APC Smart-UPS SRT Online Double-Conversion: Complete Data Center Guide
The APC Smart-UPS SRT is an online double-conversion UPS that provides zero transfer time, voltage and frequency independence, and the highest level of power protection available. It is the right choice for mission-critical data center loads, VoIP systems, and medical equipment where even milliseconds of power interruption are unacceptable.
What Is an Online Double-Conversion UPS?
An online double-conversion UPS continuously converts incoming AC power to DC (via a rectifier) and then back to AC (via an inverter). Your equipment always runs on the inverter output, never directly on utility power. This means there is zero transfer time when the power fails because the inverter is already powering the load. The battery simply takes over the DC bus role from the rectifier, with no switching delay whatsoever. The APC Smart-UPS SRT series is APC's flagship online UPS line, available from 1kVA to 10kVA in rack-mount and tower configurations.
Online vs Line-Interactive: When Does It Matter?
The standard APC Smart-UPS (SMT/SMX series) is a line-interactive UPS with a 2-4ms transfer time. For 99% of IT equipment, this transfer time is invisible because server power supplies have internal capacitors that bridge the gap. So when does the SRT's zero transfer time actually matter?
Zero Transfer Time Is Critical For:
- Active database transactions — A 2-4ms gap during a disk write can corrupt transaction logs on databases not using battery-backed write cache
- VoIP and real-time communications — Even brief power glitches cause audible artifacts or dropped calls in PBX systems
- Medical equipment — Regulatory requirements (IEC 60601) mandate zero-break power for patient-connected devices
- Industrial control systems — PLCs and SCADA systems with millisecond-sensitive processes
- Blade enclosures — High-density blade systems with thin power supply margins may not tolerate any gap
Line-Interactive Is Fine For:
- Standard rack servers (Dell, HP, Lenovo) with modern PSUs
- Network switches and routers
- Storage arrays with battery-backed cache
- Desktop workstations
- General office IT equipment
Pro tip: If you are unsure whether you need online or line-interactive, you almost certainly need line-interactive. The SMT series costs 40-60% less than the SRT series for equivalent VA ratings. Only buy online UPS when you have a specific technical requirement for zero transfer time.
APC Smart-UPS SRT Model Lineup
| Model | VA/Watts | Form Factor | Input | Runtime at 50% |
|---|---|---|---|---|
| SRT1000XLI | 1000VA / 900W | Tower / 2U Rack | 230V | 18 min |
| SRT1500XLI | 1500VA / 1350W | Tower / 2U Rack | 230V | 15 min |
| SRT2200XLI | 2200VA / 1980W | Tower / 3U Rack | 230V | 12 min |
| SRT3000XLI | 3000VA / 2700W | Tower / 3U Rack | 230V | 10 min |
| SRT3000RMXLI | 3000VA / 2700W | 2U Rack | 230V | 7 min |
| SRT5KXLI | 5000VA / 4500W | Tower / 3U Rack | 230V | 8 min |
| SRT5KRMXLI | 5000VA / 4500W | 3U Rack | 230V | 6 min |
| SRT6KXLI | 6000VA / 6000W | Tower / 4U Rack | 230V | 6 min |
| SRT8KXLI | 8000VA / 8000W | Tower / 6U Rack | 230V | 5 min |
| SRT10KXLI | 10000VA / 10000W | Tower / 6U Rack | 230V | 4 min |
Note: 120V models are available for North American deployments (SRT1000RMXLA, SRT3000RMXLA, etc.).
Key SRT Features
Unity Power Factor
Unlike the standard Smart-UPS where VA and watts differ significantly (e.g., 1500VA/1000W), the SRT series at 5kVA and above operates at unity power factor, meaning VA equals watts. The SRT10KXLI delivers a full 10,000W of real power, not a derated figure.
