Technical Training > Power Demand Formula

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.

EXAMPLES :

  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.