UPS Runtime Calculator

Connected load:

Enter the live protected equipment load from a UPS display, PDU, wattmeter, or load study.

Load power factor:

Use 1.00 for unity PF equipment; use the measured PF when the UPS reports it.

UPS watt rating:

Enter the output watt rating from the UPS nameplate or datasheet.

UPS VA rating:

Enter the UPS VA rating, for example 1500 VA or 3000 VA.

Battery capacity source:

Choose how the calculator should derive nominal battery energy.

Battery energy:

Use datasheet watt-hours when available.

Battery bank voltage:

Nominal DC battery bank voltage.

V DC

Battery capacity:

Use the Ah rating for one string. Add parallel strings below.

Parallel strings:

Count only strings that are installed and connected for runtime.

strings

UPS efficiency:

Use the on-battery efficiency estimate for the UPS topology and load range.

Battery health:

Use 100% for new batteries; lower the value when the pack is aged or hot.

Reserved energy:

Reserve is subtracted from usable battery energy before calculating runtime.

Required runtime:

Set the minimum minutes needed for orderly shutdown or operational bridge time.

min

Check the UPS runtime inputs

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High-load derating:

Leave 0 unless you want an additional conservative runtime reduction.

Extra battery modules:

Use this for external battery packs or cabinets that add known watt-hours.

modules

Energy per module:

Set 0 when no external battery modules are attached.

Wh each

Generator bridge:

Set the minimum bridge time needed before generator power is stable.

min

Display precision:

Adjust tables and chart CSV precision without changing calculations.

Metric	Value	Readout	Copy
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Check	Value	Status	Action	Copy
{{ row.check }}	{{ row.value }}	{{ row.status }}	{{ row.action }}

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UPS runtime is the amount of time a battery backup can keep a connected load running after utility power fails. The useful planning question is not only how large the UPS is, but how much stored battery energy remains after conversion losses, aging, reserve, and the actual protected load are considered.

Runtime estimates matter most when equipment needs enough time for an orderly shutdown, a generator transfer, or a short bridge through brief outages. A small network closet, a lab rack, or a workstation with storage attached may only need several minutes. A larger rack can need longer because services, disks, and dependent devices have to stop in a controlled order.

Watts and volt-amps are both part of UPS sizing. Watts describe real power consumed by the equipment. Volt-amps describe apparent power, which can be higher when the load has a power factor below 1.00. A runtime number can look acceptable while the UPS is still close to its VA limit, so both ratings deserve a separate check.

UPS runtime energy path showing nominal battery energy reduced by health, efficiency, reserve, and derating before load determines runtime.

A calculated runtime is a planning estimate, not a guarantee. Battery temperature, battery age, recharge state, discharge rate, low-battery cutoff, and the UPS model's own runtime curve can all change the actual minutes available during an outage. Use the result to size the plan, then confirm critical systems with the UPS datasheet and a controlled shutdown test when the risk justifies it.

Technical Details:

UPS runtime is an energy balance. The battery bank stores nominal watt-hours, the inverter and battery condition reduce the useful portion, reserve holds back a planned buffer, and the connected real-power load consumes the remaining energy. When load rises, runtime falls because the same usable watt-hours are being spent faster.

Power factor connects the two load ratings that appear on many UPS nameplates. Real power in watts is the load that drains the battery energy. Apparent power in VA is still a UPS output limit because the inverter and wiring must support the current. A low-power-factor load can fit the watt rating and still crowd the VA rating.

Battery runtime curves from manufacturers are usually based on model-specific tests. A watt-hour calculation is useful for planning and comparison, but it cannot know the exact low-voltage cutoff, discharge curve, thermal state, cell chemistry, or firmware estimate inside a particular UPS.

Formula Core:

The calculation first converts the load into real watts, builds nominal battery energy, then applies the runtime allowances before dividing by load.

\begin{array}{lcl} W_{load} & = & {VA}_{load} \times PF when load is entered as VA \\ {VA}_{load} & = & \frac{W_{load}}{PF} when load is entered as watts \\ E_{nominal} & = & (V_{bank} \times Ah \times N_{strings}) + (N_{modules} \times E_{module}) \\ E_{usable} & = & E_{nominal} \times η \times H \times (1 - R) \times (1 - D) \\ T_{minutes} & = & \frac{E_{usable}}{W_{load}} \times 60 \end{array}

For direct watt-hour entry, the nominal energy term starts from the entered battery watt-hours instead of voltage times amp-hours. Extra battery modules add nominal watt-hours before efficiency, battery health, reserve, and derating are applied.

UPS runtime formula symbols and units
Symbol	Meaning	Tool input or output
PF	Load power factor from 0.10 to 1.00.	Load power factor
E nominal	Battery energy before runtime allowances.	Internal battery energy plus Added module energy
η	UPS conversion efficiency as a decimal factor.	UPS efficiency
H	Battery health allowance as a decimal factor.	Battery health
R	Reserved energy held back before runtime.	Reserved energy
D	Optional high-load derating for discharge and uncertainty losses.	High-load derating

Readiness labels come from explicit boundaries. Runtime target margin is estimated runtime minus the required runtime. Rating utilization compares the calculated load with the entered UPS watt and VA limits.

