UPS Energy and Runtime Screening Calculator
Screen how long a UPS battery bank can carry your critical load before you ask for a battery selection. Enter the load, the UPS rating and efficiency, and the battery's voltage, Ah and strings with the share of its energy you can use and how much it will have aged. The answer is an energy balance — the battery maker's discharge table decides the real runtime.
What this tool estimates
How long a UPS battery bank could carry your load on an energy balance — nominal battery energy reduced by the usable share, inverter efficiency and ageing you enter — once the load is shown to fit the UPS's kW and kVA rating.
What it cannot decide
- The runtime the battery will actually give: that comes from the maker's constant-power discharge table, to the UPS's end-of-discharge voltage, at your battery temperature.
- How fast your battery will age, or when it should be replaced.
- Whether the UPS topology, bypass and changeover arrangement suit your loads.
- Battery-room ventilation, fire safety, earthing or the installation's compliance.
Result
| Step | Value |
|---|
| Scenario | Runtime | Why it matters |
|---|
An energy balance, not a discharge-curve guarantee. Lead-acid batteries give less energy at the high rates a UPS draws than their long-duration Ah rating suggests, cold reduces capacity, and the UPS stops at its own end-of-discharge voltage. Ask for the battery maker's constant-power table at your load and end voltage before relying on a runtime.
Method version 6 October 2026. A screening estimate, not a design: see the limits below.
How this calculator works — and where it stops
What it calculates
How long a UPS battery bank could carry a load on an energy balance — nominal battery energy reduced by the usable share, inverter efficiency and ageing — once the load is shown to fit the UPS's kW limit and, where both power factors are known, its kVA rating. Optionally, the battery energy a target runtime would need on the same fractions.
Inputs
- Load in W or kW — the real power on the UPS output — and, optionally, its power factor for the kVA check.
- UPS output rating in kVA or kW, and its rated output power factor (its kW ÷ kVA), which is needed when the rating is in kVA.
- Inverter efficiency on battery, as a fraction.
- Battery: nominal string voltage, rated Ah per string and the number of parallel strings (a whole number).
- Usable fraction — the share of nominal energy the UPS can draw before its end-of-discharge voltage.
- Ageing fraction — the capacity left at the point in battery life you are planning for (1 = new).
- Target runtime in minutes (optional).
Method
- Load check first: the kW limit is the kW rating, or the kVA rating × the rated output power factor. A load above it is refused. With the load's power factor, its kVA (kW ÷ PF) is checked against the kVA rating — or the kW rating ÷ the rated PF — and refused if above. An overloaded UPS goes to bypass or shuts down, so it gets no runtime.
- Nominal energy (Wh) = string voltage × Ah per string × parallel strings.
- Usable AC energy (Wh) = nominal energy × usable fraction × inverter efficiency × ageing fraction.
- Runtime (h) = usable AC energy ÷ load in W — an energy-balance estimate, labelled as one.
- Battery current per string at nominal voltage = load W ÷ efficiency ÷ (string voltage × strings), shown with current ÷ Ah (the C-rate) as a warning sign for rate effects.
- Target (optional): nominal energy needed = load W × target hours ÷ (usable × efficiency × ageing), and the Ah per string that means at the voltage and strings entered.
- Scenario comparison: the same balance with a new battery, the load 20% higher and 20% lower, the usable fraction 0.1 lower and the efficiency 0.05 lower. A scenario whose load no longer fits the UPS gets no runtime.
Assumptions
- The load is constant for the whole discharge.
- Parallel strings add their capacities; series blocks add their voltages.
- The fractions come from the battery and UPS makers' data. Lead-acid sizing practice (IEEE 485, as summarised in the IEEE IAS presentation below) treats a battery as at end of life at 80% of its rated capacity and sizes with a 1.25 ageing factor — an ageing fraction of 0.8 is that point.
- The efficiency is the inverter's on battery at about this load.
Limitations
- Not a discharge-curve result: lead-acid cells give less energy at UPS discharge rates than their long-duration Ah rating implies. The maker's constant-power table, to the actual end-of-discharge voltage, decides the runtime.
- No temperature correction: cold reduces the capacity available, and heat shortens battery life.
- Does not model chemistry differences, recharge time, or the UPS's own low-battery and end-of-discharge settings.
- Says nothing about UPS topology, bypass, redundancy, battery-room ventilation, fire safety or the installation's compliance.
Worked example
Produced by running this calculator with the inputs below.
