Why is it important to use a suitable transformer?
LED-transformers are needed when the LED's voltage is 12V or higher. The reason is that the public power supply in Germany provides a current with a voltage of 230V. As halogen lamps have a very high consumption LEDs are much more popular. However, common transformers need a minimum load which LEDs do not provide. That's why halogen lamps cannot simply be replaced by LEDs. The transformer would not recognize the new load and therefore work incorrectly or not at all. Indicators for that are for example that the LED does not shine at all, flickers or starts buzzing. Some producers do not allow the use of common transformers with LEDs. That's why suitable ones should be bought.
The difference between DC and AC LEDs and which transformers are suitable:
Common LEDs work with DC and AC voltages. That leads to the question which transformer is better. Firstly you should know that every LED is equipped with a rectifier that converts an AC current into a DC current. The original voltage of 12V then changes to 14V. This residual energy is emitted as heat and therefore unnecessary and unsuitable for an economical use. It can also decrease the LED's life-span or even destroy it instantly. For that reason it is very important to use a 12V DC transformer to run an AC-LED. This ensures longest possible operation.
How do I recognize the right transformer?
It is very easy to evaluate the necessary watts a transformer must have to run a certain illuminant. However, you should know that most producers round down this number so the actual consumption is higher than it is given on the product. Therefore it is recommended to use a transformer which has a higher performance (W) than the LED. This prevents the LED from falling out and allows the installation of stronger LEDs later. Anyway, more watts do not automatically mean more consumption.