These LED bulbs worked well without any need to to replace the existing 50w transformer, and the light given out is very close to a 50w halogen bulb. How other suppliers can claim that 4w or 5w LEDs are equivalent replacements beats me, but these are fine. Only thing to be aware of is that the rear of the bulb is much larger than a 50w MR16, and so may not fit into the existing bulb holder. In my case I ordered 3 to see if they fitted OK and/or needed a replacement transformer - in the event the existing transformer works fine with these bulbs, but the large size of the rear of these bulbs meant that they didn't fit into my existing main light holders, but they did fit as replacements for my bathroom lights which have a larger holder. I was therefore able to use them in the bathrooms but am still looking for a suitable smaller size replacement for the other MR16's in my flat.