For a user’s PC, a sudden power cut may just mean an unsaved file. For a server, it can lead to database corruption, file system damage or even hardware failure. Voltage fluctuations and brief dips, which often go unnoticed, also wear down power supplies over time. A UPS (uninterruptible power supply) is the server room’s first line of defense against these problems.
Types of UPS
- Offline or Standby: passes mains power straight through under normal conditions and switches to battery when the power goes out. It is inexpensive and suited to desktop computers, not servers.
- Line-Interactive: in addition to battery backup, it corrects voltage fluctuations to some extent. It is an acceptable option for small racks with fairly stable power.
- Online (Double Conversion): constantly converts incoming power to DC and back to AC, so the output is always stable and there is no switchover time during an outage. It is the recommended choice for server rooms, especially where mains power is unstable.
A server UPS should have a Pure Sine Wave output; some server power supplies do not work properly with a simulated sine wave.
Calculating Capacity
UPS capacity is stated with two figures: VA (apparent power) and W (real power). The number that matters to you is watts, because equipment consumption is measured in watts, and a UPS’s watt rating is usually lower than its VA rating.
- List all the equipment that needs to be on the UPS: servers, storage, switches, firewall and modem.
- Find the actual consumption of each device. The figure on a power supply label is its maximum capacity, not real consumption. You can see server consumption in iLO, iDRAC or the manufacturer’s sizing tool.
- Add up the total and leave a margin for future growth. A common rule of thumb is that a UPS should not run near its maximum capacity all the time.
- Determine the runtime you need. Runtime depends on battery capacity and load, and you can get it from the manufacturer’s tables or tools. For longer runtime, choose a model that supports external battery packs.
A UPS is not meant to keep the server room running for hours. Its job is to cover short outages, give a generator time to start, or shut servers down safely.
Automatic Shutdown: The Part People Forget
If an outage drags on and nobody is in the office, the UPS shuts off when the battery runs out and the servers experience the same sudden power loss. To prevent this:
- Connect the UPS to your management system with a Network Management Card or a USB cable.
- Configure the manufacturer’s shutdown software or tools such as NUT on virtualization hosts and servers, so that at a set battery level the virtual machines shut down first, followed by the hosts in order.
- Send power failure and low battery alerts to the responsible technician by email or messaging app.
Installation and Maintenance Tips
- Connect servers with dual power supplies to two separate power paths, ideally two UPS units or two PDUs.
- Do not plug high-draw, non-essential devices such as laser printers, heaters or air conditioners into the UPS.
- Batteries have a limited life and wear out faster in heat. Keep the UPS in a cool environment and test and replace batteries as the manufacturer recommends.
- On larger UPS units, a Bypass switch lets you service the unit without shutting down the equipment.
Checklist
- An Online UPS with Pure Sine Wave output is used for servers.
- Capacity is calculated from actual consumption with room for growth.
- Runtime matches the company’s needs and whether or not a generator is available.
- Automatic server shutdown is configured and tested.
- UPS alerts reach the responsible person.
- There is a schedule for battery testing and replacement.
ITPlus724 engineers can assess your server room load, recommend the right UPS, and set up automatic shutdown and alerting.