How many MVA of grid capacity does a 100 MW data centre need?

Tenants lease megawatts, utilities and transformers are rated in MVA, and the gap between them is where site models go wrong.

Julian R. Sterling · from The Data Center Development Handbook

Size a data centre substation from the critical IT load times the peak PUE, divided by the power factor, then add a spare transformer. For an illustrative 100 MW of IT at a peak PUE of 1.35 and a 0.95 power factor, the site needs 142.1 MVA of firm capacity, about 1.42 MVA per MW of IT. In an N+1 layout of three 75 MVA transformers, that means 225 MVA installed for 150 MVA firm.

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 →

Why MW and MVA are not the same number

A tenant leases kilowatts of IT load. A utility, a transformer and a cable are rated in volt-amperes, because the current they carry, and the heat that limits them, depends on apparent power, not on the real power that does useful work. The two are linked by the power factor: real power (MW) = apparent power (MVA) × power factor. Part of the current a site draws is reactive, here 44.4 MVAr, which does no work in the servers but still loads every transformer on the way in. A development team that writes “120 MW” in the model when the grid offer says “120 MVA” has overstated what it can sell before it has built anything.

The case and its assumptions

Illustrative campus. Ratings and factors are illustrative design assumptions, not equipment specifications.
Input	Value
Critical IT load to be leased, MW	100
Peak design PUE (hottest hour)	1.35
Annualised PUE, for comparison	1.25
Blended power factor at the point of connection	0.95
Transformer redundancy	N+1

The power factor is a blended figure. Modern UPS systems present a power factor close to one to the grid; motors, pumps and chillers less so. The design team will compute it, and the utility may set a minimum it requires at the connection point.

The calculation, step by step

Step 1, real power at peak = IT load × peak PUE = 100 × 1.35 = 135.0 MW

Step 2, apparent power = 135.0 / 0.95 = 142.1 MVA (reactive part = √(142.1² − 135.0²) = 44.4 MVAr)

Step 3, transformers in N+1: with three units, any two must carry the full load, so each needs 142.1 / 2 = 71.1 MVA. The next rating offered is 75 MVA.

Installed = 3 × 75 = 225 MVA; firm (one unit out) = 2 × 75 = 150 MVA, a headroom of 7.9 MVA

Excel: =IT_MW*PeakPUE/PF for MVA, and =MVA/(Units-1) for the rating each unit needs

Two shortcuts follow. The ratio is 1.42 kVA of grid capacity per kW of IT on these assumptions, a number worth checking against every grid offer. And using the annualised PUE of 1.25 instead of the peak gives 131.6 MVA, 10.5 MVA short on the one afternoon a year when it matters, the same trap described for capacity in how much leasable IT capacity a higher PUE costs.

The redundancy choice

Three ways to deliver 142.1 MVA of firm capacity.
Layout	Rating needed per unit	Unit chosen	Installed MVA	Firm MVA	Installed / need
N+1, three units	71.1	75	225	150	1.58
N+1, four units	47.4	50	200	150	1.41
Two units, each carrying the full load	142.1	150	300	150	2.11

All three deliver the same 150 MVA of firm capacity. What changes is how much transformer is bought and stands idle: 2.11 MVA installed for every MVA needed with two full-size units, 1.41 with four smaller ones. More units means less spare iron but more bays, switchgear and space in the substation compound, and the economics of that trade-off belong to the electrical engineer. The developer's job is to make sure the model uses firm capacity, never installed.

Running it backwards: what does a grid offer support?

The more common question on the deal side runs the other way. A utility offers 120 MVA. How much IT can be leased?

Real power = 120 × 0.95 = 114.0 MW; leasable IT = 114.0 / 1.35 = 84.4 MW

Read 120 MVA as 120 MW and you get 120 / 1.35 = 88.9 MW: 4.4 MW too much, a 5.3 per cent overstatement

At the book's illustrative turnkey rent of $105 per kW-month, the 4.4 MW that does not exist would have been $5.60 million a year of rent in the model, before anyone capitalised it into a site value.

