UPS runtime calculator
How long will these batteries carry your load? Enter the load (W, VA or a device list), the battery arrangement and the losses to see the estimated runtime and how it reacts to load changes. In reverse mode, work out the capacity needed for a target time.
Results
- Estimated runtime
- 5 h
- Load (W)
- 600 W
- Power drawn from the batteries
- 652.17 W
- 600 W / 92 %
- Battery current
- 13.59 A
- 652.17 W / 48 V
- Battery string, nominal energy
- 4.8 kWh
- 48 V · 100 Ah · 4 batteries (4 series × 1 parallel)
- Usable energy
- 3.26 kWh
- Share of nominal capacity left after 80% DoD and 85% adjustment: 68 Ah
Sensitivity to load changes
| Load | Power | Runtime |
|---|---|---|
| 50 % | 300 W | 10 h 1 min |
| 75 % | 450 W | 6 h 40 min |
| 100 % | 600 W | 5 h |
| 125 % | 750 W | 4 h |
As load rises, the runtime shortens faster than linearly (efficiency and Peukert effect).
The manufacturer's runtime curve (runtime-per-kW table) is authoritative: this tool gives a general energy balance and does not replace a product-specific curve. Use the manufacturer's table for purchasing and sizing.
Calculation and assumptions
- Load (W) = W or VA × power factor. DC power = load / inverter efficiency. DC current = DC power / string voltage.
- String voltage = battery V × series count. String capacity = battery Ah × parallel strings. Nominal energy (Wh) = string V × string Ah.
- Usable Ah = string Ah × DoD × adjustment. Runtime (no Peukert) = usable Ah / DC current.
- Peukert: runtime = H × (usable Ah / (current × H))^k; H is the discharge time the capacity is quoted for. Reverse: usable Ah = current × H × (target / H)^(1/k).
The values you enter are processed in your browser; they are not sent to a server and not stored.
This is a preliminary estimate: it assumes constant-power load and constant inverter efficiency; end voltage, battery age, temperature and the real discharge curve change the result. The Peukert model is an approximate correction. For critical systems rely on the manufacturer's runtime table and a field discharge test.
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How to use
A
Enter the load in watts, VA (with power factor) or as a device list; enter the battery voltage, capacity and series/parallel arrangement.
B
Set the inverter efficiency, DoD and the ageing/temperature adjustment from your own documents; enter a Peukert exponent if you want one.
C
See the runtime and the sensitivity table for 50–125% load, or switch to reverse mode to find the capacity needed for a target time.
02
How is runtime calculated?
The basic idea is an energy balance: the usable energy in the batteries is divided by the power the load draws from them. Because the inverter has losses, the power drawn from the batteries is larger than the load (load / efficiency). Usable energy is only a part of nominal energy: DoD is the margin left to protect the battery, and the adjustment factor represents ageing and temperature losses.
This tool calculates through battery current: runtime = usable Ah / current. You must enter the arrangement as series and parallel, because voltage grows with the series count, capacity with parallel strings, and the current for a given load is inversely proportional to voltage.
03
The Peukert exponent and why it is optional
Lead-acid batteries deliver less than their rated capacity at high current; the Peukert relation describes this approximately. The exponent is specific to the battery and comes from manufacturer data or a discharge measurement; this tool does not suggest a ready value. If you do not know it, leave it at 1: the result is then linear and may be optimistic at high loads.
When you use Peukert you must also enter the discharge time the capacity is quoted for (e.g. the 20-hour rate). The result also shows the runtime without Peukert, so you can see the difference and judge the sensitivity.
04
The manufacturer's curve is authoritative
UPS manufacturers usually publish measured runtime tables for a given battery and load; they include end voltage, the inverter's load curve and the real discharge behaviour. If this tool's result differs greatly from your table, trust the table; the tool exists to make assumptions visible and to compare quickly.
For a new installation also plan for ageing: capacity falls over time, so try the adjustment factor with an end-of-life value instead of leaving it at 100% for new batteries. If the time falls short, consider separating non-critical loads rather than only adding capacity.
FAQ
- Is my input sent anywhere?
- No. The calculation runs in code in your browser; values are not sent to any server and are not stored.
- What is the difference between VA and W?
- VA is apparent power, W is real power; W = VA × power factor. A UPS rating may be given in both; what determines runtime is the real power (W) drawn from the batteries.
- Why is DoD not 100%?
- Fully discharging a battery shortens its life, and dropping below the end voltage shuts the UPS down. The usable share depends on the manufacturer and chemistry; enter it from the datasheet.
- Why does runtime not halve when the load doubles?
- As load rises, current rises; in lead-acid batteries capacity falls at high current (the Peukert effect) and inverter efficiency changes too. The sensitivity table therefore calculates each load value separately; without a Peukert exponent it stays linear.
- How do I find the number of batteries?
- First find the UPS DC bus voltage; series count = bus voltage / battery voltage. Then switch to reverse mode: the tool gives how many parallel strings (total batteries) the required string capacity needs.
- Is the result enough for a critical system?
- Use it for preliminary sizing. For critical systems rely on the manufacturer's runtime table, battery ageing and regular discharge tests.
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