UPS Runtime Calculator
Estimate how long a UPS can support a connected load from battery-bank voltage and amp-hour capacity, with optional real-world loss and loading checks.
Calculator is for informational and preliminary sizing purposes only. Terms and Conditions
This energy-balance estimate is adjusted only by the factors you enter; enable Peukert correction when you have a defensible battery exponent and rated capacity period.
Enter the UPS load and battery bank
Use the actual connected real-power load in watts. Enter total battery-bank voltage and bank amp-hour capacity, not the UPS VA rating.
For series batteries, add voltage but do not add Ah. For parallel strings, add Ah at the same bank voltage.
Estimated UPS Runtime
Runtime is an estimate; manufacturer runtime curves for the exact UPS and battery configuration should take precedence when available.
Result details
- Nominal battery energy—
Estimate basis
Show calculation steps Review battery energy, losses, discharge correction, reserve, and load checks
- Enter valid values to see the complete calculation.
Runtime vs Load
Compare estimated runtime at 25%, 50%, 75%, and 100% of the connected load you entered. This is a model comparison, not a manufacturer-tested UPS curve.
- Enter valid values to populate the runtime comparison.
Method, Sources, and Assumptions
Calculation basis, authoritative references, limitations, and final verification requirements.
Runtime is estimated from battery energy and connected real-power load. Optional correction factors are applied only when entered. Manufacturer runtime data for the exact UPS and battery configuration is preferred for final equipment decisions.
- Enter valid values to see the active assumptions and limitations.
Calculator guide
How to Calculate and Interpret UPS Runtime
The UPS Runtime Calculator above estimates how long a battery-backed UPS can support a connected real-power load. The minimum inputs are connected load in watts, total battery-bank voltage, and battery-bank amp-hour capacity; the primary output is estimated runtime in minutes or hours. The tool can also apply the entered efficiency, usable-capacity, battery-health, runtime-reserve, and optional Peukert settings.
The basic relationship is energy divided by power: battery voltage multiplied by amp-hours gives nominal watt-hours, and usable watt-hours divided by load watts gives runtime. That result is a planning estimate, not a model-specific runtime guarantee, because actual UPS behavior depends on the exact battery, discharge rate, inverter, shutdown threshold, and UPS design.
- Minimum inputs
- Load watts, battery-bank voltage, and battery-bank Ah
- Main output
- Estimated planning runtime in minutes or hours
- Best verification
- Exact manufacturer runtime curve for the UPS and battery configuration
How to Use the UPS Runtime Calculator
Start with the three values that define the basic runtime estimate, then use Advanced Options only when you have defensible information for those settings. The calculator updates automatically, so you can immediately see how load, battery capacity, and assumptions change the result.
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Enter the connected real-power load
Use the watts actually expected while the UPS is on battery. The calculator accepts W or kW and converts the physical quantity when the unit selector changes. Do not enter a UPS VA rating as though it were load watts.
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Enter the complete battery-bank voltage and Ah
Use the nominal voltage and amp-hour capacity of the entire bank. Series batteries add voltage without adding Ah; parallel strings add Ah at the same bank voltage.
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Use Advanced Options only when the values are known
Efficiency, usable capacity, battery health, reserve, UPS watt/VA ratings, power factor, and Peukert inputs can materially change the estimate. Leave optional expert values at their disclosed example state or blank when you do not have better project- or manufacturer-specific data.
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Verify the result against the exact UPS model
Use the result as a transparent planning estimate, then compare it with the manufacturer runtime graph, table, or selector for the exact UPS and battery configuration whenever that information is available.
Inputs and Outputs That Control UPS Runtime
Use actual UPS load and battery-bank information whenever possible. The calculator treats the connected load as real power in watts and the battery values as the nominal voltage and Ah of the complete bank.
- Connected Load
- The real power used by the protected equipment while the UPS is on battery. Use watts or kW. If you only know VA, determine real power or power factor rather than entering VA as watts.
- Battery Bank Voltage
- The nominal DC voltage of the complete bank. Series-connected batteries add voltage; for example, four 12 V batteries in series form a 48 V bank.
- Battery Bank Capacity
- The Ah capacity of the complete bank at its stated rating period. Series batteries do not add Ah; parallel strings do add Ah when the strings operate at the same bank voltage.
- UPS Efficiency
- An optional loss factor between the DC battery side and the AC load. The calculator uses the value entered rather than assuming a universal efficiency for every UPS.
- Usable Capacity and Battery Health
- Optional fractional reductions applied to nominal battery capacity. Use values supported by the battery condition, test data, or a documented planning assumption rather than treating the example defaults as universal.
