1. Definition and Purpose
- Watt is the real power drawn by the equipment. The Watt rating to determine the actual power purchased from the utility company and the heat loading generated by the equipment.
- The VA rating is used for sizing wiring and circuit breakers.
2. IT Equipment Switching Power Supply Types
- Power Factor Corrected supply (PFC)
- Capacitor Input supply
3. Computer Equipment Watt and VA Rating
- Large computer equipment such as routers, switches, arrays, and servers made after 1996 use the PFC supply. Watt rating = VA rating
- PCs, small hubs, PC accessories, and larger computer equipment made prior to 1996 typically use Capacitor Input supply. Watt rating = 0.55 ~ 0.75 * VA rating
4. UPS Watt an VA Rating
- Small UPS: Watt rating = 60% * VA rating (a de-facto standard in the industry)
- Larger UPS: Watt rating = VA rating
5. Conclusion
- Neither the Watt nor the VA rating of a UPS may be exceeded
- Sizing the VA rating of a load to be no greater than 60% of the VA rating of the UPS ensures the Watt rating of a load not to exceed the Watt rating of the UPS. This conservative sizing approach will typically oversize the UPS and provide additional run time.
Source: APC White Paper #15 “Watts and Volt-Amps: Powerful Confusion”
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