UPS Runtime Calculator

Shutdown {{ resultsReady ? targetStatus.label : '—' }} W load {{ resultsReady ? formatPercent(computation.values.analysis.watt_load_percent) : '—' }} Usable {{ resultsReady ? formatWh(computation.values.analysis.usable_runtime_wh) : '—' }}

Connected load:

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

Load power factor:

Use a measured value when possible; valid range is 0.10 to 1.00.

UPS watt rating:

Used only for output-capacity headroom; it does not add battery energy.

UPS VA rating:

Enter zero only when the nameplate VA rating is unavailable.

Battery capacity source:

Choose the source that matches the battery or UPS datasheet.

Battery energy:

Use the datasheet watt-hour value when it is available.

UPS efficiency:

A conservative estimate is safer than a peak-efficiency figure.

Battery health:

Use 100% only for a new, fully charged battery under suitable conditions.

Reserved energy:

The sample holds 15% of conditioned energy outside the estimate.

Required runtime:

Use 0 when no explicit shutdown target is being assessed.

min

Additional runtime derating:

Neutral default: 0%. Use a manufacturer curve instead when one is available.

Extra battery modules:

Neutral default: 0 connected modules.

Energy per module:

Neutral default: 0 Wh; enter a positive value when module count is above zero.

Wh each

Generator bridge:

Neutral default: 0 minutes, which omits this readiness check.

min

Display precision:

Neutral sample default: 1 decimal place.

Runtime plan

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Energy path:

This is a planning energy balance, not a manufacturer discharge curve.

runtime min = usable Wh ÷ real load W × 60

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The chart renderer is unavailable. The same runtime values remain available in the plan and ledger.

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An uninterruptible power supply (UPS) has to pass two separate tests. Its inverter must support the connected load without exceeding either the watt or volt-amp rating, and its battery must store enough usable energy to keep that load running until power returns, a generator starts, or systems shut down safely.

Watts measure real power consumed by the equipment. Volt-amperes (VA) measure apparent power carried by the UPS. Power factor links them: watts equal VA multiplied by power factor. A low-power-factor load can fit below the watt rating and still approach the VA limit, so both nameplate ratings matter.

Battery capacity is commonly stated as watt-hours (Wh) or as bank voltage and amp-hours (Ah). Multiplying volts by amp-hours gives nominal watt-hours for one string; parallel strings add capacity at the same voltage. Nominal energy is not the same as usable runtime energy. Conversion loss, battery age, reserve, temperature, discharge rate, and low-battery cutoff all reduce what reaches the load.

UPS quantities and their planning roles
Quantity	Planning role
Watt rating	Checks the UPS real-power output limit.
VA rating	Checks apparent-power capacity after power-factor conversion.
Battery Wh	Provides the nominal energy available before loss and reserve factors.
Required runtime	Defines the minimum shutdown or bridge window to compare with the estimate.

Runtime falls as load rises, often more sharply than a simple energy division suggests because real batteries have manufacturer-specific discharge curves. An energy-balance estimate is useful for early sizing and sensitivity checks, but a UPS vendor's runtime chart or a controlled load test is better evidence for a final purchase or shutdown plan.

Leave operating margin for workload peaks, battery aging, and shutdown time. A result that barely clears the target under ideal inputs is not a resilient design, especially for servers, storage, networking, medical equipment, motors, or other loads with consequential interruption or high inrush current.

How to Use This Tool:

Use measured load and battery data when available, then make conservative allowances for losses and age.

Enter Connected load in watts or VA. Add the measured Load power factor, then enter the UPS watt and VA ratings so both output limits can be checked.
Choose Battery capacity source. Enter datasheet watt-hours directly, or use DC bank voltage, amp-hours per string, and the number of installed parallel strings.
Set UPS efficiency, Battery health, and Reserved energy. Enter the minimum Required runtime for shutdown or bridging; zero leaves the target comparison unset.
Review Runtime plan for estimated minutes and usable Wh. Check Readiness ledger for watt, VA, battery, reserve, target, and optional generator-bridge status. When the target is short, the ledger reports the additional nominal Wh required under the same assumptions.

