UPS Runtime and Shutdown Calculator
Enter the measured UPS load and battery-bank details to estimate available runtime, then reserve enough of it for an orderly shutdown. The calculation accounts for battery condition, usable discharge depth, inverter losses, idle draw, and the higher-current penalty described by Peukert’s law.
Estimated battery runtime10 minutes
- Begin shutdown after
- 2 minutes
- Shutdown readiness
- Ready for the entered shutdown and reserve
- Battery bank voltage (V)
- 24
- Condition-adjusted bank capacity (Ah)
- 7.2
- Estimated battery current (A)
- 18.94
- Ideal linear runtime
- 18 minutes
- Peukert runtime reduction
- 44.81%
Uses nominal battery voltage, a fixed load, and the selected Peukert exponent.
Compare this planning estimate with the UPS manufacturer’s runtime curve or configuration tool.
How to use this calculator
- Measure the real watt load at the UPS output during the condition you need to protect.
- Enter one battery block’s voltage, amp-hour rating, rating period, and the series and parallel arrangement.
- Use a conservative measured capacity result and the UPS battery-mode efficiency when those values are available.
- Enter the longest tested shutdown duration and a separate safety reserve, then use the shutdown-start result in the outage plan.
How UPS runtime and shutdown time are calculated
The load at the UPS output is not the same as the demand on its battery. The calculator first converts output load to battery-side demand by dividing it by battery-mode efficiency, then adds the UPS electronics’ idle draw. Two 12 V blocks in series make a 24 V bank; adding a second identical string in parallel keeps the bank at 24 V but doubles its amp-hour capacity.
The main relationships are:
V = Vb x S; C = Cb x N x h; Pd = P / eta + P0; I = Pd / V
T = D x H x (C / (I x H))^k; Tlinear = D x C / I
- V is nominal bank voltage, from block voltage Vb and series count S.
- C is condition-adjusted capacity, from block capacity Cb, parallel strings N, and remaining-capacity fraction h.
- Pd and I are battery-side watts and nominal battery current.
- T is Peukert-corrected runtime in hours; D is usable discharge fraction, H is the rating period, and k is the Peukert exponent.
The result converts T to minutes, then calculates begin shutdown after = max(0, runtime - shutdown duration - safety reserve). If runtime is shorter than shutdown duration plus reserve, the calculator says to start immediately. The linear comparison sets k = 1.00; the percentage difference shows the rate-related reduction.
What changes the estimate most
Actual watts, battery health, and discharge current usually matter more than a battery’s nameplate amp-hour value. A 9 Ah block rated at C20 is rated at a comparatively gentle current, so a UPS drawing many amps can have much less available capacity. Victron’s battery-monitor documentation notes that faster discharge reduces available capacity and that a fixed Peukert exponent is only a rough approximation at very high current.
Use the capacity rating period printed in the battery datasheet. A C20 rating means the published amp-hours were measured over 20 hours, while C10 uses 10 hours. Series blocks raise voltage and lower current for a given battery-side watt demand; parallel strings add amp-hours. Neither relationship makes mismatched, aged, or poorly connected batteries behave like new matched strings.
What this calculator leaves out
This is a planning model, not a model-specific runtime curve. It does not model cutoff voltage, battery temperature, internal resistance, cell imbalance, wiring loss, load steps, startup surges, prior discharge history, or the UPS controller’s behavior. Lithium and proprietary cartridge systems can depart substantially from a lead-acid Peukert model. Use the UPS manufacturer’s runtime curve or configuration tool and a witnessed runtime test before relying on an automated shutdown setting.
Worked example
Consider a 12 V, 20 Ah battery bank at 100% health and 100% usable discharge depth supplying a 24 W load through a 100% efficient UPS with no idle draw. With one 12 V block, the battery current is 24 / 12 = 2 A. At C20, with a Peukert exponent of 1.20, runtime is 20 x (20 / (2 x 20))^1.20 = 8.7055 hours, or about 8 hours 42 minutes.
The linear comparison is 20 / 2 = 10 hours, so Peukert correction reduces the estimate by about 12.94%. With a 5-minute shutdown duration and a 3-minute reserve, begin shutdown after about 8 hours 34 minutes. The runtime is longer than the 8 minutes required for shutdown and reserve, so it is ready by this calculation.
Common questions
How long will my UPS run at its current watt load?
Enter the measured real watts at the UPS output, not the sum of equipment nameplate ratings. The estimate converts that load into battery-side current, applies the battery bank arrangement, and reduces runtime for the chosen battery condition, usable discharge depth, UPS losses, and Peukert exponent. A model-specific UPS runtime chart remains the better answer when one is available.
When should a server begin shutting down during an outage?
Begin the shutdown when the estimated runtime remaining equals the longest observed orderly shutdown duration plus a separate safety reserve. This calculator reports the elapsed outage time at which that point arrives. Test the full shutdown sequence under realistic load and use a larger reserve when the estimate is uncertain.
What Peukert exponent should I use for an AGM UPS battery?
Use the value supplied by the battery manufacturer when it is available. Victron describes 1.25 as an acceptable average for many lead-acid batteries, while 1.00 represents an ideal battery with no rate-related capacity loss. The default 1.15 is a planning assumption, not a substitute for battery discharge data.
What do C20 and C10 mean on a battery?
They state the discharge period used to assign the published amp-hour capacity. A 100 Ah battery rated at C20 is tested to deliver its rated capacity over 20 hours, or roughly 5 A, while C10 uses a 10-hour test. Enter the period that accompanies the amp-hour rating you used.
How do series batteries differ from parallel battery strings?
Series-connected blocks add their voltages while the amp-hour capacity remains that of one string. Parallel strings retain the voltage of one string while their amp-hour capacities add. Only combine batteries that are compatible, matched, and installed according to the UPS and battery manufacturers’ instructions.
Why can calculated UPS runtime be longer than observed runtime?
Actual batteries lose capacity with age, temperature, and previous use, and a UPS may stop at a voltage threshold before the simple amp-hour model is exhausted. Wiring resistance, load surges, inverter behavior, unequal blocks, and a current-dependent discharge curve can also shorten runtime. Treat a runtime test and the manufacturer curve as the final check.