- Runtime Reserve
- A planning deduction applied after the runtime estimate. It is a user-selected margin, not a physical battery property.
- UPS Watt and VA Ratings
- Optional output-capability checks. They do not create battery energy, but they can reveal that the load exceeds the UPS real-power or apparent-power rating.
- Estimated UPS Runtime
- The planning runtime after the active efficiency, capacity, health, Peukert, and reserve settings are applied. Compare it with manufacturer runtime data before relying on it for equipment selection.
| Battery arrangement | Bank voltage | Bank capacity | Nominal energy |
|---|---|---|---|
| One 12 V, 9 Ah battery | 12 V | 9 Ah | 108 Wh |
| Two 12 V, 9 Ah batteries in series | 24 V | 9 Ah | 216 Wh |
| Two 12 V, 9 Ah batteries in parallel | 12 V | 18 Ah | 216 Wh |
| Two series strings in parallel, two batteries per string | 24 V | 18 Ah | 432 Wh |
UPS Runtime Formula and Calculation Method
The calculator uses an energy-balance estimate by default. It starts with nominal battery energy, applies only the correction factors entered in the tool, divides by connected load, and then applies the selected runtime reserve.
Nominal battery energy
Plain language: nominal watt-hours equal battery-bank volts multiplied by battery-bank amp-hours.
Adjusted planning runtime
Plain language: multiply nominal battery energy by UPS efficiency, usable-capacity fraction, and battery-health fraction; divide by load watts; then deduct the selected runtime reserve.
This is a simplified planning model. It does not recreate the exact discharge curve, low-voltage cutoff, self-consumption, or firmware behavior of a specific UPS.
Optional Peukert-corrected runtime
When both a Peukert exponent and the Ah capacity-rating period are entered, the calculator estimates battery-side current and applies the entered high-rate discharge correction before the reserve. In this form, \(C_{eff}=C_{Ah}f_u f_h\).
Victron Energy notes that available battery capacity depends on discharge rate and that Peukert behavior is an approximation, especially at very high current. Use the battery datasheet or manufacturer discharge data when available.
- \(V_{bank}\)
- Nominal battery-bank voltage, V
- \(C_{Ah}\)
- Nominal battery-bank capacity, Ah
- \(C_{eff}\)
- Effective Ah used by the optional Peukert model after usable-capacity and battery-health factors: \(C_{Ah}f_u f_h\)
- \(P_{load}\)
- Connected real-power load, W
- \(\eta\)
- Entered UPS efficiency, expressed as a decimal fraction
- \(f_u\)
- Entered usable-capacity fraction
- \(f_h\)
- Entered battery-health fraction
- \(r\)
- Entered runtime-reserve fraction
- \(H\)
- Battery capacity-rating period, h
- \(k\)
- Entered Peukert exponent
- \(I_{bat}\)
- Estimated battery-side current, A
UPS Runtime Calculation Example
The calculator loads with a clearly labeled example so the result is visible immediately. This example uses a 500 W load on a 24 V, 18 Ah bank with 90% efficiency, 90% usable capacity, 100% battery health, and a 10% planning reserve. Peukert correction is not enabled in the default example.
Calculate adjusted energy available to the load
Nominal battery energy is 432 Wh. After the entered efficiency, usable-capacity, and battery-health factors, the simplified model represents 349.92 Wh as adjusted energy available to the load before the runtime reserve.
Calculate runtime and apply reserve
Result
Estimated planning runtime: 37.79 minutes
This matches the calculator’s displayed default result before any Peukert correction or optional UPS W/VA rating checks are added.
How to Interpret the Runtime Result
Treat the result as the estimated time available at the entered load and assumptions. A runtime number is useful only if the UPS can also support the load within its watt and VA limits and if the battery assumptions resemble the actual battery condition.
Load strongly controls runtime
With the simple energy-balance method and all other inputs held constant, runtime is inversely proportional to load. Halving a 500 W load to 250 W doubles the simplified runtime; Peukert correction can make the high-load case fall even faster for lead-acid batteries.
Check both W and VA
Schneider Electric distinguishes watts from VA when sizing UPS equipment. If a VA rating is entered in the calculator, power factor is required so apparent power can be estimated with \(S=P/PF\).
Compare with the exact UPS curve
Eaton publishes model-specific runtime graphs with different load and battery-pack configurations. Those curves are better evidence for a known UPS model than a generic battery-energy estimate.
What Changes Real UPS Runtime?
