In simple terms, watts (W) is the amount of power consumed by a device which is determined by the voltage (V) multiplied by the current measured in amps (A). Watts = Volts x Amps
If you reverse this equation you can work out how much current is being drawn from a particular devise as well.
An example could be a 2000W heater, if we wanted to know how much current was being drawn from the grid the equation would be 2000W divided by 240V equals the current in amps.
2000W / 240V = 8.33A
On your electricity bill, the unit rate is kilowatt hours (kWh), to work out how much power a device is using you simply multiply the wattage rating divided by 1000 and then times by the hours of operation.
Example of this is a 50W halogen downlight bulb, if you use 4 of these bulbs in a room for 5 hours a night you can work out the cost of this by: 50W x 4 bulbs x 5 hours / 1000 = 2.25 kWh, multiply that by your unit price of electricity (20 cents) and then you have your cost.
LED technology is very different from traditional lighting technology and as such it’s difficult to get your head around the new types of transformers or drivers as they're called now.
A transformer or driver is a device which is used to transform the mains power, which is typically 240V AC in Australia, to a lower controlled voltage and current designed to power the light.
Light emitting diodes are generally direct current (DC) powered, and as such require a constant or controlled current to emit light correctly and as well as maintain their estimated life time. The drivers that go hand in hand with LEDs are generally constant current.
There are however constant voltage drivers that power retrofit lamps which typically operate from 12 or 24 volts, in which you would need to select a driver with a large enough power output to accommodate the multiples of lamps you are intending to use.
Example of this would be 10x 12V lights each consuming 7 Watts of power.
Using the equations above you would determine that 10 lamps x 7W = 70 Watts of power at 12V.
Therefore you would need a 12V driver with more than 70 watts of output to power all 10.
Often people ask why LED transformers aren’t interchangeable, and simply it’s because of the different current and voltage supplied to the LED.
Typically LEDs operate from 50 milliamps (mA) up to 1.5A depending on the configuration, given that LEDs are constant current driven the voltage is determined after the power consumption.
For example, a 350mA driven LED that consumes 15W would mean the voltage needs to be above 43V DC.
15W / 0.35A = 42.85V
If you want to know how much current it’s using from mains power, you go the opposite way and do 15W / 240V = 0.0625A.
Drivers also have additional components which can allow use with dimmers, have surge or spike protection and integrate with sensors such as motion or photo electric.