UPS and backup power

Autonomy quoted at full load and autonomy measured on site are two different numbers.

Parent division: Power and backup supply

The problem

A UPS is not an energy reserve: it is the time needed for the backup source to take over.

A UPS is often bought as an energy reserve, with the idea of riding through an outage. That is not its function. Its function is to cover the interval between the loss of the grid and the takeover by another source, and to filter disturbances on the incoming supply.

Two symmetrical mistakes follow from that misunderstanding. Too little autonomy, and equipment stops before the generator starts. Too much, and the organisation ties up a costly, bulky battery bank that has to be cooled and periodically replaced.

Batteries are, besides, the one component of the system that ages with certainty. Their capacity falls with time and with ambient temperature; an installation never tested since commissioning does not know its real autonomy.

Real autonomy and its margin, schematic

01Connected load: The load actually connected and measured, not the machine’s nominal rating.
02UPS: Filters incoming disturbance and covers the interval, not the duration of the outage.
03Battery: The one component that ages with certainty. Capacity falls with time and with temperature.
04Backed-up distribution: Backed-up circuits physically marked and separated from ordinary ones.
05Maintenance bypass: Without it, replacing a battery means shutting down what the UPS protects.
06Clean shutdown at a threshold: When the backup source does not come, what remains is to stop cleanly — proven at least once.

Schematic. The accented path is the margin between measured autonomy and the backup source’s restoration time.Typical sizing figures. Autonomy covers generator start-up and clean shutdown; it is calculated on the real load, never on the rated power.

Scope

What the service covers

Autonomy study: Load actually connected, restoration time of the backup source, margin, room temperature and ageing taken into the calculation, rather than catalogue autonomy.
Topology and redundancy: Conversion technology chosen, single or parallel machines, single or dual supply paths, and a maintenance bypass allowing work without an outage.
Batteries: Technology, sizing, layout and ventilation of the room, state-of-health monitoring, replacement programme and disposal route.
Backed-up distribution: Backed-up boards and circuits identified and labelled, strict separation from non-backed-up circuits, residual current protection and discrimination between levels.
Interaction with the generator: Compatibility of the UPS with the replacement source, behaviour at transfer, and a test of the complete chain from grid to generator, on load.
Monitoring and testing: Fault and battery status reporting, clean server shutdown at an autonomy threshold, periodic tests and a register of results.

Situations

The most frequent situations

Server room: The load is known and steady. What matters is the margin between real autonomy and generator restoration time, and the ability to shut systems down cleanly if restoration does not happen.
Supervision or guard post: The post must stay in service throughout the outage. Autonomy is calculated not from generator restoration but from how long surveillance must be held.
Network and security equipment: Switches, cameras, access controllers and telephony. Backing up some without the others produces a partly live system, usually unusable.
Site with an unstable grid: Microbreaks and voltage variation wear equipment down. Here the UPS serves as much to filter as to back up.

Requirements

What to require, of us as of anyone

These requirements hold whichever supplier is appointed. Written into a tender, they filter out the responses that will not hold.

An autonomy calculation based on the load actually connected, not on the machine’s nominal rating.
The margin between autonomy and the backup source’s restoration time, verified by an on-load test.
A maintenance bypass, without which any servicing means an outage for the protected equipment.
Battery state-of-health monitoring and a forecast replacement schedule.
Physical separation and labelling of backed-up and non-backed-up circuits.
Automatic clean shutdown at an autonomy threshold, proven at least once.
The disposal route for spent batteries, which is an obligation rather than a convenience.

Pitfalls

Common mistakes, and what they cost

Taking catalogue autonomy at face value: It is quoted at a reference load and temperature, on new batteries. The real autonomy of an installation in service for a few years is appreciably lower, and nobody discovers it before the outage.
Connecting unplanned loads: A portable heater, a printer or an air conditioner plugged into a backed-up socket divides the autonomy. That is why backed-up sockets are physically marked.
Omitting the maintenance bypass: Without a bypass, replacing a battery means shutting down what the UPS protects. Servicing is then postponed, until the battery fails on its own.
Never testing the complete chain: UPS and generator are tested separately and both work. The transfer between them is not, and that is precisely where the incompatibilities observed in real conditions sit.

Questions

Questions asked before consulting

How much autonomy is needed?

Enough to cover detection, starting and restoration of the backup source, with a margin for battery ageing. With no backup source, autonomy must allow systems to be shut down cleanly, which is a matter of minutes rather than hours. Longer autonomy is justified only for equipment whose service must be held throughout the outage.

How long do batteries last?

Their life depends mainly on room temperature and on the number of cycles they have been through. A sustained rise of a few degrees above the reference temperature shortens life appreciably. That is why the room is ventilated or cooled, and why battery health is monitored rather than assumed.

UPS or generator set?

They answer different needs and complement each other. The UPS covers the instant, without a break, and conditions the voltage. The generator covers duration, with a starting delay. A critical site needs both; a site whose equipment tolerates a short break may need only the generator.

Can batteries be added to an existing UPS?

Often, within the limits allowed by the machine and by the room. Adding them means checking the UPS charging capacity, the floor loading, ventilation and protection of the new bank. Mixing new and old batteries on one string degrades the whole, and is handled by full replacement rather than by topping up.

How do you know the installation will hold?

By an on-load discharge test, run periodically and recorded. It is the only check that measures autonomy rather than declaring it. It is run within an agreed window, with a rollback procedure, because a test is also the moment a weakness reveals itself.

What happens to spent batteries?

They follow a regulated disposal route and cannot be treated as ordinary waste. The route chosen, the carrier and the traceability of collection are part of the service, and are documented like the installation itself.

Evidence

Where we have applied it

Energy and critical infrastructure2024
Mining site — server room and backup power
Construction of a secure technical room and its power chain, on an isolated site with no reliable public grid.
Weeks of measurement before sizing
4
Load tests per year
12
Reference KP-2024-008
International organisations2022
European diplomatic mission — multi-system security
Complete overhaul of access control, video surveillance and backup power for a chancery in service.
Systems merged
3
Service interruptions
0
Reference KP-2022-014