Frequency Conversion Mode
The SRT can operate in frequency conversion mode, converting 50Hz input to 60Hz output (or vice versa). This is valuable for:
- International deployments where equipment is designed for a different power frequency
- Generator testing where output frequency may be unstable
- Specialized equipment with strict frequency requirements
Scalable Runtime with External Battery Packs
Every SRT model supports external battery packs for extended runtime:
| Battery Pack | Compatible Models | Added Runtime (50% load) |
|---|---|---|
| SRT48BP | SRT1000/1500 | +20 min per pack |
| SRT72BP | SRT2200 | +18 min per pack |
| SRT96BP | SRT3000 | +15 min per pack |
| SRT192BP | SRT5000/6000 | +20 min per pack |
| SRT192BP2 | SRT8000/10000 | +15 min per pack |
You can daisy-chain up to 10 external battery packs for hours of runtime. This is commonly used in environments where generator start time is unpredictable or where the UPS must power equipment through an entire outage without generator backup.
Parallel Operation
SRT models 5kVA and above support parallel operation for N+1 or 2N redundancy. Two SRT10KXLI units can be paralleled for 20kVA total capacity with automatic load sharing and failover.
Efficiency Considerations
Online double-conversion UPS operates at 90-94% efficiency in double-conversion mode, compared to 97-99% for line-interactive. For a 10kVA UPS running at 50% load, this difference translates to:
- SRT10KXLI (online, 93% efficient): 5,000W load, 5,376W from utility, 376W lost as heat
- SMT equivalent (line-interactive, 98% efficient): 5,000W load, 5,102W from utility, 102W lost as heat
The extra 274W of heat requires additional cooling, which further increases power costs. Over a year, the efficiency difference at 50% load costs approximately $240 in electricity (at $0.10/kWh).
ECO Mode
To address efficiency concerns, the SRT supports ECO (economy) mode, which operates as a line-interactive UPS during normal conditions (98% efficiency) and switches to online double-conversion only during power anomalies. Transfer time in ECO mode is 2-4ms, same as a standard Smart-UPS.
ECO mode is suitable for environments where zero transfer time is not a constant requirement but is needed during power events. However, many data center operators disable ECO mode and accept the efficiency penalty for guaranteed zero transfer time.
Installation Best Practices
Electrical Requirements
SRT units 5kVA and above require hardwired electrical connections (no standard plug). A licensed electrician must:
- Install a dedicated circuit with appropriate breaker (30A-50A depending on model)
- Wire the input and output connections per APC's installation guide
- Verify grounding meets local electrical code
Rack Mounting
SRT rack-mount models are heavy (80-150 lbs). Installation requires:
- Rack rated for the weight (check your rack's static load rating)
- Two people for lifting and positioning
- APC rail kit (included with RMXLI models)
- Adequate rear clearance for airflow (minimum 2 inches)
Network Management
All SRT models include an AP9631 network management card pre-installed. This provides:
- SNMP v1/v2c/v3 monitoring
- Web interface for configuration and monitoring
- Email alerts for power events
- PowerChute Network Shutdown integration
- Environmental monitoring (with optional AP9335T temperature probe)
SRT vs SMT: Cost Comparison
| Capacity | Smart-UPS SMT (Line-Interactive) | Smart-UPS SRT (Online) | Price Premium |
|---|---|---|---|
| 1500VA | SMT1500RM2UC — ~$1,150 | SRT1500XLI — ~$1,800 | +57% |
| 3000VA | SMT3000RM2UC — ~$1,950 | SRT3000RMXLI — ~$3,200 | +64% |
| 5000VA | N/A (max SMT is 5kVA) | SRT5KRMXLI — ~$4,850 | — |
| 10000VA | N/A | SRT10KXLI — ~$8,500 | — |
The 50-65% price premium for online UPS is justified only when your application genuinely requires zero transfer time. For standard server rooms, the Smart-UPS SMT provides excellent protection at a significantly lower cost.
Pro Disk Network stocks the full APC Smart-UPS SRT lineup from 1kVA to 10kVA, plus external battery packs and replacement batteries. All units ship same-day from our US warehouse with free shipping over $150. Contact sales@prodisknetwork.com for data center power planning assistance and volume pricing.