Technical Support
How to estimate the autonomy of a UPS battery bank for IT loads — the formula, the deep-discharge coefficient, worked examples and reference configurations.
A UPS does not store energy; its battery bank does. The same UPS model can deliver five minutes or one hour of autonomy depending only on how the battery bank is configured. For this reason, backup time should always be quoted together with three pieces of information: the battery capacity (Ah), the number of blocks in the string, and the load level at which the figure applies.
The type of load also matters. The method described here applies to IT loads — servers, storage, network equipment and other devices powered by switch-mode power supplies, which behave as an approximately constant-power load.
| Symbol | Meaning | How it is obtained |
|---|---|---|
| T | Backup time | Result, in minutes. |
| AH | Battery capacity in hours | Rated capacity of one block, converted to the minute basis (AH = Ampere × 60 minutes). |
| V | Battery bank voltage | 12 V × number of blocks in the string. |
| pf | Power factor | Power factor of the connected load. |
| W | Consumption power | Actual load in watts. For a 100% load figure, use the rated active power of the UPS. |
| D | Deep discharge coefficient | See section 3. |
A battery never delivers its full nameplate capacity during a short, high-rate discharge. The rated Ah figure is measured over a 10-hour or 20-hour discharge; when the same battery is emptied in 30 minutes, only part of that capacity is available.
| Battery configuration | Coefficient D |
|---|---|
| Standard battery built into the UPS cabinet | 60 – 70 % |
| External battery cabinet or battery rack | 65 – 75 % |
V = 40 × 12 V = 480 Vdc · AH = 100 · W = 60,000 W · pf = 1.0 · D = 65 %
V = 40 × 12 V = 480 Vdc · AH = 33 · W = 20,000 W · pf = 1.0 · D = 65 %
V = 20 × 12 V = 240 Vdc · AH = 9 · W = 10,000 W · pf = 1.0 · D = 60 %
| UPS | Battery bank | Bank voltage | D | Backup time |
|---|---|---|---|---|
| 60 kVA / 60 kW | 40 × 12 V / 100 Ah, external | 480 Vdc | 65 % | 31.2 min |
| 60 kVA / 60 kW | 36 × 12 V / 150 Ah, external | 432 Vdc | 65 % | 42.1 min |
| 30 kVA / 30 kW | 40 × 12 V / 75 Ah, external | 480 Vdc | 65 % | 46.8 min |
| 20 kVA / 20 kW | 40 × 12 V / 33 Ah, external | 480 Vdc | 65 % | 30.9 min |
| 20 kVA / 20 kW | 40 × 12 V / 65 Ah, external | 480 Vdc | 65 % | 60.8 min |
| 10 kVA / 10 kW | 20 × 12 V / 9 Ah, built-in | 240 Vdc | 60 % | 7.8 min |
