ATS and UPS — Brief Definition
ATS (Automatic Transfer Switch / Automatic Transfer Switchgear) is a switching panel that automatically transfers the load to the generator within 5–30 seconds when the utility power fails, and switches back to the utility when it returns. UPS (Uninterruptible Power Supply), on the other hand, is a system that contains a battery bank and an inverter, comes online within 0 milliseconds during a utility outage to supply sensitive electronic loads, and can run them on battery power for anything from a few minutes to hours.
The two technologies do not replace each other; on the contrary, they complement each other. In a critical facility, both are typically present: the UPS provides continuity at the millisecond-to-second level, while the ATS brings the generator online for long outages at the minute-to-hour level.
Detailed Comparison Table
| Criteria | ATS | UPS |
|---|---|---|
| Transfer time | 3–30 seconds | 0–4 milliseconds (online) |
| Backup duration | As long as fuel lasts (8–72 hours) | 5–60 minutes (standard) |
| Power source | Generator (diesel/natural gas) | Battery (VRLA / Lithium) |
| Rated current/power | 16–6300 A | 1–5000 kVA |
| Investment/kVA | Low (excluding generator) | High |
| Maintenance | Monthly generator test | Annual battery test |
| Service life | 15–25 years | UPS 7–12 years, battery 3–7 years |
| Efficiency | 99%+ (passive device) | 92–96% (online) |
| Cooling | Passive | Active (fan-cooled) |
| Footprint | Small | Medium–Large |
| Output voltage regulation | NONE | Full regulation (±1%) |
| Harmonic filtering | NONE | Yes (online type) |
1. ATS (Automatic Transfer Switch)
Operating Principle
- Utility fails → controller detects it (~0.5 s delay)
- Generator start command is issued
- Generator warm-up time (~10–20 s)
- Voltage and frequency stabilize (~5 s)
- ATS disconnects the utility from the load
- ATS transfers the load to the generator side
- When utility returns, the reverse process (transfer time 30–60 s)
Types
- Open-transition ATS: Transfer with a brief interruption (common)
- Closed-transition ATS: Seamless via paralleling (CTTS)
- Soft-load ATS: Ramp-load transfer (critical for motors)
Advantages
- Low cost (1/5 the price of a UPS)
- Long backup duration (unlimited as long as fuel is replenished)
- High power (MW level)
- Low efficiency loss (passive device)
- Broad load support (motors, heaters, lighting)
Disadvantages
- Transfer interruption (even the fastest ATS is 3 s)
- Generator noise and emissions
- Fuel storage requirement (~a 24-hour tank is mandatory)
- Periodic generator testing is mandatory (once a week at idle, once a month under load)
Applications
- Plant-wide supply in factories
- Hotels, shopping mall common areas
- Main backup for elevators, HVAC, lighting
- Shelters, cold storage facilities
- Municipal traffic lights
2. UPS (Uninterruptible Power Supply)
UPS Types
Offline (Standby) UPS:
- 4–10 ms transfer time
- Normally the load is fed from utility; switches to battery during an outage
- Suitable for home/office computers
- Price: Low-cost (1–3 kVA range)
Line Interactive UPS:
- 2–6 ms transfer time
- Voltage fluctuation correction via AVR (Automatic Voltage Regulator)
- Small offices, POS systems, recording devices
- Price: Mid-range (1–10 kVA range)
Online (Double Conversion) UPS:
- 0 ms transfer time (true uninterrupted power)
- Load is always fed from the inverter
- Full voltage regulation + harmonic filtering
- Data centers, hospital operating rooms, manufacturing PLCs
- Price: Premium (3–5000 kVA range)
Advantages
- 0 ms continuity (online)
- Stable output (±1% voltage, ±0.1% frequency)
- Harmonic cleaning (THD <3%)
- Smart management (SNMP, dry contact, Modbus)
- Eco mode (98% efficiency, for non-sensitive loads)
Disadvantages
- High cost (15–30x ATS per kVA)
- Limited backup time (defined by battery capacity)
- Periodic battery replacement (every 5–7 years, increases cost)
- Space requirement (especially long-runtime models)
- Cooling requirement (air conditioning is mandatory)
Applications
- Data center (always with 2N redundancy)
- Hospital operating rooms, intensive care
- Production line PLCs, robot control
- Bank branch server rooms
- Broadcast studio equipment
- Sensitive laboratory devices (chromatography, MRI)
3. Combined Use: UPS + Generator + ATS
Most professional installation scheme:
Utility ─┬─ ATS ─┬─ UPS ─── Critical Loads (0 ms)
│ │
Generator ───────┴────────── General Loads (15–30 s)
In this configuration:
- Utility fails → UPS supplies from battery (0 ms)
- ATS transfers to generator (~20 s)
- UPS charging is supplied by the generator → battery is preserved
- Utility returns → ATS switches back → UPS recharges
4. Cost Comparison (Turkey 2026)
Comparison for 100 kVA power:
| Solution | Investment | 10-year TCO | Backup |
|---|---|---|---|
| Generator + ATS only | 380,000 TL | 580,000 TL | 24 hours |
| UPS only (60 min autonomy) | 740,000 TL | 1,180,000 TL | 1 hour |
| UPS (15 min) + Generator + ATS | 670,000 TL | 970,000 TL | 24+ hours |
Conclusion: In critical facilities, a UPS+ATS combination is both the safest and the most economical.
5. Certifications
- ATS: IEC 60947-6-1, UL 1008
- UPS: IEC 62040 series, EN 50091
- TSE: In Turkey, TSE certification should be preferred for both ATS and UPS
- EMC: Certification is mandatory (CE EMC EN 50091-2)
DOA Enerji ATS and UPS Service
DOA Enerji is an authorized solution partner for Schneider Galaxy, ABB PowerWave, Eaton 93PM, APC Symmetra UPS systems and ABB OTM, Socomec, Atlas, Aksa Power ATS panels. Projects including synchronization panels, soft transfer, parallel UPS have been completed. For emergency critical power projects, 24/7 on-site support is provided.
Frequently Asked Questions
Is a UPS alone sufficient, or is a generator necessary?
If the UPS battery capacity is less than 1 hour, a generator + ATS is absolutely required. For 24/7 critical services, the UPS is used only as a bridge; long-duration backup is provided by the generator.
Is ATS + generator alone sufficient?
For sensitive electronics such as PCs, servers, POS systems, ATMs, and recording devices, the transfer time (5–30 s) will shut devices down and cause data loss. For such loads, a UPS is mandatory.
What is the difference between an Online UPS and an Offline UPS?
An Online UPS always feeds the load from the inverter and has zero transfer time; an Offline UPS normally feeds from the utility and switches to battery during an outage (4–10 ms). Online is preferred for all sensitive loads; Offline is sufficient at the home/office level.
How long do UPS batteries last?
Standard VRLA (lead-acid) batteries last 3–5 years; premium ones last 5–7 years. Lithium-ion UPS batteries last 10–15 years and offset the extra investment over the long term.
Where in a hospital is a UPS mandatory?
By regulation, a UPS is mandatory for operating rooms, intensive care, hemodialysis, imaging (MRI, CT), pharmacy refrigerators, and laboratory auto-analyzer devices. These loads are considered Group C and require 0 ms continuity.