Inputs
- Load 1,000 W at power factor 0.9
- UPS 2 kVA, rated output power factor 0.8
- Inverter efficiency 0.9
- Battery 48 V × 100 Ah, 1 string
- Usable fraction 0.8, ageing fraction 0.8
- Target runtime 180 minutes
Working
- UPS kW limit = 2 kVA × 0.8 = 1.6 kW. The 1 kW load is 62.5% of it, and its 1.11 kVA is 55.6% of the 2 kVA rating — it fits.
- Nominal energy = 48 × 100 × 1 = 4,800 Wh.
- Usable AC energy = 4,800 × 0.8 × 0.9 × 0.8 = 2,764.8 Wh.
- Runtime = 2,764.8 ÷ 1,000 = 2.7648 h — about 2 h 46 min on an energy balance.
- Battery current ≈ 1,000 ÷ 0.9 ÷ 48 = 23.1 A per string (0.23 C).
- For 180 minutes: 1,000 × 3 ÷ 0.576 = 5,208 Wh nominal, about 108.5 Ah per string at 48 V — the bank as entered is 14 minutes short.
Result. About 2 h 46 min (2.7648 h) on an energy balance — 14 minutes short of a 3-hour target, which would need about 108.5 Ah per string. The battery maker's constant-power table at the real discharge rate and end voltage decides the actual runtime.
Sensitivity — what moves the answer
- Ageing: the same bank new gives 3.456 h — 25% more than at the 0.8 ageing fraction — which is why runtime should be checked at end of life, not on the day the battery is installed.
- Load: 20% more load gives 2.304 h and 20% less gives 3.456 h. On an energy balance runtime is inversely proportional to load; a lead-acid battery falls further than that at higher rates.
- Usable fraction 0.1 lower (0.7) gives 2.4192 h; inverter efficiency 0.05 lower (0.85) gives 2.6112 h.
How engineers use the result
To screen whether a proposed battery bank is in the right range for the bridge the UPS must provide — usually the DG's start and transfer time plus a margin, or a controlled shutdown — before asking the battery maker for a selection at the real discharge rate.
When a professional design must replace it
Before ordering batteries or relying on a runtime: the battery maker's constant-power or constant-current discharge data for the actual load, end-of-discharge voltage and lowest expected temperature, with ageing and design margins, should replace this balance, and the UPS's kW and kVA ratings, topology and bypass should be confirmed against the measured load.
Sources
- IEEE 485-2020, IEEE Recommended Practice for Sizing Lead-Acid Batteries for Stationary Applications — the full sizing method this balance simplifies
- IEEE IAS Atlanta Chapter: Stationary Battery Sizing (L. Varga, P.E., 20 Apr 2015) — end of life at 80% of rated capacity and the 1.25 ageing factor; temperature correction; UPS batteries rated in W per cell for a time to an end voltage; parallel strings add capacity; battery kW = kVA × PF ÷ inverter efficiency
- IEC 62040-3:2021, Uninterruptible power systems (UPS) — Part 3: Method of specifying the performance and test requirements — how UPS performance is specified and tested; this tool does not classify a UPS
- CEA (Measures relating to Safety and Electric Supply) Regulations, 2023 (notified 8 Jun 2023) — the safety regulations the UPS installation, its earthing and protection come under; this tool assesses none of it
Questions people ask before using it
What information do I need before using the UPS runtime calculator?
The real power of the loads on the UPS output in watts or kW — measured, or from the equipment's input ratings — and their power factor if you want the kVA side checked; the UPS's output rating and rated output power factor (its kW ÷ kVA) from the datasheet; the inverter's efficiency on battery at about your load; the battery string's nominal voltage, its Ah rating and how many strings run in parallel; the share of the battery's energy the UPS can draw before its end-of-discharge voltage; and the capacity you expect the battery to keep late in its life. Lead-acid sizing practice treats a battery as at end of life when it is down to 80% of its rated capacity.
Which site conditions could change the runtime?
Battery temperature: cold reduces the capacity available, heat shortens battery life. The discharge rate: lead-acid cells give less energy at the high rates a UPS draws than their long-duration Ah rating implies — in the maker's table quoted in the IEEE IAS presentation listed in the method's sources, one lead-acid UPS cell gives about 0.22 kWh over 5 minutes against about 0.55 kWh over 20 minutes, both to 1.67 V per cell. Also battery age and maintenance, the UPS's end-of-discharge voltage setting, load growth, and loads with a poor power factor or high inrush that use up the UPS's kVA.
What should I send an engineer for a project-specific review?
The single-line diagram showing what the UPS feeds and where the DG changeover sits; the list of loads on the UPS with measured kW and power factor; the UPS datasheet and the battery datasheet with its constant-power or constant-current discharge table; the battery's age and any recent capacity or impedance test results; the battery room's temperature record; and the runtime you need — usually the DG's start and transfer time plus a margin, or the time a process needs to shut down safely.
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