In Short
A UPS and a generator do different jobs. The UPS uses batteries to take over instantly and carry the load for a short time, while the generator starts within moments and then supplies power for as long as it has fuel. Most critical sites need both, because each covers what the other cannot.
When the grid fails, two machines keep a critical load alive, and they are often confused. The uninterruptible power supply (UPS) is a battery and an inverter that take over instantly and carry the load for minutes; the diesel generator needs a few seconds to start and accept load and then carries it for as long as there is fuel. Neither replaces the other: the UPS bridges the gap, the generator provides the autonomy. This guide explains what each one does, how they hand over and how a site decides what it needs.
What a UPS Does
A UPS sits between the supply and the load. In normal operation it conditions the power - filtering spikes, sags and frequency variations - and keeps its batteries charged; at a failure its inverter carries the load from the batteries without a break. Batteries are expensive and heavy, so a UPS carries the load for minutes only - enough to ride through short interruptions and to let a generator start. Rotary UPS systems store energy in a flywheel instead of batteries and bridge seconds rather than minutes; they are used in large plants where the generator is counted on to start every time.
What a Generator Does
A standby generator starts automatically when the supply fails - within seconds - and the automatic transfer switch (ATS) connects the load to it once the voltage and frequency are stable. It then runs for hours or days on the fuel in its tanks: a belly tank for shorter autonomy, a bulk tank and transfer system for longer. It cannot bridge the start-up gap, which is why loads that cannot blink - servers, telecoms, medical equipment - have a UPS in front of them, while loads that can - cooling, lighting, pumps - go directly to the generator.
How They Work Together
The sequence at a grid failure: the UPS carries the critical load from the first millisecond; the generator starts and comes up to speed; the ATS transfers the site to the generator; the UPS goes back to conditioning and recharges its batteries from the generator. When the grid returns and is stable, the ATS transfers back - with a short interruption that the UPS bridges, or without one on a closed-transition switch - and the generator cools down and stops. The generator has to suit the UPS's input, the cooling and all the other loads it will carry, including the motors that start at transfer.
In a data center the whole chain - utility, UPS, generators, switchgear, fuel - is duplicated to the redundancy level the tier requires, and the generators are tested under load bank on a schedule because they are the one part that sits idle.
Deciding What a Site Needs
A site with loads that tolerate a short break - a factory, a water works, a building - needs a generator and an ATS. A site with loads that do not - IT, telecoms, hospitals, laboratories - needs a UPS in front of those loads and a generator behind it. A site with a few minutes of non-critical interruption tolerance and no long outages may live with a UPS alone, which is rare. AXON builds the generator side of the system: containers and enclosures, fuel systems, exhaust and silencers, switchgear with the generator breaker, the ATS and the MCC, and load banks for testing - around generator sets of any make.
In short
- UPS: instant takeover, minutes of autonomy, power conditioning
- Generator: starts within seconds, hours to days of autonomy, needs fuel
- The ATS transfers the site to the generator and back
- Loads that cannot blink get a UPS; loads that can go to the generator
- The generator has to suit the UPS input, the cooling and the motors
- AXON builds the generator package: enclosure, fuel, exhaust, switchgear, load bank
What to send with an enquiry
- Critical and non-critical loads in kW / kVA
- UPS rating and type, if there is one
- Required autonomy in hours at full load
- Redundancy required: single set, N+1, 2N
- Site: indoor, outdoor, container, sound limit
- Grid connection and the transfer scheme wanted
Questions we are often asked
Can a generator replace a UPS?
No: a generator needs a few seconds to start and accept load, and loads that cannot tolerate that break need a UPS to bridge it. A generator replaces the UPS's long autonomy, not its instant response.
Can a UPS replace a generator?
Only where the outages are short and the batteries cover them. For hours of autonomy a battery UPS becomes very large and expensive; the generator is the economical source for long outages.
How big must the generator be for a UPS?
Larger than the UPS rating: the UPS input draws more than its output, its rectifier puts extra strain on the alternator, and the cooling and the other loads come on top. The UPS and generator makers give the figures for their equipment.
What is an automatic transfer switch?
The switch that senses the supply failure, starts the generator and transfers the load to it, then transfers back when the supply returns. Open-transition switches break briefly; closed-transition switches synchronise and transfer without a break.
How often should the generator be tested?
A regular short run, a periodic run under load - real or load bank - and a full-load test every year; critical sites follow their own standards.
Products
- Containers for Power Generators
- Circuit Breaker Panel
- Auxiliary Cabinet (MCC)
- Load Bank for Generator Testing
- Generator Belly Tank (Base Frame Fuel Tank)