UPS runtime status boundaries
Status cue	Boundary	Meaning
Target met	Runtime target margin is at least `5 min`.	The estimate clears the required shutdown target with room to spare.
Tight margin	Runtime target margin is from `0 min` up to less than `5 min`.	The estimate reaches the target, but small input changes could erase the margin.
Runtime short	Runtime target margin is below `0 min`.	Reduce load, add battery energy, or lower the required runtime.
High load	Watt or VA utilization is above `80%` and no more than `100%`.	Runtime may still calculate, but the UPS rating margin is narrow.
Over rating	Watt or VA utilization is above `100%`.	The load exceeds the entered UPS output rating, so runtime should not be trusted.

Everyday Use & Decision Guide:

Start with measured protected load whenever possible. A UPS display, managed PDU, wattmeter, or short load study is usually better than adding nameplate maximums. Enter the load as watts when you have real power. Use VA only when the source gives apparent power, then set Load power factor from the UPS or equipment report.

Use Voltage x Ah bank when the battery bank is described by DC voltage and amp-hour capacity. Use Watt-hours entered when the datasheet already lists battery energy. Do not enter AC line voltage as the battery bank voltage; the field expects the nominal DC bank voltage such as 24 V, 48 V, 72 V, or 192 V.

A practical first pass is to leave High-load derating at 0%, use a measured UPS efficiency for the load range if available, keep Battery health below 100% for aged packs, and set Reserved energy high enough for low-battery cutoff and shutdown uncertainty. The default reserve style is conservative enough for planning, but critical systems need a field test.

Runtime Ledger shows real load, VA load, rating utilization, battery energy, reserved energy, usable runtime energy, estimated runtime, and required runtime.
Shutdown Readiness turns the runtime estimate into target, generator bridge, rating, health, reserve, and added-capacity checks.
Runtime Load Curve shows how minutes change as load percent rises against the current UPS watt rating basis.
JSON keeps the normalized inputs, calculations, readiness values, and runtime curve rows together for review.

Stop and verify when Runtime short, Tight margin, High load, or Over rating appears. A long runtime estimate does not mean the UPS is correctly sized if the watt or VA rating check is over the limit. It also does not prove the batteries are healthy enough for a real outage.

Use the result to decide whether the current UPS can cover a shutdown script, whether an external battery module changes the plan enough, or whether load should move to another UPS. Before relying on the number for production equipment, compare Estimated runtime with the manufacturer's runtime chart at the same load and confirm that the shutdown process finishes before the low-battery cutoff.

Step-by-Step Guide:

Work from the connected load to battery energy, then read the runtime and readiness checks together.

Enter Connected load and choose W or VA. The Connected real load row should show watts, and its readout should show the matching apparent load at the selected power factor.
Set Load power factor. If the load was entered in watts, UPS VA utilization changes because apparent load is calculated from watts divided by power factor.
Enter UPS watt rating and UPS VA rating. In Shutdown Readiness, Real-power rating and Apparent-power rating should move from Not checked to a rating status.
Choose Battery capacity source. For Voltage x Ah bank, fill Battery bank voltage, Battery capacity, and Parallel strings; for Watt-hours entered, fill Battery energy.
Set UPS efficiency, Battery health, Reserved energy, and Required runtime. The summary should show UPS runtime estimate, a primary runtime value, and target or rating badges.
Open Advanced when you need High-load derating, Extra battery modules, Energy per module, Generator bridge, or Display precision. Added modules should appear in Added module energy.
If Check UPS runtime inputs appears, fix the listed field before using the result. Common recovery paths are entering a positive connected load, keeping power factor between 0.10 and 1.00, and using positive battery voltage and Ah values in bank mode.
Read Runtime Ledger first, then Shutdown Readiness. Open Runtime Load Curve after the inputs are valid to see how runtime changes at lower and higher load percentages.

Interpreting Results:

Estimated runtime is the headline, but Usable runtime energy explains why it changed. Efficiency, battery health, reserve, external modules, and derating all move usable watt-hours before the load is applied.

Required runtime should be read as a planning gate. Target met means the estimate is at least five minutes above the target. Tight margin means the estimate reaches the target with less than five minutes to spare. Runtime short means the entered plan does not reach the target.

