UPS Runtime Estimator.
Critical load in. UPS VA, battery string count and the Peukert-corrected runtime curve out. The model accounts for power factor, ambient, chemistry and redundancy — the four variables that decide whether the bank holds for the promised minutes.
- Chemistries
- VRLA · LFP
- Topology
- Online VFI
- Redundancy
- N · N+1 · 2N
- Headroom
- 25%
· Load vs runtime · Peukert-corrected
Target 15 min @ 100%
UPS sizing · VRLA (sealed lead-acid)
Modular cabinet — 60 kVA modular (Vertiv APM / Schneider Galaxy VM)
With 1 battery string of 48 kWh each, runtime at full load lands at 136 minutes — ambient-corrected, Peukert-corrected.
Recommended VA
30 kVA
Online double-conversion · N+1 (one spare module)
Battery strings
1
48 kWh per string
Runtime @ 100%
136 min
vs 15 min target
Effective battery life
4.5 yr
at 28 °C ambient
- Runtime @ 75% load194 min
- Runtime @ 50% load323 min
- UPS capex (indicative)₹ 16.1 L
- Battery capex (indicative)₹ 6.7 L
- Total capex₹ 22.8 L
Assumptions driving this recommendation↓ expand
- Topology
- Online double-conversion (VFI)
- VA headroom
- 25%
- Discharge model
- Peukert with chemistry exponent
- Max DoD
- 80%
- Cell voltage
- 2 V
- Cells / string
- 240
- Module Ah
- 100 Ah
- Hot-zone ambient
- 30 °C
- Arrhenius life rule
- Halves per 8 °C above hot-zone
- Redundancy
- N+1 (one spare module)
Engineering caveats
- VRLA is the default for sub-5-minute critical-load UPS — cheap, well-understood, replaced every 4–5 years.
- Common enterprise tier — one extra UPS module so the bank survives one module fault without dropping load.
Operationally sensible ecosystem
Brands grouped by engineering role — not random logos.
UPS / power conditioning
Single + 3-phase + transfer
- APC Symmetra LXModular 3-phase mid
- Vertiv Liebert APMModular 3-phase mid
BESS battery
LFP packs + BMS + enclosure
- Delta LFP CabinetCommercial cabinet
- Fuji Electric LFPCommercial mid-scale
Server racks
Equipment racks + PDU + cooling
- Vertiv VRMid-enterprise rack
- Rittal TS ITEuropean enterprise enclosure
Indicative — production sizing uses the manufacturer's published runtime tables, which take cell ageing, charging-current limits and string-balancing into account. Refresh batteries on the conservative half of the calculated life when ambient regularly exceeds the hot-zone.
Translate into a briefPeukert's law isn't linear
A VRLA string rated 100 Ah at the 10-hour rate delivers only 60–70 Ah at a 1-hour rate. The model corrects for this; specifying a string at its nameplate rate is the most common UPS sizing mistake.
Heat is the silent killer
Battery life halves for every 8 °C above the chemistry hot-zone (30 °C for VRLA, 40 °C for LFP). A 35 °C summer battery room turns a 5-year VRLA bank into a 3-year bank.
LFP is not always right
For runtimes under 5 minutes, LFP is over-engineered — VRLA gives the same outcome at ~40% capex. The chemistry crossover is around 15 min of runtime.
N+1 over N for ≥ 100 kW
Above 100 kW critical load, a single module fault drops too much load to be acceptable. N+1 modular adds one extra UPS module to the bus — survives one fault without dropping load.
· Engineering advisory · UPS Runtime
What this answer means for the deployment.
The runtime number is the start of the engineering conversation, not the end. The four notes below frame what the answer predicts about the building beyond the calculated minutes.
Deployment observations
- A 15-minute target runtime is a different deployment than a 30-minute target — the first is a graceful-shutdown window for IT loads, the second is a generator-start ride-through for mission-critical scope. Specify against the operational requirement, not the catalogue convenience.
- VRLA vs LFP is decided by the eight-year total-cost picture, not the day-one capex. Above 20 kVA new-install and above 15-minute runtime, LFP is the disciplined choice; below 10 kVA single-rack scope, VRLA remains defensible on a 5-year refresh budget.
- The Peukert-corrected runtime curve is the answer the operator actually inherits — the nameplate runtime is the answer the catalogue advertises. The two diverge most at high C-rate / short-runtime points, which is where the design risk concentrates.
Redundancy posture
- N+1 protects against a single module fault; 2N protects against a single distribution-path fault. The two answer different failure modes and the cost premium is not the same — N+1 sits at ~1.34× of N, 2N sits at 2.00×+.
