Size the tanks on the load the generators actually carry, cooling included, and the 48 hours turn into a logistics question as much as a storage one.
Backup fuel is facility load, not IT load, times the hours of autonomy times the engines' fuel burn per kWh. An illustrative 60 MW IT building at a 1.35 peak PUE puts 81 MW on its generators; at 0.26 litres per kWh it burns 21,060 litres an hour, so 48 hours of autonomy needs 1.01 million litres of diesel, about 267,075 US gallons. Beyond the tanks, staying on generators means 14.9 road tankers a day.
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 →
All figures are illustrative. Specific fuel consumption, the litres a generator burns per kWh it produces, comes from each engine's data sheet and rises at part load.
| Input | Value |
|---|---|
| Critical IT load | 60 MW |
| Peak PUE, hot day, full load | 1.35 |
| Autonomy required | 48 hours |
| Fuel burn at full load | 0.26 L per kWh |
| Fuel burn at half load | 0.29 L per kWh |
| Generator rating | 3.0 MW |
| Diesel price | $1.10 per litre |
| Road tanker capacity | 34,000 litres |
Forty-eight hours is a common design target in tenant specifications and lender technical reviews, not a universal rule; some sites are built to 24 hours, some to 72 or more.
Facility load = IT load × peak PUE = 60 × 1.35 = 81 MW = 81,000 kW
Running generators = 81 / 3.0 = 27
The chillers, pumps, fans and UPS losses all move to backup with the servers. The design case is the hot day at full IT load, because that is when the grid is most likely to fail and the cooling works hardest.
Fuel per hour = 81,000 kW × 0.26 L per kWh = 21,060 litres (780 per generator)
Fuel per day = 21,060 × 24 = 505,440 litres
Fuel for 48 hours = 21,060 × 48 = 1,010,880 litres = 267,075 US gallons (at 3.785 litres per gallon)
Excel: =IT_MW*1000*PUE_peak*SFC*Hours
That is 16,848 litres per MW of IT load, or 37,440 litres per running generator, which is roughly the size of a large belly tank under each set. The fuel follows the load, not the engine count: spreading 81 MW over more generators changes the tank layout, not the litres.
Fuel inventory = 1,010,880 × $1.10 = $1,111,968
Carrying cost at 8 per cent a year = $88,957
Tankers to keep running = 505,440 / 34,000 = 14.9 a day, one every 1.6 hours
The inventory is trivial beside a building that costs several hundred million dollars. What matters is the logistics: 29.7 tanker loads would be needed to refill the tanks after a 48 hour event, during a regional outage, when every hospital and telecom site is calling the same distributors. Fuel supply contracts with priority clauses, and fuel polishing to keep stored diesel usable, are part of the diligence, not an operational footnote.
Real outages rarely hit at full design load. At half load the engines burn more per kWh, so fuel does not fall in proportion.
| Load, share of 81 MW | L per kWh | 24 h | 48 h | 72 h | 96 h |
|---|---|---|---|---|---|
| 50% | 0.290 | 0.28 | 0.56 | 0.85 | 1.13 |
| 75% | 0.275 | 0.40 | 0.80 | 1.20 | 1.60 |
| 100% | 0.260 | 0.51 | 1.01 | 1.52 | 2.02 |
At half load the 48 hour figure is 55.8 per cent of the full-load figure, not 50, because of the part-load penalty. Moving to 72 hours of autonomy at full load needs 1,516,320 litres, $1.67m of fuel and half as much tank space again, which on a tight site competes with the generator yard for land.
A building sized on full load stores more hours than its name says on an ordinary day: a 48 hour tank lasts about 86 hours at 50 per cent load. Lenders and tenants specify the design case precisely so that the label means the same thing on the worst day.
Sizing autonomy on IT load. Forty-eight hours at 60 MW is 748,800 litres. At the 81 MW the building actually draws on a hot day, that lasts 35.6 hours: the tanks are 25.9 per cent short, which is exactly the PUE gap. The opposite error is sizing on installed nameplate. With one spare per block of five, the site has 33 generators and 99 MW of nameplate; 48 hours at nameplate is 1,235,520 litres, 22.2 per cent more than the load needs and $0.25m of extra stock, plus the tanks and the land to hold it. Spares are there for redundancy, not to run at the same time as the units they back up.
The same PUE that sets the fuel also caps how much IT load a grid connection supports, worked in how much leasable IT capacity a higher PUE costs. Where the generators run continuously rather than as backup, the economics change entirely, as in what on-site gas power costs per MWh.
The book's power and site work, with the underwrite of its 180 MW case, is in the free workbook for this case.
At an illustrative specific fuel consumption of 0.26 litres per kWh at full load, a 3.0 MW generator burns 780 litres an hour, about 18,720 litres a day. At half load consumption per kWh rises, to an illustrative 0.29, so the engine burns less fuel in total but less efficiently. Manufacturer data sheets give the curve for each engine.
Because refuelling at full load is a logistics operation, not a phone call. The illustrative 60 MW building burns 505,440 litres a day: 14.9 road tankers of 34,000 litres, one every 1.6 hours, during the same event that caused the outage. Storage buys time for supply contracts with priority delivery to work.
On facility load. Cooling, UPS losses and lighting run on the generators too. Sizing 48 hours on the 60 MW IT load stores 748,800 litres, which lasts 35.6 hours at the 81 MW the site actually draws: a 25.9 per cent shortfall.
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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