UPS DESIGN
UPS Sizing, Runtime and Installation Playbook
How to calculate protected load, check watts and VA, use manufacturer runtime data and avoid common UPS sizing mistakes.
Measure the load you actually intend to protect
Schneider Electric's UPS selector guidance recommends adding the expected power draw of the protected equipment and notes that nameplate ratings can be higher than real draw. For a professional design, use measured or manufacturer load data where possible and keep future expansion as a clearly stated planning assumption.
Watts and VA are different limits
A UPS normally has both a real-power limit in watts and an apparent-power limit in VA. The connected load must fit both limits. Do not size only from VA because a load can exceed the watt rating first.
Runtime cannot be guessed accurately from battery Ah alone
UPS runtime depends on battery system, inverter efficiency, battery condition, temperature and load. Use the manufacturer's runtime curve or selector for the exact UPS and load. If the manufacturer does not publish runtime for the model/load, state that runtime is unverified rather than inventing a number.
PoE changes UPS load
If a UPS protects a PoE switch, include the downstream powered devices in the expected switch input demand. A switch with many cameras or APs can draw much more under full PoE load than when tested empty on the bench.
Installation checks
- Confirm input plug/circuit requirements and local electrical installation requirements.
- Check output connector compatibility and PDU loading.
- Provide ventilation clearances required by the UPS manufacturer.
- Record battery installation date and replacement history.
- Configure network management and shutdown integration where required.
- Test a controlled power failure before handover.
Bypass and maintenance
For critical installations, consider how equipment will remain powered while the UPS is serviced. A maintenance bypass architecture must be designed for the actual electrical system and should be installed by appropriately qualified personnel.
Client benefit
Correct sizing gives the client predictable support for the intended load, avoids nuisance overload shutdowns and makes runtime expectations traceable to manufacturer data rather than sales estimates.
UPS & Power Knowledge Centre | Electrical Load and Heat Calculator | UPS categories at Server Warehouse
Technical references
Training manual: turn load data into a defensible UPS selection
Step 1: build the protected-load schedule
| Equipment | Quantity | Expected W each | Expected VA each | Critical? |
|---|---|---|---|---|
| Firewall | User input | Verified input | Verified input | Yes / No |
| PoE switch | User input | Include planned PoE load | Verify | Yes / No |
| NVR / server | User input | Measured or published | Verify | Yes / No |
Step 2: check both ratings
The selected UPS must support both the total real power in watts and the total apparent power in VA. If only one of those values is known, the design is incomplete.
Step 3: apply project growth as an explicit assumption
If the client expects expansion, document the reserve separately. Example: current measured load 900 W, user-selected 25 percent planning reserve, planning load 1125 W. The 25 percent is a design assumption, not a universal standard.
Step 4: verify runtime
Use the exact manufacturer runtime table or selector for the chosen UPS at the planning load. Runtime is nonlinear, which is why battery capacity alone is not a reliable shortcut.
Step 5: test the whole chain
- Confirm normal utility operation.
- Record UPS load percentage and battery state.
- Perform a controlled input-power failure if the site permits.
- Confirm critical switches, firewall, storage and endpoints remain powered.
- Confirm network connectivity remains operational.
- Restore power and confirm normal recharge/online state.
Client benefit
The client receives a documented protection design with a traceable load, known capacity and runtime expectation from the manufacturer rather than a vague promise that "the UPS should last about an hour".