An uninterruptible power supply (UPS) keeps critical equipment running through a power failure with no break, using stored energy, usually batteries, until the utility returns or a generator takes over. It also protects that equipment from sags, surges and other disturbances on the supply. In buildings, UPS systems range from a small unit under a desk to large systems serving entire server rooms, control rooms or medical equipment.
Why a generator alone is not enough
A standby generator takes time to start, reach speed and take the load through its automatic transfer switch. During that gap, which is usually several seconds, the load has no power. Lighting, heating and most motors tolerate it. Servers, network equipment, phone systems, building controls, security systems and some medical and laboratory equipment do not: they reboot, lose data or fail. A UPS bridges the gap, and a generator then carries the load for as long as fuel lasts. For long outages, the two work together.
Types of UPS
Standby (offline)
The load runs directly from the utility. When the supply fails, the UPS switches to its inverter and batteries. The switch takes a few milliseconds. These units are common for single workstations and small equipment.
Line-interactive
Similar to standby, but with a transformer or regulator that corrects moderate voltage sags and swells without going to battery. It suits small server closets and network equipment where the supply is generally good.
Double conversion (online)
The incoming AC is converted to DC, which charges the batteries and feeds an inverter. The inverter makes clean AC for the load all the time. When the supply fails, the inverter simply keeps running from the batteries, with no transfer at all. This is the usual choice for data centres, larger server rooms and critical facility loads. It costs more and has some conversion losses, though many units have efficiency modes that reduce them.
| Type | Transfer on failure | Supply conditioning | Typical use |
|---|---|---|---|
| Standby | Brief switch to battery | Basic surge protection | Single workstations, small devices |
| Line-interactive | Brief switch to battery | Corrects moderate sags and swells | Network closets, small server rooms |
| Double conversion | None: the inverter is always running | Load fully isolated from supply disturbances | Server rooms, data centres, critical equipment |
Batteries
The battery is the part of a UPS most likely to fail and the part that sets how long the system can run. Two families are common:
- Valve-regulated lead-acid (VRLA). Familiar, widely available and lower in up-front cost. Life is shortened considerably by heat, so battery rooms and cabinets need temperature control. They are typically replaced on a schedule well before they fail outright.
- Lithium-ion. Smaller and lighter for the same energy, with longer life and better tolerance of temperature, but higher up-front cost. They come with a battery management system and have specific fire protection considerations that the design must address.
Runtime is chosen to suit the plan. If a generator will take over, a UPS may only need enough runtime to cover the generator start and a margin for a failed start, plus time for an orderly shutdown if the generator does not come on. Without a generator, runtime must cover either the expected outage or a controlled shutdown.
Sizing a UPS
- List the loads. Measure or record the actual power drawn by the equipment to be protected, not the nameplate maximum of each power supply.
- Allow for growth. Server rooms grow. Plan for expected additions over the life of the UPS.
- Check kW and kVA. A UPS has both ratings, and the load must fit within both.
- Choose redundancy. A single UPS is a single point of failure. Critical sites use N+1 modules, so one can fail or be serviced, or two separate UPS systems feeding equipment with dual power supplies.
- Set runtime and battery size from the outage plan and generator arrangement.
- Check the generator. A UPS drawing power to recharge its batteries while carrying a full load can present a large and non-linear load to the generator. The generator must be sized with this in mind. See how generators are sized.
Bypasses and maintenance
Every UPS of any size should have a way to keep the load powered when the UPS itself is out of service. An internal static bypass transfers the load to the raw supply automatically if the UPS faults or is overloaded. An external maintenance bypass, a separate switch arrangement, allows the UPS to be completely isolated for service or replacement. Without one, servicing the UPS means shutting down the load it protects.
- Regular visual checks of alarms, load level and battery status
- Preventive maintenance by a qualified service technician on the manufacturer's schedule
- Battery testing to find weak cells before an outage does
- Planned battery replacement before the end of expected life
- Temperature monitoring in the UPS and battery space
- Periodic tests under real conditions, coordinated with the generator
- A written procedure for switching to and from maintenance bypass
A typical case
A small office has a server closet with a rack-mounted UPS bought years ago. During a short outage, the servers go down within seconds even though the UPS shows it is on battery. Investigation finds batteries at the end of their life and a load that had grown well beyond what the UPS was bought for. When the owner considers a replacement, the review also finds that the closet runs hot, which shortens battery life, and that there is no way to service the UPS without shutting down the rack. The replacement includes a correctly sized line-interactive or online unit, a simple external maintenance bypass, better cooling for the closet, and a battery replacement plan with dates.
Stored energy is a hazard. UPS systems and batteries remain energized when the input is switched off. Only qualified workers should open UPS equipment or battery cabinets, following the manufacturer's procedures and the site's lockout and arc flash practices.
How ehZee can help
Our emergency and standby power design work includes UPS systems: sizing, redundancy, battery choice, bypass arrangements, and coordination with the generator and distribution. For server rooms, it fits with our data centre power design work.
Common questions
How long should a UPS run on battery?
Long enough for the plan: covering the generator start and a failed-start margin if there is a generator, or allowing a controlled shutdown if there is not. Longer runtimes cost more in batteries, space and cooling.
How often should UPS batteries be replaced?
It depends on the battery type, temperature and use. Follow the manufacturer's guidance and test results, and plan replacement before the end of expected life rather than waiting for a failure.
Can a UPS run from a generator?
Yes, and they are commonly used together. The generator must be sized and specified to handle the UPS as a load, including battery recharging and its effect on generator voltage and frequency.
Should the UPS be on the emergency or standby generator circuit?
A UPS protecting business equipment is normally a standby load, not a life safety load, so it is usually fed from the standby side. Your engineer will confirm the arrangement, especially where a UPS also serves equipment that the code treats as emergency load.