Leasable IT MW per 100 MVA of firm grid capacity.
Power factor	PUE 1.20	PUE 1.35	PUE 1.50
0.90	75.0	66.7	60.0
0.95	79.2	70.4	63.3
0.99	82.5	73.3	66.0

The grid is the same in every cell; the leasable capacity ranges from 60.0 to 82.5 MW. Peak PUE moves it more than the power factor, but both are worth a line in the model rather than a hard-coded MW figure. The inverse table, MVA needed for 100 MW of IT, runs from 121.2 MVA at the best corner to 166.7 at the worst.

The common mistakes

MVA treated as MW. The commonest error, and it always flatters the site.
Installed capacity treated as firm. A substation with 225 MVA of transformers does not support 225 MVA of load if it is to survive the loss of one unit.
Annual PUE used for sizing. Capacity is fixed by the worst hour, energy by the average one. Annual energy is the subject of how much electricity a 100 MW data centre uses.
Future density ignored. Higher-density halls with liquid cooling change both PUE and the mix of loads, so the factor used today may not hold for the second phase.

Power procurement and grid interconnection are at the centre of the book, and the free workbook for the book's 180 MW site builds the campus economics on the same turnkey rent.

Takeaway

Grid MVA × power factor / peak PUE is what you can lease; IT MW × peak PUE / power factor is what you must connect. Here 100 MW of IT needs 142.1 MVA of firm capacity, delivered by 225 MVA of transformers in an N+1 layout, and a 120 MVA offer supports 84.4 MW, not 88.9.

Questions readers ask

How do you convert MVA to MW for a data centre?

Multiply MVA by the power factor. A 120 MVA connection at a 0.95 power factor delivers 114.0 MW of real power. Dividing by a peak PUE of 1.35 then gives 84.4 MW of IT load that can be leased. Treating the 120 MVA as 120 MW would overstate leasable capacity by 4.4 MW, or 5.3 per cent.

How many kVA per kW of IT does a data centre need?

Peak PUE divided by power factor. At 1.35 and 0.95 it is 1.42 kVA of firm grid capacity per kW of IT. With a 1.20 peak PUE and a 0.99 power factor it falls to about 1.21, and at 1.50 and 0.90 it rises to about 1.67, so the ratio varies by more than a third across plausible designs.

What does N+1 mean for data centre transformers?

That the site can lose any one transformer and still carry its full load. With three units, two must carry 142.1 MVA, so each needs 71.1 MVA and a 75 MVA rating is chosen: 225 MVA installed but 150 MVA firm. Only the firm figure should be used to size what can be leased.

Read the whole case

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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Also on this site

How much leasable IT capacity does a higher PUE cost?Divide the grid connection by peak PUE. On 120 MW, a 1.40 peak instead of 1.25 leaves 85.7 MW to lease, not 96.0: $12.96m of rent a year lost.
How much electricity does a 100 MW data centre use a year?IT MW x utilisation x PUE x 8,760 hours. At 70% utilisation and a 1.30 PUE, 100 MW of IT draws 797,160 MWh a year, about $55.8m at $70 per MWh.
How much does a data centre cost to build per MW?Divide the all-in cost by critical IT load, not utility capacity. An illustrative 60 MW campus costs $670.2m: $11.17m per IT MW, but only $8.59m per utility MW.
How does load factor change a data centre's demand charge per MWh?Demand charge per MWh = rate x 1,000 / (load factor x hours). At $12 per kW-month it is $17.54 per MWh at a 95% load factor and $55.56 at 30%.
Should a data centre be built in one phase or several?On an illustrative 120 MW campus leasing 40 MW every two years, three phases add $80 million of NPV despite costing 6.3 per cent more to build.
What starting rent matches a 3% data centre lease escalator?Over 15 years, $105 per kW-month at a 3% escalator equals $112.93 at 2%: one point is worth 7.5% of face rent, but only 5.7% to a seven-year seller.

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