Two UPS systems with the same nominal battery watt-hours can produce different actual runtimes. Product-specific inverter behavior, battery condition, temperature, charge state, discharge rate, and shutdown thresholds all influence how much of the nominal battery energy is actually delivered before the UPS turns off.
Battery age and condition
An older or degraded battery may deliver less usable capacity than its nameplate rating. The calculator’s Battery Health field is a planning factor; it does not automatically measure state of health.
State of charge
A battery that is not fully charged starts with less available energy. Generic runtime calculations usually assume the entered capacity represents the available starting condition.
Temperature and environment
Battery performance and aging are temperature-sensitive, and the effect depends on chemistry and manufacturer data. Do not apply one universal temperature correction when the battery documentation provides a better basis.
Discharge rate
Higher load means higher battery current. For lead-acid batteries, effective capacity generally falls as discharge rate rises, which is why the optional Peukert model and manufacturer runtime curves matter most in high-load cases.
UPS self-consumption and efficiency
The inverter, controls, cooling, and internal electronics consume energy. UPS efficiency can also change with load, while this calculator uses the single efficiency value entered by the user.
Shutdown voltage and firmware behavior
A UPS stops supporting the load before a battery reaches literal zero stored energy. Product-specific low-voltage cutoffs, protection logic, and firmware therefore affect measured runtime.
How Much UPS Runtime Do You Need?
The right runtime is determined by what must happen after utility power is lost. Do not choose a UPS from a generic minute target alone; define the outage response first, then make sure the estimated and manufacturer-verified runtime covers that sequence with appropriate margin.
Graceful shutdown
For computers, servers, storage, or controls that only need time to save state and shut down, base the requirement on the actual shutdown sequence plus margin rather than on the longest runtime available.
Network continuity
Routers, switches, firewalls, ONTs, access points, and communications equipment may need to remain online through short outages. Measure the combined real load of only the devices that must stay powered.
Generator bridge
If a generator will take over, the UPS needs enough runtime for outage detection, generator start, stabilization, transfer, and a reasonable contingency. Use the actual sequence and transfer timing for the site.
Extended backup
Long-duration operation usually requires more battery energy, a UPS designed for extended runtime, or manufacturer-supported external battery packs. A higher VA rating alone does not guarantee longer runtime.
Common UPS Runtime Calculation Mistakes
Most large errors come from mixing power and energy ratings, entering the battery bank incorrectly, or treating an ideal energy estimate as manufacturer-tested runtime.
Using VA as if it were Wh
VA is an apparent-power rating, not stored energy. A 1500 VA nameplate alone cannot determine runtime because it does not tell you the battery-bank watt-hours or the actual load.
Adding Ah for batteries in series
Series connection raises bank voltage while Ah stays the same. Adding both voltage and Ah for a series string double-counts stored energy and can greatly overstate runtime.
Using maximum equipment nameplate values as actual load
Runtime should be based on the power actually expected during battery operation. A UPS display, management software, monitored PDU, or watt meter can provide a more useful load value when available.
Ignoring high discharge rate
Victron Energy explains that battery capacity decreases as discharge rate increases and that lead-acid batteries are more affected than lithium batteries. If the Ah rating period and exponent are known, the calculator can apply the entered Peukert correction.
Assuming the battery is still at nameplate condition
The battery-health field is a planning factor, not an automatic battery test. Use measured capacity, maintenance data, or manufacturer guidance when battery condition matters.
Treating reserve as extra battery loss
The runtime reserve is deliberately applied after the estimate as a planning margin. It should not be confused with inverter efficiency, battery health, or a manufacturer discharge factor.
Assumptions, Peukert Correction, and Limits
This is a preliminary engineering estimate. The calculator intentionally exposes its assumptions instead of pretending one generic equation can reproduce every UPS model and battery pack.
Constant real-power load
The calculation treats the entered watt load as constant during the outage. Real computers, servers, network equipment, motors, and power supplies can change load over time.
Nominal battery values
Voltage and Ah are treated as nominal bank ratings. The model does not recreate the full battery voltage curve, cell imbalance, internal resistance, or the UPS low-voltage cutoff.
Peukert is optional and approximate
The calculator enables Peukert correction only when both the exponent and capacity-rating period are supplied. Victron describes Peukert behavior as an approximation and notes that fixed-exponent prediction can still be optimistic at very high current.
UPS self-consumption is not separately modeled
At very low connected loads and long estimated runtimes, control electronics and UPS self-consumption can become more significant relative to the protected load.
