Definition
UPS sizing is the process of determining the required apparent power (VA) and battery capacity (Ah) based on the actual load, its power factor, and the target backup time. The calculation works backward from the desired runtime, applying temperature‑adjusted discharge curves to ensure continuous power supply meets project reliability criteria. Subject to actual project.UPS Selection Checklist (printable)
| # | Step | Action | Pass Criterion | Note |
|---|---|---|---|---|
| 1 | Collect name‑plate data | Record rated power (W) and current (A) for every device in the weak‑current room (cameras, NVRs, switches, access controllers, displays, etc.) | Data completeness ≥ 100 % | Use technical manuals from Hikvision, Dahua, Axis, etc. |
| 2 | Measure real‑time current | Use a clamp meter or power analyzer at peak load to capture actual current per device class | Measured current ≤ 1.2 × name‑plate current (investigate larger gaps) | Field‑specific; “to be verified / subject to official release” if no baseline exists |
| 3 | Calculate total real power | Sum the measured watts of all devices | ΣW_total ≤ 0.85 × UPS rated power (15 % reserve) | Adjust reserve upward if future expansion is planned |
| 4 | Apply power‑factor conversion | Retrieve each device’s power factor (PF); default to 0.9 when unspecified | Compute apparent power VA = W / PF for each device | Results feed the vendor’s VA input field |
| 5 | Determine required VA | Sum all apparent powers | ΣVA_total ≤ 0.85 × UPS rated VA | Must satisfy both real‑power and apparent‑power limits |
| 6 | Set backup duration | Define required runtime (e.g., 15 min, 30 min, 1 h) according to business continuity SLA | Target time T_target matches project specification | Historical outage data can inform the choice |
| 7 | Reverse‑calculate battery capacity | Input load (W), PF, and T_target into the vendor’s sizing software (e.g., Hikvision UPS Configurator) | Obtained Ah ≥ software‑recommended Ah | The tool incorporates discharge curves automatically |
| 8 | Apply temperature correction | Identify the highest ambient temperature expected (e.g., 35 °C) and apply the manufacturer’s temperature coefficient | Corrected capacity = Ah × (1 + ΔT coefficient) | ΔT coefficient taken from the battery data sheet; subject to actual project |
| 9 | Bypass configuration | Decide between manual or automatic bypass switch | Automatic bypass recommended when critical load ≥ 10 kW | Bypass rating must meet or exceed UPS rated power |
| 10 | Tiered power design | Classify loads as critical, normal, or low priority and connect them to UPS, bypass, or utility respectively | Critical load ≤ 70 % of UPS rated power | Refer to the “Tiered Power” example in the weak‑current solution guide |
| 11 | Remote alarm & monitoring | Enable SNMP, Modbus, or the vendor’s cloud platform; set thresholds for battery temperature, state‑of‑charge, and input voltage | Alarm limits comply with local O&M SOPs | Example: Hikvision “Smart Power” platform |
| 12 | Battery replacement schedule | Log the initial in‑service date; plan replacement based on manufacturer life (typically 3‑5 years) | Generate a work order 30 days before expiry | After replacement, recalibrate capacity in the monitoring system |
| 13 | On‑site acceptance test | Verify: (a) real‑power ≤ 0.85 × UPS rating, (b) bypass switch time ≤ 5 s, (c) remote alarms reachable | All three items pass → Acceptance OK | Use the standard “UPS Selection and Acceptance Form” |
Additional Calculation Guidance (optional)
- Load budgeting – Add a 10 % safety margin to the summed measured power before applying the 0.85 factor, especially when the system will host future cameras or additional displays.
- Power‑factor range – If a device’s PF varies (e.g., 0.8 – 0.95), use the lowest value for the most conservative VA estimate.
- Battery derating – For every degree Celsius above 25 °C, most lead‑acid batteries lose roughly 0.5 % of capacity; verify the exact curve in the data sheet.
- Efficiency impact – UPS efficiency at full load (≈ 93 %) reduces the net usable energy; the sizing tool usually accounts for this, but manual checks can be performed by dividing required Ah by the efficiency factor.
Comparison of UPS Topologies
| Attribute | Online Double Conversion | Line‑Interactive (Online/Bypass) | Offline/Standby |
|---|---|---|---|
| Input voltage tolerance | ±1 % (tight) | ±5 % (moderate) | ±10 % (loose) |
| Transfer time | < 2 ms (seamless) | 5‑10 ms (brief) | 10‑20 ms (perceptible) |
| Full‑load efficiency | 92‑95 % | 88‑91 % | 85‑88 % |
| Ideal load type | Power‑quality‑sensitive core surveillance, servers | General weak‑current devices (cameras, access control) | Low‑power backup (lighting, few sensors) |
| Relative cost | High | Medium | Low |
| Maintenance level | Higher (periodic calibration) | Medium | Low |
FAQ
Q1: The measured power is much higher than the name‑plate rating. Which value should I use? A: Rely on the measured power because real‑world conditions (temperature, load variation) affect consumption. If the measured value exceeds the name‑plate rating by more than 20 %, investigate device configuration or consider a higher‑capacity UPS.
Q2: Is the VA = W / PF conversion always accurate when PF is 0.8? A: The formula provides a theoretical apparent power. In practice PF can shift with load level. Input a PF range (e.g., 0.8‑0.95) into the sizing tool; the tool will use the most conservative (lowest) PF for VA calculation.
Q3: How much does battery capacity drop at temperatures above 35 °C? A: Typical lead‑acid batteries lose about 5‑10 % of capacity at 35 °C. Use the temperature‑correction coefficient supplied by the battery manufacturer; the sizing software will apply it automatically.
Q4: Must an automatic bypass be installed? A: Not mandatory, but advisable when critical load exceeds 30 % of total load or when the SLA demands uninterrupted power. Automatic bypass reduces reliance on manual intervention during an outage.
Q5: Is a vendor‑specific cloud platform required for remote alarms? A: No. If an enterprise network management system (NMS) is already in place, SNMP or Modbus integration is sufficient. Vendor cloud platforms offer quicker deployment but are optional.