An uninterruptible power supply, also uninterruptible power source, UPS or battery/flywheel backup, is an electrical apparatus that provides emergency power to a load when the input power source, typically mains power, fails.

*Note: A UPS will beep continuously when power is off. This is normal.

The UPS is only a temporary power backup (5 to 15 minutes) and it is not intended for continuous use.  You will need to switch over to a generator or power your PC down.

An inverter is meant for continuous power and will not beep continuously.

Offline UPS

Offline UPS connects the computer to the ESKOM's power grid (or to a generator) until the voltage falls below a certain level, in which case, the UPS switches over to battery backup. The switch-over time can be as long as 25 milliseconds depending on the amount of time it takes the standby UPS to detect the lost utility voltage. The UPS will be designed to power certain equipment, such as a personal computer, without any objectionable dip or brownout to that device.

Online UPS

Online UPS connects the computer directly to the battery backup at all times. This type of UPS is able to tolerate continuous under-voltage brownouts and over-voltage surges without consuming the limited reserve battery power. It instead compensates by automatically selecting different power taps on the auto-transformer.

Double conversion UPS

The primary power path is the inverter / battery and it is not the AC mains. This means that the output power is supplied by the battery all the time and no switch-over happens if power fails (0ms switch over)

If the battery / inverter fails, the AC is used as the backup power.

The advantage is that there is no switch over if AC fails, but the disadvantages are additional heat, lower efficiency and reduced life-span of the components in the UPS.

Other UPS features may include:

• AVR - Automatic Voltage Regulator
• Monitoring ports and software
• Built-in Surge Protection
• Audible alarms for power outage and low battery

What is the difference between VA and Watts

UPS are rated in VA (Volt-Amps).

This is a basic rating for UPS and is the Voltage multiplied by the Amperage the UPS handles.

At 220Volts, a UPS that can supply 1Amp would be rated 220VA.

This however is not the real power for AC devices because AC power rating requires the power factor to be taken into account.

The industry standard rule-of-thumb: plan for a power factor of 60% (this is an average conservative power factor).  The power that AC products can draw from the UPS is therefore approximately 60% of the VA rating.

A 1000VA UPS would therefore have an AC power rating of approximately 600Watts.

What is power factor?

In AC circuits, the power factor is the ratio of the real power that is used to do work and the apparent power that is supplied to the circuit.

The power factor can get values in the range from 0 to 1.

When all the power is reactive power with no real power (usually inductive load) - the power factor is 0.

When all the power is real power with no reactive power (resistive load) - the power factor is 1.

Typical power factors
Resistive load = 1
Fluorescent lamp = 0.95
Incandescent lamp = 1
Induction motor full load  = 0.85
Induction motor no load  =  0.35
Resistive oven  = 1
Synchronous motor = 0.9

UPS keeps switching on and off while using a generator