How to act on UPS runtime output fields
Result field	Trust cue	Follow-up
Estimated runtime	Useful when the load, battery energy, health, and reserve assumptions are realistic.	Compare with a vendor runtime chart before relying on it for critical equipment.
UPS watt utilization	Within rating stays at or below 80%; High load is above 80%; Over rating is above 100%.	Reduce real-power load or use a higher-watt UPS when the rating is crowded.
UPS VA utilization	Shows whether apparent load fits the entered VA rating.	Use measured power factor when VA margin is tight.
Battery energy for target	Add capacity appears when nominal battery energy is not enough for the target under the current assumptions.	Use the additional Wh value as a planning clue, then check compatible battery modules.
Generator bridge	Shows whether estimated runtime covers the entered generator start and stabilization time.	Leave it unset when generator transfer is not part of the plan.

Do not treat a clean result as a battery test. The calculation cannot detect a weak cell, a UPS with stale runtime calibration, a load that changes during shutdown, or a battery pack that has not fully recharged after a recent outage.

Worked Examples:

Network closet with a comfortable shutdown margin

A 320 W protected load at 0.90 power factor on a 900 W / 1500 VA UPS uses a 48 V, 9 Ah battery string, 88% efficiency, 90% battery health, and 15% reserve. Connected real load is 320.0 W, UPS watt utilization is 35.6%, and UPS VA utilization is 23.7%. Usable runtime energy is 290.8 Wh, so Estimated runtime is 54.5 min. With Required runtime set to 10.0 min, the readiness check shows Target met and 44.5 min above target.

VA-heavy load with a short target

An 850 VA load at 0.65 power factor becomes 552.5 W real load. With a 600 W / 1000 VA UPS, 480 Wh entered battery energy, 85% efficiency, 80% battery health, 10% reserve, 15% high-load derating, and a 30 min target, Estimated runtime is 27.1 min. UPS watt utilization is 92.1% and UPS VA utilization is 85.0%, so both rating checks read High load. Graceful shutdown target reads Runtime short, and Battery energy for target says to add about 51.0 Wh nominal battery energy or reduce load.

External battery module that changes the plan

A 500 W load at 0.95 power factor on the same 48 V, 9 Ah internal battery has 290.8 Wh of usable runtime energy and an Estimated runtime of 34.9 min. With Required runtime set to 45.0 min, that is 10.1 min short and Battery energy for target calls for about 125.0 Wh more nominal energy. Adding one external module at 432 Wh changes Added module energy to 432.0 Wh, raises Estimated runtime to 69.8 min, and changes the target check to 24.8 min above target.

Input that blocks the result

If Battery capacity source is Voltage x Ah bank and Battery bank voltage is left at zero, the summary changes to Check UPS runtime inputs with Input needed. The listed error says Battery bank voltage must be greater than zero. Entering the DC bank voltage lets Runtime Ledger, Shutdown Readiness, and the curve return.

FAQ:

Why does 1500 VA not always mean 1500 W?

VA is apparent power, while watts are real power. The calculator uses Load power factor to convert between them, so a 1500 VA load at 0.90 PF is treated as 1350 W, while a 900 W load at 0.90 PF is checked as 1000 VA.

Which battery capacity source should I choose?

Choose Watt-hours entered when a datasheet gives battery Wh directly. Choose Voltage x Ah bank when you know the DC bank voltage, amp-hour rating, and number of parallel strings.

What does High load mean?

High load appears when watt or VA utilization is above 80% but not above 100%. Over rating appears above 100%, and the readiness action tells you to reduce load or select a higher-rated UPS.

Why does reserve reduce the runtime?

Reserved energy is subtracted after efficiency and battery health. It represents energy held back for shutdown buffers, low-battery cutoffs, and estimate uncertainty before Estimated runtime is calculated.

How do I fix an Input needed result?

Read the error list under Check UPS runtime inputs. The result is blocked until required values are positive and power factor, efficiency, battery health, reserve, and derating are inside their allowed ranges.

Does the result prove the UPS will last that long?

No. The result is an estimate from the entered load, battery energy, efficiency, health, reserve, and derating. Battery condition, temperature, recharge state, actual discharge curve, and shutdown behavior still need a datasheet check or controlled test for critical systems.

Glossary:

UPS: Uninterruptible power supply, a battery-backed power device used to bridge outages and voltage events.
VA: Volt-amps, the apparent-power load that must fit the UPS VA rating.
Power factor: The ratio between real power in watts and apparent power in VA.
Watt-hour: A unit of stored or delivered energy equal to one watt for one hour.
Nominal battery energy: Battery watt-hours before efficiency, health, reserve, and derating are applied.
Usable runtime energy: Battery watt-hours left for runtime after all entered allowances are applied.
Derating: A conservative reduction used for high discharge current, Peukert effect, temperature, or unknown runtime-curve losses.
Graceful shutdown: An orderly stop of protected systems before battery power is exhausted.

References:

The difference between VA and Watts, Eaton.
Why is the estimated runtime displayed on my Smart-UPS LCD not accurate?, Schneider Electric, updated 2026-04-22.
How to find APC UPS runtime Graph?, Schneider Electric, updated 2026-04-26.
What is the expected life of my APC UPS VRLA battery?, Schneider Electric, updated 2026-02-17.