800 tons of effective capacity.
Step 4, add redundancy. N+1 means one more machine than the load needs, so the plant can lose a chiller on the design day and still hold the room: 9 chillers, 9,000 tons nominal, 450 tons per MW of IT.
| Step | Tons | Tons per MW of IT |
|---|---|---|
| IT load only | 5,686.9 | 284.3 |
| Plus losses and other gains | 6,056.5 | 302.8 |
| Effective capacity of N = 8 derated chillers | 6,800 | 340 |
| Installed nominal, N+1 = 9 chillers | 9,000 | 450 |
Without the derate, 6,056.5 tons would need 6.06 machines, so 7 plus a spare: 8,000 tons, 400 per MW. The hot-day derate costs a whole extra chiller here because the load sits just above a multiple of the derated size. That cliff is common in plant sizing, and it is why the chiller size is chosen with the load, not before it.
The ratio of tons to MW does not depend on rack density. A 20 MW hall at 10 kW per rack and one at 80 kW per rack reject the same 20 MW of heat. Density changes how the heat is collected, by air or by liquid, and at what temperature, not how much there is.
| Design choice | Derated tons each | N | Installed tons | Tons per MW |
|---|---|---|---|---|
| N+1, no derate | 1,000 | 7 | 8,000 | 400 |
| N+1, 10% derate | 900 | 7 | 8,000 | 400 |
| N+1, 15% derate | 850 | 8 | 9,000 | 450 |
| N+1, 20% derate | 800 | 8 | 9,000 | 450 |
| N+2, 15% derate | 850 | 8 | 10,000 | 500 |
| 2N, 15% derate | 850 | 8 | 16,000 | 800 |
The heat load scales linearly: 303 tons for 1 MW of IT, 1,514 for 5 MW and 15,141 for 50 MW on the same overheads. The installed plant scales in steps, which is why per-MW cooling cost is lumpy for small halls and smooth for large campuses built in repeated blocks.
Tons also set the power the cooling consumes. At an illustrative 0.80 kW of plant power per ton on the design day, 6,056.5 tons draw 4,845 kW, about 0.24 of the IT load. Averaged over a year, with the hall at 70 per cent of its design load and economiser hours bringing the plant to 0.45 kW per ton, the draw is 1,908 kW and 16,712 MWh a year, around 0.136 of the average IT load. That mechanical overhead is the largest piece of PUE above 1.0, and it is power the site cannot sell; how a higher PUE cuts leasable capacity works through what it costs.
Start from 284.3 tons per MW, add the heat that shares the room, divide by the derated chiller size, round up and add the spare: 450 tons per MW of IT for this hall. The Data Center Development Handbook follows site selection, power procurement and grid interconnection on the deal side, and the free workbook for this case carries the underwriting models. For the other utilities a site has to secure, see how much water a data centre uses.
One ton of refrigeration removes 12,000 BTU an hour, and one kW is about 3,412 BTU an hour, so a ton is 3.517 kW. Turned round, 1 MW of heat needs 284.3 tons. The name comes from the heat absorbed by melting a short ton of ice over a day, but in data centre design it is simply a unit of heat removal rate.
No. Heat removed equals power consumed, so 20 MW of IT rejects the same heat at 10 kW or 80 kW per rack: about 5,686.9 tons before overheads. Density changes the method, liquid loops instead of air, and the temperature of the heat, which can cut the plant power per ton. It does not change the tons.
Enough to hold the room on the design day after losing the planned number of machines. In the illustrative 20 MW hall, a 6,056.5-ton load on 1,000-ton chillers derated to 850 tons needs 8 machines; N+1 installs 9,000 tons, N+2 installs 10,000 and 2N installs 16,000, or 450, 500 and 800 tons per MW of IT.
This article is one calculation from The Data Center Development Handbook. The book takes the same case from first principles to the decision, chapter by chapter, and every figure it prints is a live formula in the free companion workbooks.
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