Battery Backup Calculator — Size Your UPS and Battery Bank Correctly

A UPS (uninterruptible power supply) provides battery backup power to critical electrical loads when the mains supply fails. UPS sizing means selecting a UPS with the correct power rating (kVA or kW) and battery capacity (Ah) to support the connected loads for the required runtime during a power outage.

Getting UPS sizing wrong has direct consequences. An undersized UPS will overload and shut down, defeating its purpose. An oversized UPS wastes capital expenditure, occupies unnecessary floor space, and operates at poor efficiency. Incorrect battery sizing leads to runtime shorter than expected — a server room that was supposed to have 30 minutes of backup actually runs out of battery in 15 minutes.

Electricians encounter UPS installations in server rooms, data centres, hospital critical care areas, retail point-of-sale systems, security installations, emergency lighting central battery systems, and increasingly in domestic settings for home offices and network equipment. The maximum demand calculator helps determine the total load that the UPS needs to support, while the cable sizing calculator ensures the cables feeding the UPS are correctly rated.

The first step in UPS sizing is accurately assessing the load. You need two values: the load in watts (W) and the load in volt-amperes (VA). These are different because most electrical loads have a power factor less than 1.0.

Watts (W) is the real power consumed by the load — the power that does useful work and generates heat. Volt-amperes (VA) is the apparent power — the product of voltage and current. The relationship is: W = VA x Power Factor.

Computer servers typically have a power factor of 0.9-0.99 (with active PFC power supplies). Older computer equipment may have a power factor of 0.6-0.7. UPS units are rated in VA (or kVA) and in watts (or kW). You must ensure that neither the VA rating nor the W rating of the UPS is exceeded.

For example, a 3 kVA UPS with a 0.9 power factor can deliver 2,700 W. If your load is 2,500 W at 0.95 power factor, the VA load is 2,500 / 0.95 = 2,632 VA. This is within the 3 kVA rating and within the 2,700 W rating, so the UPS is adequate. The power factor calculator helps you convert between watts and VA if you only have one value.

Runtime is the length of time the UPS can sustain the connected load on battery power alone, with no mains supply. It depends on the battery capacity, the load current, and the UPS inverter efficiency.

This formula gives a theoretical runtime. In practice, battery capacity decreases at high discharge rates (Peukert's effect), in cold temperatures, and as batteries age. A new battery rated at 100 Ah may deliver only 80 Ah at a high discharge rate, and only 70 Ah after three years of service.

For critical applications, designers typically apply an ageing factor of 0.8 (assuming the battery has 80% of its rated capacity at end of design life) and a temperature derating factor if the ambient temperature exceeds 20 degrees Celsius. Elec-Mate applies these factors automatically, giving you a realistic runtime estimate rather than an optimistic theoretical value.

Common runtime requirements include 5-10 minutes for a graceful server shutdown, 30 minutes for extended operation during short outages, and 2-8 hours for critical healthcare and security systems. The load schedule calculator helps you list and total all the loads that need battery backup.

Battery capacity is measured in amp-hours (Ah) at a specific discharge rate. A battery rated at 100 Ah at the C10 rate means it can deliver 10 amps for 10 hours. However, the same battery may only deliver 65-75 Ah at the C1 rate (65-75 amps for 1 hour) due to Peukert's effect — the faster you discharge a lead-acid battery, the less total energy you get.

The formula to calculate the required battery capacity is:

For example, a 1,000 W load requiring 30 minutes (0.5 hours) of runtime from a 48 V battery bank: Ah = (1,000 x 0.5) / (48 x 0.9 x 0.8) = 500 / 34.56 = 14.5 Ah. You would select a battery of at least 17-18 Ah to provide margin.

Most UPS systems use sealed lead-acid (SLA/VRLA) batteries with a design life of 3-5 years (standard) or 10 years (long-life). Lithium-ion batteries are increasingly used in modern UPS systems, offering longer life, lower weight, and better performance at high discharge rates. The choice affects both the initial cost and the ongoing replacement schedule. Understanding three-phase power calculations is essential when sizing UPS systems for larger commercial installations.