- Battery redundancy is independent of UPS-module redundancy. A 2N UPS with a single battery bank shares a failure domain with the battery; 2N at the UPS layer requires twin battery strings on twin paths for the redundancy claim to be honest.
Environmental considerations
- Ambient temperature is the single biggest derating factor for VRLA — every 10 °C above the rated 25 °C halves the battery's calendar life. An electrical room running at 35 °C in summer needs the runtime sized against that ambient, not against a 25 °C nameplate.
- Humidity and particulate load drive the UPS-room maintenance interval — high-RH or dusty environments need quarterly filter and connection inspections; standard offices can hold to half-yearly.
- LFP indoor installations are NFPA 855-compliant without bespoke fire suppression for LFP chemistry; NMC chemistry requires dedicated suppression for indoor banks. Chemistry choice carries a fire-and-life-safety implication, not only a cost one.
Commissioning discipline
- Battery internal-resistance and capacity test at commissioning — the documented baseline lets every subsequent AMC visit catch degradation against the original measurement, not against the catalogue spec.
- Generator-start coordination with the UPS is a commissioning-stage exercise — the UPS rides the load for the generator-start window (typically 10–30 seconds), then transfers to generator. The transfer behaviour is rehearsed under load, not assumed from documentation.
- Configuration baseline (UPS settings, BMS integration map, alarm thresholds) is exported offline at commissioning and after every configuration change — recovery from a UPS controller fault is a same-day exercise against the saved baseline.
Lifecycle implications
- VRLA banks need a planned full replacement every 4–5 years; LFP banks hold for 7–10 years. The AMC calendar carries the replacement window, not the failure event — reactive replacement is multiple times the cost of preventive.
- UPS module capacitors drift on a 10–12 year horizon — the annual ESR measurement on the AMC catches drift before it shows on the inverter's THD measurement.
Expansion readiness
- Sizing with a 25% headroom on the calculated VA gives the deployment foreseeable expansion against an IT-load growth curve — the headroom is the difference between a 5-year and an 8-year refresh window.
- A modular UPS topology lets the deployment add capacity without re-cabling — single-module additions live within the existing distribution path; rack-and-paralleling is a configuration exercise, not a re-installation.
· Why the curve matters
A 40 kW critical load with a 15-minute target runtime sized at 100% with VRLA usually needs four battery strings — but if real-world load only hits 70%, runtime doubles to over 30 minutes. The reverse is also true: a bank sized at 75% expected load with Peukert-corrected sizing collapses to under 8 minutes if load momentarily spikes to 100%. The curve makes that asymmetry visible at the design stage.
· Frequently asked
UPS Runtime —
what people ask first.
Why online double-conversion only?
Line-interactive and offline UPS topologies transfer load to the inverter only when mains fail — the transfer takes 4–10 ms, which is enough to glitch sensitive IT loads. Online double-conversion always runs the load through the inverter, so battery-supported runtime starts from a stable bus. For critical-load sizing the difference matters; for less critical loads, the sizing logic still applies but the topology can be relaxed.
Why is VRLA so penalised at high discharge rates?
Lead-acid chemistry's Peukert exponent sits around 1.25 — a 100 Ah string rated at the 10-hour rate delivers maybe 60–70 Ah at a 1-hour rate. LFP sits at ~1.05, which is why it stays close to the spec sheet even at high C-rates. Specifying a battery at its nameplate capacity ignoring Peukert is the most common UPS sizing mistake; this tool corrects for it explicitly.
When does LFP overtake VRLA on lifetime cost?
Around 15 minutes of runtime and a 5-year horizon. LFP capex is 2.5× VRLA, but VRLA needs one full replacement at year 4.5 and the LFP bank runs ~10 years. Add the labour of a battery swap (which is non-trivial in an occupied building) and LFP wins on lifetime cost above 15 min runtime in most cases. Below 5 min runtime, VRLA wins comfortably.
What does N+1 actually mean?
N is the minimum number of UPS modules required to support the critical load. N+1 adds one extra module so the bus survives a single module fault without dropping load. 2N is two completely independent UPS rooms with twin distribution paths — used in mission-critical contexts like ICUs, command centres and Tier-III data centres. The cost premium of N+1 over N is ~34%; the premium of 2N over N is 100%+.
How accurate are the numbers?
Accurate enough for brief-stage budgeting, not for final procurement. Production sizing always uses the manufacturer's runtime curves, which account for cell ageing, charging-current limits, string balancing and BMS firmware. The tool's job is to get the budget conversation and the rough specification right; the BOM sizing is a different exercise.
· Begin
Take the brief to a
UPS workshop.
A site walkthrough lets us load-profile the critical bus (peaks, ramps, harmonics) and validate the chemistry choice against your actual ambient — not just summer-mean.