Rated versus actual power consumption

The power consumption figure quoted by your LED luminaire supplier will almost always exceed the actual power which will be consumed

Knowing the power drawn by your luminaire is important, since often the financial viability of a project may depend on a particular reduction in electricity consumption e.g. replacing glass neon with Neon Flex. You may also be planning the power supplies you require – and you don’t want to waste money by buying power supplies which are far too large for what you require, meaning they will idle. Let’s deal the latter concern right up front: It is always safer to over-spec a power supply than under-spec one – meaning that if you base your power supply on the rated power drawn you will always be safe and you are unlikely to overwhelm your power supply and damage it.

The reason for the higher power consumption figure lies in the difference between rated power and actual power.

In electrical terms Power (P) is a factor of Voltage (V) and Current (I).

P = VI

When you’re told your 12V flexible strip draws 12W per meter, you would expect that when you measure it yourself you will get a similar figure. You will be surprised to hear that’s not usually the case. In fact it’s rarely the case. The power figure quoted is always the “rated power” – sometimes called theoretical power. Firstly, it is theoretical, calculated by adding up the wattage of each series of LEDs at a particular voltage. Secondly, it is calculated at a higher voltage than the luminaire will typically operate.

Fact 1: As voltage increases the amperage increases exponentially

Fact 2: Rated power is calculated at a higher voltage than you would typically operate your luminaire

An LED is a diode, which means it is a non-linear device – current and voltage do not follow a linear relationship. Until the forward voltage is achieved, no current will flow. Thereafter, increasing the voltage will see an exponential rise in the current flow until the LED fails as the pn-gate is burnt.

Fact 3: As the voltage increases past the forward voltage, the current will rise exponentially

Forward current versus forward voltage relationship

A luminaire will always be designed to operate at higher voltages than what the luminaire will operate at. A 12V luminaire will generally operate just fine at 14.2V. The rated power will be calculated at the higher voltage. The luminaire may operate slightly brighter at the higher voltage, but the LEDs will decay faster (more heat). Reducing the voltage from 14.2V to 12V won’t reduce the power consumed linearly, because of the exponential nature of the Voltage/Amperage chart, rather the power drop will be massive.

For example, at 14.2V the power drawn may be 12W however at 12V it may be as low as 6W. The brightness may be visibly similar, yet far less heat will be produced at 12V than at 14.2V – a double bonus right? Your luminaire will last longer and consume less power.

The relationship between current and voltage differs for the different colour LED chips, as you can see in the chart below:

Forward current versus forward voltage relationship – blue and red LEDs

Fact 4: Luminaire manufacturers will generally quote a single rated power figure across the range of colours

What this all means is that the rated power figure which you are given, is a theoretical number, calculated at a higher voltage than you will most likely run your luminaire at, and generally will represent the rated power of the highest power consuming colour LED chip.

If you want to know exactly how much power your luminaire will consume, you should test your luminaire yourself – hook up a sample (e.g. 1m of neon flex) to a power supply with a known, measured constant voltage, and use your ammeter to measure the current drawn, then calculate the actual power consumed.

Leave a Reply