Power Demands of a System

When installing any new fire, security, CCTV or access control system, one of the first and most important considerations is power demand. Knowing this allows efficient and cost-effective matching of power supply to the system requirements. The same is true when a new device is added to an existing system. Will the system's power supply be able to handle the additional power requirements? The following equation is a helpful tool to determine what the total power demand of a system will be:

Power (in Watts) = Current (in Amps) x Voltage (in Volts)

VA is power in watts described for AC systems. 100 VA equals 100 Watts.

To determine the power requirements for a system, add the total AMPs required by each device in the installation and multiply by the Voltage <the result is how many Watts, or VA are needed to run the system.


1. Given a relay coil with a resistance of 300 Ohms and an Activation Voltage of 12 Volts, how much current is needed? Divide 12 Volts by 300 Ohms. This equals .04 AMPs, or 40 milliamps.
NOTE: 1000 milliamps equals 1 AMP.

2. Given a device that draws a 350 milliamp current at 12 volts, what is the power requirement of the system? Multiply .35 AMPs by 12 to get 4.2 Watts.

3. Given a 24 VDC system with 5 devices that draw 100 Milliamps each, and one device that draws 1 AMP, what is the total power demand of the system? In this case, five devices at .1 AMPs each (100 milliamps) plus one device at 1 AMP equals .1+.1+.1+.1+.1 + 1.0, which is equal to 1.5 AMPs. Multiplied by 24Volts, the total power demand of the system is 36VA, or 36 Watts.

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