Uninterruptible power supplies protect critical loads from mains failure, voltage sags, surges, and frequency variations. From small desktop units to large three-phase systems, this guide covers UPS types, sizing, installation requirements, earthing considerations, maintenance, and how to test and commission a UPS installation to BS 7671.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
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An uninterruptible power supply (UPS) is a device that provides emergency power to connected equipment when the mains supply fails. Unlike a standby generator, which takes 10-30 seconds to start and reach stable output, a UPS switches to battery power instantaneously (or near-instantaneously), ensuring that sensitive equipment experiences no interruption.
UPS systems are used wherever a power interruption would cause data loss, equipment damage, safety risks, or financial loss. Common applications include server rooms, data centres, medical equipment, industrial control systems, CCTV and security systems, point-of-sale terminals, and telecommunications equipment.
A UPS typically contains three main components: a rectifier/charger (which converts mains AC to DC and charges the batteries), a battery bank (which stores energy for use during a mains failure), and an inverter (which converts the DC battery power back to AC to supply the load). The arrangement and interaction of these components determines the UPS type and its performance characteristics.
A UPS and a generator serve different purposes. A UPS provides immediate, short-duration power (minutes to hours) to bridge the gap during a mains failure. A generator provides long-duration power (hours to days) but takes time to start. In critical installations, both are used together — the UPS provides immediate protection while the generator starts, then the generator takes over for extended outages.
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The three main UPS types differ in how they handle the mains supply during normal operation and how quickly they transfer to battery during a mains failure.
The simplest and most affordable type. During normal operation, the load is supplied directly from the mains — the UPS only activates when a mains failure is detected. Transfer time is typically 5-12 milliseconds. Suitable for desktop PCs, home networking equipment, and non-critical loads. Not suitable for sensitive equipment that cannot tolerate any transfer time or voltage/frequency variations.
Adds an autotransformer (voltage regulator) to the offline design. This allows the UPS to correct voltage sags and swells without switching to battery, extending battery life. Transfer time is typically 2-4 milliseconds. Suitable for small server rooms, network equipment, and point-of-sale systems. The most popular type for small to medium business applications.
The load is always supplied by the inverter, regardless of mains status. The mains supply charges the batteries and powers the rectifier, but the load is completely isolated from the mains by the double conversion process (AC to DC, then DC back to AC). Transfer time is zero — there is no transfer because the inverter is always running. Provides complete protection against all mains disturbances. Essential for data centres, medical equipment, and any critical load that cannot tolerate any power quality deviation.
Correct UPS sizing is critical. An undersized UPS will overload during normal operation or fail to provide the required runtime on battery. An oversized UPS wastes money and may operate inefficiently at light loads.
Add up the VA (volt-ampere) rating or wattage of every piece of equipment that will be connected to the UPS. Use the equipment's rated power consumption, not the fuse rating or circuit breaker size. For IT equipment, the nameplate rating is often significantly higher than the actual consumption — measuring with a power meter gives a more accurate figure.
UPS systems are rated in VA (apparent power) and kW (real power). The ratio between them is the power factor. Most modern IT loads have a power factor of 0.9-1.0, while older equipment may be 0.6-0.8. The UPS must be sized to handle both the VA and kW requirements of the load. A 10kVA UPS with a 0.9 power factor can supply a maximum of 9kW of real power.
How long must the UPS support the load during a mains failure? For UPS systems with a generator backup, 5-10 minutes may be sufficient (enough for the generator to start and stabilise). For standalone UPS systems, 30-60 minutes or more may be needed. Longer runtime requires larger (or additional) battery banks.
Always add 20-30% headroom above the calculated load to allow for future expansion, inrush currents during equipment startup, and battery degradation over time. A UPS running at 70-80% of its rated capacity is in the optimal efficiency range — both higher and lower loading reduce efficiency.
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UPS installation involves more than simply plugging in a box. Larger systems (above 3kVA) require dedicated circuits, appropriate cable sizing, mechanical ventilation, and compliance with manufacturer specifications and BS 7671.
Earthing a UPS system requires careful consideration because the UPS may or may not create a separately derived source. The earthing arrangement affects RCD operation, fault loop impedance, and the overall safety of the installation.
Most single-phase UPS systems pass the neutral straight through from input to output without any transformer isolation. In this configuration, the UPS output is not a separately derived source — the earth reference comes from the incoming supply. The earth conductor from the load connects back to the distribution board or consumer unit earth bar as normal. This is the simplest arrangement and is common for small to medium UPS systems.
Some larger UPS systems (particularly three-phase) include an isolation transformer on the output. This creates a separately derived source — the output neutral is not connected to the input neutral. In this case, a new earth reference must be established at the UPS output transformer, with a neutral-earth bond and a local earth electrode if required. This arrangement must be clearly documented on the EIC and the earthing arrangement verified during commissioning.
In all cases, the UPS chassis must be bonded to the installation earth. Any external battery cabinets must also be bonded. The earth continuity through the UPS must be verified during testing — an open earth on the UPS output would leave connected equipment without a safety earth, creating a serious danger.
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Start free trialA UPS will eventually need maintenance — battery replacement, firmware updates, or component repair. Without a bypass switch, maintaining the UPS means switching off the load, which defeats the purpose of having a UPS in the first place.
The batteries are the most critical — and most failure-prone — component of any UPS system. A UPS with degraded batteries provides a false sense of security, as it will fail to support the load when a mains failure actually occurs.
Most UPS systems use valve-regulated lead-acid (VRLA) batteries — also called sealed lead-acid (SLA) or maintenance-free batteries. Standard VRLA batteries have a design life of 3-5 years; long-life VRLA batteries have a design life of 8-10 years. Larger data centre UPS systems may use nickel-cadmium (NiCd) batteries (longer life but higher cost) or lithium-ion batteries (smaller, lighter, longer life, but significantly more expensive and requiring battery management systems).
Battery life is halved for every 10 degrees C above the recommended operating temperature of 20-25 degrees C. A battery rated for 5 years at 20 degrees C will last only 2.5 years at 30 degrees C and potentially less than 2 years at 35 degrees C. This is why UPS room ventilation and cooling are critical — the room temperature directly affects the battery replacement cycle and operating costs.
Regular battery testing is essential. Most UPS systems have a built-in battery test that can be run automatically or manually. This test discharges the batteries under load and measures the voltage drop to assess capacity. However, a battery test only confirms whether the batteries can support the load right now — it does not predict when they will fail. Battery impedance testing (using a handheld battery analyser) is more predictive and should be carried out annually. Plan to replace batteries at 80% of their design life — do not wait for them to fail.
Testing a UPS installation includes both the standard electrical tests required by BS 7671 and UPS-specific functional tests.
Elec-Mate's digital EIC form handles UPS installation certification including supply characteristics, circuit details, and all test results.
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