Calculator 05 · Backup power

How many generators, how much diesel?

Pick the outage load, generator size, redundancy and runtime. See the generator yard, the fuel to store and the tanker loads to refill it.

Inputs

Facility loadMW

Load carried in an outage%

Share of facility load the generators must carry.

Generator size

Redundancy

Runtime on stored fuelhours

Fuel burn at loadgal/kWh

Check the generator spec sheet; this starts at an example value.

Tanker truck sizegal

Generator yard

Carrying loadRedundant

Generators

20 to carry 60 MW, plus 1 for redundancy

Installed capacity

63.0MW

Fuel on site

Each truck = one tanker load · 48 hours at load

Diesel stored

201,600gal

Burn at load

4,200gal/hr

Tanker loads to refill

For early planning only. Results follow from the inputs and the stated assumptions; they are not engineering, financial or legal advice.

How the math works

Generators to carry the load

The outage load is the facility load times the share the generators must carry. Dividing by the generator size and rounding up gives the units needed (N). Redundancy adds one or two spare units, or a full duplicate set for 2N.

Fuel = load × burn rate × hours

Diesel burn at load is the outage load in kW times gallons per kWh, which starts at an example value; generator spec sheets give the real figure at each load step. Multiplying by the runtime gives the fuel to store on site.

Refilling the tanks

Tanker loads divide the stored fuel by the truck size you enter. Fuel supply contracts during a regional emergency matter as much as tank size.

Questions

How many generators does a 60 MW data center need?

With 3 MW generators carrying the full load, 60 MW needs 20 units, plus one spare at N+1 for 21. At 2N it would be 40.

What is N+1 versus 2N?

N is the number of generators needed to carry the load. N+1 adds one spare so any single unit can fail; 2N doubles the whole set so an entire system can fail or be taken out of service.

How much diesel is typical to store?

Designs commonly hold enough fuel for one to several days at load. The right runtime depends on the reliability target, the local fuel supply and permit limits on storage and runtime.

Do generators need air permits?

Usually. Large diesel generator fleets are regulated sources of air emissions, and permits can cap their annual runtime. See the guide on air permits for data center generators.