An emergency power supply is a device which supplies the connected loads on mains failure by drawing previously stored power from the battery. On mains presence, the system recharges the batteries, whilst on mains failure it supplies external loads. There are different types of emergency power supplies to meet the requirements of different possible loads. They are first classified according to whether they produce a d.c. or a.c. output. In a.c. output units, an inverter converts the direct current supplied by the battery into alternating current. The most common applications of emergency power supplies are to power safety and alarm systems and emergency lighting systems.

To choose the power of an emergency power supply it is not enough to add the rated loads of the connected units. It is necessary to consider the efficiency and, in particular, the peak currents absorbed during the starting phase. The following selection guide applies to emergency power supplies with a square wave output:

The following articles cannot be powered by a square wave:

• power factor correction condenser
• power factor corrected loads
• electronic loads
• refrigerators

The following selection guide applies to emergency power supplies with a sine wave output:

1. Nominal duration is obtained with 80% of nominal power in Watts;
2. Limit only applies to fluorescent tube with starters;  
3. Motor nameplate current rating.

N.B. With electro-mechanical starters, fluorescent tubes take 5 to 15 seconds to switch on.