How to Choose an LED Power Supply? With Transformer Calculator

For operation of 12V LED lamps and spotlights on the mains a power supply or transformer is needed. How cay you find the right transformer within all offered variants? In this guide you can learn what is important when choosing an LED power supply. We will also show you how to calculate the required output power and select the right LED transformer.

LED power supply dimensioning

In addition to 120V LED lights, there are also various LED spots, spotlights and other light sources, which are operated with low-voltage. Usual operating voltages are 12V and 24V. An LED power supply is required to operate the low-voltage lamps on the 120V mains. This transforms the mains voltage down to the required LED voltage. Instead of the term power supply, the following terms are also common:

  • Transformer
  • Low-voltage transformer
  • LED power supply

LED transformers are available in a wide variety of power classes. However, it makes no sense to simply buy an oversized transformer without calculating the actual power required in advance. Many transformers have a minimum load and would not provide any voltage at all if the load is below this limit. You should therefore dimension the transformer exactly for your application.

Calculate LED Power Supply

The required power of the LED power supply can be calculated quite easily. For most light sources and LED spots the power consumption in watts (W) is given. You will find this information both on the packaging and directly printed on the lamp. For example, if you only want to operate a single low-voltage lamp with 10W, the transformer must also be able to supply at least 10W + power reserve.

The operation of several lamps on one transformer is also very common and very economical. Here, the power consumption of all LED’s must be added together.

Example: Power supply calculation for multiple spotlights

Six 12V LED spots with 6 watts each shall be operated:

6 watts · 6 (quantity) = 36 watts

Add 20% power reserve:

36 watts + (0.2 · 36) = 43.2 watts

→ A 45 watts power supply would be a good choice here.

Calculate power via current consumption

In some special cases, the power consumption of the LED’s is unknown. Instead, the current consumption is given in amperes (A). Then the power can be determined by multiplying voltage and current. The result can then be used to calculate the transformer as described above. Examples for power calculation from voltage and current:

  • 12V · 2.5A = 30W
  • 24V · 0.8A = 19.2W

Calculate power supply for LED strips

LED stripes are often sold by the foot. This leads to the following specialty. The power consumption in the shop or in the data sheet is therefore usually given in watts per foot (W/ft). For example, if you want to operate a 5ft LED strip, the transformer can be calculated as follows:

Calculate power reserve

It is not good idea to permanently operate an LED transformer with 100% load. On the one hand, the power supplies fuse could be triggered by the inrush current of the lamps. Furthermore, the power supply unit could heat up above average, which probably would reduce its service life. A reasonable reserve also provides scope for an extension of the lighting installation.

A power reserve of 20% is recommended in most cases. If additional spotlights are to be added later, the reserve should be increased accordingly. The power calculation with reserve usually results in crooked values. It is advisable then to choose the next larger power supply unit.

LED power supply calculator

The power calculation of an LED transformer has been described in detail before. It is even easier with the online calculator. Here you can enter the power consumption of all LED lamps to be operated on the power supply as well as the desired power reserve. Alternatively, the transformer can also be determined by entering the LED operating voltage and the total current of all lamps.

LED Power Supply Calculator
The tools in this website are provided "as is" without any warranty of any kind.

What to pay attention to with LED power supply units?

The calculated power is an important criterion when selecting the LED transformer. To find the right supply unit, the following points should also be considered.

LED transformer or halogen transformer?

Sometimes there is the question whether an existing halogen transformer can be re-used when converting to LED. If the operating voltage of LED and halogen lamps is identical, this seems to be possible at first glance. However, it is not recommended, as many halogen transformers have at least one of the following issues:

  • High minimum load → LED lamps remain dark or flicker
  • No constant output voltage → Voltage peaks damage the LED
  • AC output voltage → see next section
If possible, use an LED transformer

AC or DC – alternating voltage or direct voltage?

There are LED transformers that generate AC voltage and there are variants that provide DC voltage at the output. Most low-voltage LED lamps have an integrated rectifier and can be operated with both an AC and DC transformer. However, this is not always visible from the outside. Therefore, the transformer should always be selected to match your LED light source.

On the light source or in the data sheet it is always indicated whether the LED is operated with DC or AC voltage.

Select AC/DC transformer depending on LED illuminant

Dimmable transformers

If the brightness of the LED’s should be regulated, the LED transformer can be connected to a dimmer. But the dimming of LED lamps can be problematic, if not all components in the chain are designed for it. If you want to dim the low voltage lamps on the transformer, both the LED’s, the dimmer and the transformer must be designed for this purpose. Only then there is a good chance that no problems will occur.

The dimmability must be listed in the product descriptions of all components. If a non-dimmable transformer is connected to a dimmer, the light may remain dark, flickering or humming.

Select a dimmable transformer when connecting to a dimmer


Calculating the output power of an LED transformer is not difficult. With the help of the examples and the online calculator you can now dimension an LED power supply for your application. In addition, you know which further criteria are important when choosing an LED transformer.