An Uninterruptable Power Supply or UPS, provides electrical power to a device when Utility Power fails. It does this by using an inverter that converts the direct current (DC) provided by a series of Batteries in to alternating current (AC), the type found in a electrical outlets in your home or office.
There are two basic types of UPS devices:
A Stand-by UPS, which has a 2ms to 4ms delay in sensing loss of Utility Power and switching over from the Utility Power to Battery Power for the protected equipment; and a True On-line UPS, where instead of a Switch between the Utility Power and the Battery Power, the UPS is always providing perfectly filtered Battery Power to the protected equipment. (i.e. a Computer, Router, Network Switch, etc.)
In an On-line UPS, the Utility Power continuously charges the Battery that in turn sends electrical current through the Inverter to the protected equipment. Think straight line: Utility Power, Battery, Inverter, Protected Equipment for an On-line UPS. For a Stand-by UPS, think of a fork in the road: Utility Power OR Battery Power to Inverter to the Protected Equipment.
The key advantage of a True On-line UPS is that the since the power to the protected equipment is always passing through the Battery to the Inverter, the quality of the power is stable and perfect. This can be especially important when your Power Source is unstable, such as that produced by a Generator instead of normal Utility Power.
A Stand-by UPS is suitable for infrequent Utility Power Outages as a fail-safe to allow you time to properly shut-down your Computer – either automatically if supported by the software of the UPS or manually by you. Or, to keep protected equipment, like a phone system, operational until the Batteries in the UPS run down.
Most Stand-By UPS units do NOT have any form of Power Conditioning. This means that if you are in an area with frequent Utility Power fluctuations, as in the lights in your home of office “dim” or appear to oscillate throughout the day, your equipment is getting “dirty power.” This potentially damages the electronics in your equipment and may cause premature failure. If the Utility Power actually dips below a certain level for a few milliseconds, the Stand-by UPS will kick in and flip over to Battery Power. One specific thing to watch for is if you hear the Stand-by UPS frequently cycling between Utility Power and Battery Power. When a UPS is “on-battery” usually there is both a visual indicator, an audible alarm, and most certainly a soft “hum” from the Inverter.
As stated, electricity produced by a Generator is very dirty. The power output of the Generator is significantly affected by the amount of load placed on the Generator. The more devices you have plugged in, the greater the draw of electricity or load, the harder the Generator has to work, the greater the fluctuation in the quality of power. The same visual effect can be seen with lights attached to a Generator. When a heavy load device, such as a Refrigerator, Freezer, or Air Conditioner, being powered by a Generator turns-on, you can audibly hear the Generator increase in speed to attempt to produce a higher “Surge” output of electricity to meet the demand. At the same time, you can see the lights dim or if you have a computer monitor or TV Set, the picture momentarily flicker.
To counteract the “dirty power” effect of a Generator, always use a True On-line UPS which will provide power conditioning and filtering to provide “clean power” to protect your most critical and expensive electronic devices. This would include Flat Panel TV’s, Internet Routers, Network Switches, Set-Top Boxes, Phone Systems, and Desktop or Server Computers.
When using a Generator as the Power Source, reserve the use of a Stand-by UPS for any device that has a charger, like a Notebook Computer or Phone Charger where you want to be able to continue to charge the devices when the Generator is off for maintenance or refueling. A Stand-By UPS is also useful for temporary Lighting and non-critical, low draw loads, such as the electronic starter on Gas Range, a Hot Water Heater, or a Thermostats.
The only exception to the above is for mechanical loads such as Refrigerators, Freezers, Air Conditioners and Forced Air Heating/Cooling Systems. Or, in the case of Electronics, high load devices like Laser Printers, Copiers, and Fax Machines. In general, these devices should never be plugged in to Uninterruptible Power Supply units unless the UPS is specifically designed to handle these types of higher demand loads. These devices should be plugged directly in to the Power Source, either Utility Power or the Generator.