Once you know the load (in both W and VA) and the required runtime, you can select the UPS. The UPS rating must satisfy three requirements:

• VA rating — the UPS VA rating must be at least 125% of the total load VA to allow for inrush currents and future growth. A 2,000 VA load requires a 2,500 VA (2.5 kVA) UPS minimum.
• Watt rating — the UPS watt rating must exceed the total load watts. A 3 kVA UPS with 0.9 power factor delivers 2,700 W — do not connect a 2,800 W load to it.
• Battery runtime — the UPS must either have internal batteries sufficient for the required runtime, or support external battery cabinets to extend runtime.

Standard UPS sizes for single-phase installations are 600 VA, 1 kVA, 1.5 kVA, 2 kVA, 3 kVA, 5 kVA, 6 kVA, and 10 kVA. For three-phase installations, sizes range from 10 kVA to 800 kVA or more. The UPS input circuit must be protected by an appropriately rated MCB and the cables sized for the input current, which includes the battery charging current in addition to the load current.

BS 7671:2018+A4:2026 Chapter 57 governs stationary secondary battery installations when used as a source of supply for electrical installations. Knowing where the Chapter 57 boundary falls is essential before specifying or certifying any battery backup system.

Fixed external battery cabinets and stationary battery banks that supply a building are squarely within Chapter 57 scope. Four requirements apply to every in-scope installation:

• BS EN IEC 62485 conformance (Reg 570.6.1.1.1) — stationary secondary battery installations shall conform to the relevant parts of the BS EN IEC 62485 series covering safety requirements for stationary batteries.
• Adequate ventilation (Reg 570.6.3 / 570.6.7.202) — the location or enclosure shall be adequately ventilated in accordance with the manufacturer's instructions and safety data sheets. Ventilation shall not create a hazard — gases may need to be discharged to an outdoor space.
• Warning notices (Reg 570.6.8.201) — a notice indicating the presence and location of the battery system shall be fixed at: (a) the origin of each electrical installation, (b) each metering position that is remote from the origin, and (c) each consumer unit or distribution board supplied from the battery.
• Isolation provision (Reg 462.1) — each electrical installation shall have means for isolation from every supply, including the on-site battery source. The battery must be isolatable independently of the mains supply.

For domestic installations, Reg 570.6.7.203 additionally requires that stationary secondary batteries in dwellings are installed in a suitable location in accordance with the manufacturer's instructions and PAS 63100. For non-dwelling premises, the fire strategy for the building governs siting and fire protection requirements.

The following errors appear repeatedly in practical UPS and battery bank installations. Awareness of these before starting work reduces rework and certification failures.

• Failure to segregate battery DC cables from AC mains cables — DC battery and AC output conductors shall be routed in separate containment. Running them in the same tray or conduit creates both EMC problems and an increased fire risk. This is the most frequently recorded common mistake across UPS installation records.
• Insufficient clearance around batteries — battery cabinets and battery rooms require minimum clearances for ventilation air movement and safe maintenance access. Blocking clearances by routing cables too close, or positioning adjacent equipment without reference to the manufacturer's layout drawings, restricts air flow and impedes safe cell replacement.
• Undersizing input cables by ignoring charging current — the UPS input current is higher when the battery is in a discharged state and recharging simultaneously with supplying the load. Input cables must be sized for the maximum input current with a discharged battery, not just the steady-state load current. Manufacturer datasheets publish separate figures for charged and discharged battery input current; use the higher value.
• Under-torqued terminals — bolted busbar connections and cable lugs on battery terminals and UPS busbars must be torqued to the manufacturer's specified values. Under-torqued connections cause resistance heating, voltage drop under load, and eventual failure. Always use a calibrated torque wrench and record values on the commissioning sheet.
• Routing mains and bypass conductors in the same containment — UPS bypass and input mains conductors shall be routed in segregated containment to prevent cross-coupling, maintain circuit integrity during fault conditions, and comply with wiring segregation requirements.
• Failing to commission with a discharge test — the installation is not complete until the UPS is tested under rated load and the battery is confirmed to deliver the specified runtime. A discharge test validates the Ah capacity, confirms battery connections are correct, and verifies the inverter output voltage under load.