LED lights are widely recognised as one of the most energy-efficient lighting technologies available today. There can however be some confusion as to how much power they use and how much light they produce for the amount of power used.  LED bulbs use a lot less wattage compared to incandescent or compact fluorescent bulbs to produce the same amount of light.  A 7W LED bulb for example produces the same amount of brightness as an incandescent 60W bulb.

Key Factors Affecting Power Use

The exact power consumption (which is measured in Watts) of a LED bulb depends on a few different factors which will list below:

• Lumen Output: A higher amount of lumens usually requires more power.  This is not the case with LED bulbs.  For this reason lumens will almost always be advised under the manufacturer specifications for LED lighting.
• LED Quality and Efficiency: High quality LEDs have higher efficacy, meaning they produce more light per Watt consumed. 
• Driver Efficiency: This is an electronic component that will need to be connected to LED bulbs with a lower wattage than 240v to regulate the voltage getting to the LED bulb.  This will also consume a small amount of power.

Comparative Power Consumption

The following table lists the approximate power usage of different bulb types to achieve a similar light output.  This light output is measured in lumens and this unit will be the same no matter which sort of lighting technology is being used:

A LED bulb designed to replace a standard 60-watt incandescent bulb will generally use 7 to 10 watts of electricity and will still produce the same amount of light.

Calculating Energy Costs

To calculate the annual energy cost of a LED bulb you can use the below formula:

Energy Cost (£) = (Wattage / 1000) × Hours Used Per Year × Cost Per kWh

If we take an 8-Watt LED bulb used for 4 hours per day (1,460 hours per year) at an electricity rate of £0.15 per kilowatt-hour (kWh):

• Energy Used (kWh) = (8 W / 1000) × 1,460 hours = 11.68 kWh
• Annual Cost = 11.68 kWh × £0.15/kWh = £1.75 per year

A 60-Watt incandescent bulb under the same conditions would cost:

• Energy Used (kWh) = (60 W / 1000) × 1,460 hours = 87.6 kWh
• Annual Cost = 87.6 kWh × £0.15/kWh = £13.14 per year

Do Dimmable LEDs Use Less Power When Dimmed?

This is correct. Dimmable LED systems use less power when dimmed as the light output is reduced. The reduction in power consumption will usually be proportional to the reduction in light output.

Do LED Bulbs Use Power When Turned Off?

When a LED bulb is switched off a small amount of power may be used if it is connected to a LED driver.  This amount is not measurable by the majority of electronic measuring devices.

What is Efficacy (Lumens per Watt)?

Efficacy is the best measure of an LED's energy efficiency.  It is a measure of how effectively a bulb converts the wattage into lumens.  The higher the efficacy rating the more efficient the bulb will be.