Data center backup power

Two problems that look like one

“Backup power” covers two jobs that a single machine cannot do. When the utility drops, something has to carry the load in the milliseconds before anything else reacts — that is stored energy, a UPS or a flywheel. Separately, something has to carry the load for the hours or days that follow, and that is generation: a diesel or gas engine, or on the largest campuses a turbine.

Confusing the two is the most common sizing error. A generator takes time to start, come up to speed and accept load; it does nothing for the gap at the instant of transfer. A UPS covers that gap but has minutes of runtime, not hours. Every real design uses both, and the interesting decisions are about how they hand over.

Specifications that decide the selection

Selection criteria in practice

• Size the UPS for the gap, not the outage. Its job is to carry load until the generator accepts it. Runtime beyond that is expensive battery you will replace on a cycle.
• Check step load acceptance against your actual block loads. Cooling equipment restarting at once is usually the hardest step, and it is the one that gets missed.
• Interrogate the fuel assumption. Diesel means on-site storage, fuel polishing and a refuelling plan. Natural gas removes storage but adds dependence on a supply that may be affected by the same event.
• Confirm the redundancy target applies to both layers. N+1 generation with a single UPS path is not N+1 overall.
• Plan for testing, not just for outages. NFPA 110 exercise requirements and permit hour limits shape the installation as much as the emergency case does.

Typical applications

A conventional facility pairs a static UPS covering seconds of ride-through with diesel gensets sized for the full critical load plus cooling, transferring on loss of utility. Natural-gas sets appear where fuel storage is impractical or emissions limits favour them. Flywheel or DC UPS suits sites that want ride-through without a battery replacement cycle and can accept shorter autonomy. Turbines only make sense at campus scale, where the machine is closer to a generating asset than to backup equipment.

Work the numbers before you specify

Related reading

Common questions

Why can't a generator alone provide backup power?

Because it takes time to start, reach speed and accept load. During those seconds the critical load has no source. A UPS exists specifically to cover that window; the generator covers everything after it.

How long should UPS runtime be?

Long enough to cover generator start and load acceptance, with margin for a failed first start attempt. Sizing for a long outage is generally the wrong instinct — that is the generator's job, and the extra battery is capital you replace on a cycle.

Is natural gas better than diesel for standby?

It depends on what you are protecting against. Gas removes on-site fuel storage and its maintenance burden, but it makes you dependent on a supply that may be disrupted by the same regional event causing the outage. Diesel is self-contained but brings storage, polishing and refuelling logistics.

What does “step load acceptance” mean and why does it matter more than kW?

It is how much load the set can take in a single block without the voltage or frequency dipping unacceptably. Data centre loads restart in large blocks — particularly cooling — so a set with adequate kW but insufficient step capability will still fail the transfer.

Family reference pages

Each family below has its own page: how it is specified, the standards it is built to, its indicative lead time and market price band.