Interpreting Results:

Estimated battery runtime is the time supported by modeled usable energy at the entered real-power load. The shutdown status is Comfortable at five minutes or more above the target, Tight margin from zero through less than five minutes above it, and Short below the target.

Check watt and VA load percentages independently. A value above 100% means the corresponding rating is exceeded; above 80% through 100% is marked high. Extra battery capacity can extend runtime but does not raise the UPS output rating. Reduce the load or select a larger UPS when either power limit is exceeded.

Technical Details:

The estimate converts the connected load to both watts and VA, builds nominal battery energy, applies four multiplicative allowances once, and divides the remaining Wh by real-power load. UPS ratings affect readiness checks only; they do not create battery energy.

Formula Core

Power factor converts between real and apparent power. Battery voltage and amp-hours provide nominal energy when a direct Wh figure is unavailable.

\begin{aligned} Watts & = VA \times PF \\ E_{internal} & = V \times Ah \times strings \\ E_{nominal} & = E_{internal} + (module count \times module Wh) \\ E_{usable} & = E_{nominal} \times η \times H \times (1 - R) \times (1 - D) \\ Runtime minutes & = \frac{E_{usable}}{Load watts} \times 60 \end{aligned}

PF is power factor; η is UPS efficiency; H is battery health; R is reserved energy; and D is additional derating. Percentages are converted to decimal factors. Direct battery Wh replaces the voltage × Ah path rather than being added to it.

Required nominal energy reverses the same calculation for a target runtime:

E_{required nominal} = \frac{\frac{Target minutes}{60} \times Load watts}{η \times H \times (1 - R) \times (1 - D)}

Additional required Wh is the positive difference between required and installed nominal energy. A zero target reports no required-energy comparison.

Rule Core

UPS runtime readiness thresholds
Check	Condition	Status
Shutdown margin	Margin ≥ 5 minutes	Comfortable
Shutdown margin	0 ≤ margin < 5 minutes	Tight
Shutdown margin	Margin < 0	Short
Watt or VA load	Load > 100%	Over
Watt or VA load	80% < load ≤ 100%	High
Battery health	Health ≥ 90%	Healthy
Battery health	75% ≤ health < 90%	Aging
Battery health	Health < 75%	Weak
Reserve	Reserve ≥ 15%	Conservative
Reserve	5% ≤ reserve < 15%	Modest
Reserve	Reserve < 5%	None
Generator bridge	Runtime ≥ entered bridge minutes	Covered
Generator bridge	Runtime < entered bridge minutes	Short

The load-shedding curve holds usable Wh constant and divides it by a range of loads. It demonstrates inverse load sensitivity under this energy model; it is not a manufacturer discharge curve. Full precision is retained until the selected display precision is applied.

Accuracy Notes:

Runtime depends heavily on battery chemistry, discharge current, temperature, age, charge state, inverter cutoff, and the UPS model's efficiency curve.

Prefer the manufacturer's runtime chart for the exact UPS and battery configuration. Use additional derating when only nominal Wh is known.
Use the protected load measured on the UPS, power distribution unit, or wattmeter. Nameplate maxima can greatly overstate ordinary load.
Count only compatible, installed battery modules and parallel strings. Series batteries raise bank voltage; they do not multiply Ah in the same way as parallel strings.
Verify a shutdown plan with a controlled test under safe operating procedures. Do not disconnect critical equipment merely to prove an estimate.

Worked Examples:

Aging 48 V battery bank

A 48 V, 9 Ah single-string bank provides 432 nominal Wh. At 92% efficiency, 85% battery health, 15% reserve, and 10% additional derating, about 258.44 Wh remains usable. A steady 300 W load therefore has an estimated runtime of about 51.7 minutes. A 50-minute shutdown target fits, but the 1.7-minute margin is classified as tight.

References:

UPS sizing guide, Eaton.
The difference between VA and watts, Eaton.