Example defaults are illustrative
The preloaded 500 W, 24 V, 18 Ah example is there to demonstrate calculator behavior. Its efficiency, usable-capacity, health, and reserve settings are not universal design values for all UPS systems, and the example’s 1.29C discharge-rate warning is a reminder that nameplate Ah may overstate effective high-rate capacity.
Manufacturer data takes precedence
For a known UPS model, use the manufacturer runtime table, graph, or configuration tool for the exact battery arrangement and load. That data reflects product-specific behavior a generic model cannot fully reproduce.
Sources and Calculation Checks
The calculator method was cross-checked with dimensional analysis, reverse calculation of the default example, manufacturer guidance on UPS W/VA and runtime behavior, and a battery technical reference for discharge-rate effects.
- Schneider Electric UPS Buying Guide — Supports the distinction between UPS watts and VA, power factor, and the principle that added connected load reduces runtime.
- Eaton UPS Battery Runtime Graphs — Provides model-specific runtime graphs and shows that runtime depends on UPS model, load, and optional battery-pack configuration.
- Victron Energy: Battery Capacity and Peukert Exponent — Supports Ah and C-rate concepts, reduced available capacity at higher discharge rates, and the limitations of fixed-exponent Peukert estimation.
The worked example was recomputed from the calculator’s production defaults: 24 V × 18 Ah = 432 Wh; after the entered 90% efficiency and 90% usable-capacity factors, 349.92 Wh remains in the simplified model; applying 500 W load and 10% reserve gives 37.79136 minutes, displayed by the calculator as 37.79 min.
UPS Runtime Calculator FAQ
These questions address common UPS runtime searches that are not fully answered by the calculator result alone.
How is UPS runtime calculated?
Start with battery-bank energy, \(Wh=V\times Ah\). Apply the efficiency and usable-capacity factors that are justified for the system, divide by connected load watts, and apply any planning reserve. For lead-acid batteries at high discharge rates, a verified Peukert correction or manufacturer discharge curve can materially change the estimate.
How long will a 1500 VA UPS run?
There is no single runtime for a 1500 VA UPS. The 1500 VA rating describes apparent-power capacity, not stored battery energy. Runtime also depends on battery voltage and Ah, actual watt load, efficiency, battery condition, discharge rate, and the specific UPS design.
Does a larger VA rating always mean longer UPS runtime?
No. A larger VA rating means the UPS can support more apparent power, but it does not by itself specify battery watt-hours. Two UPS units with the same VA rating can have different batteries and different runtime curves.
Why does UPS runtime fall as load increases?
Even in the simple energy-balance model, runtime is inversely proportional to load: the same usable watt-hours are consumed faster at higher watts. Lead-acid batteries can lose additional effective capacity at higher discharge current, which is why manufacturer runtime curves are nonlinear and why Peukert correction can matter.
Do batteries in series increase amp-hours?
No. Identical batteries in series add voltage while the string Ah remains the same. Parallel strings at the same voltage add Ah. Enter the voltage and Ah of the complete bank, not the sum of every battery’s Ah regardless of connection.
Should I use Peukert correction for every UPS?
No. Use it only when you have a defensible Peukert exponent and battery capacity-rating period for the battery being modeled. The calculator leaves it optional because a guessed exponent can create false precision, and manufacturer discharge data is preferable when available.
Why is my actual UPS runtime shorter than the calculator result?
Possible causes include a higher real load than entered, lower battery capacity than assumed, high-rate discharge loss, UPS self-consumption, battery voltage sag, shutdown thresholds, and product-specific inverter behavior. Compare the inputs with measured load and the exact manufacturer runtime curve.
How long will a UPS run at 500 watts?
A 500 W load is not enough information by itself. Runtime also depends on battery-bank voltage and Ah, efficiency, usable capacity, battery condition, discharge rate, and the UPS design. With this calculator’s default 24 V, 18 Ah example and current assumptions, the displayed planning estimate is 37.79 minutes, but the same state also produces a 1.29C high-discharge warning that should be checked against battery or manufacturer runtime data.
Can I calculate UPS runtime from VA alone?
No. VA describes apparent-power output capability and does not tell you how much battery energy is stored. To estimate runtime you need battery energy or model-specific runtime data plus the expected real-power load.
What is the most reliable way to estimate runtime for a specific UPS model?
Use the manufacturer runtime table, graph, or selector for the exact model and battery configuration at the expected load. Use the calculator as a transparent planning and cross-check tool, especially when you know the battery-bank specifications or want to test sensitivity to load and battery assumptions.