I am not sure if I am preaching to the converted (and informed) but I thought I would let you know how I got on, replacing MR16 halogens (12v a/c) in our kitchen and dining room with LEDs.

The brightest LEDs I could find were Cree 5w MR16, claiming about 200lumens, realistically about the same as a 20w/35w halogen. I fitted them in 2 arrays, one of 4 lamps, and one of 9 lamps, so in theory a 'demand' of 20watts and 45watts respectively.

However, it was not easy to power them....

The first transformers I used were 'electronic' 20va-60va 12v a/c, they overheated in about 10mins, causing the LEDS to pulse and flicker.
So I got some dedicated LED drivers, 12v d/c, as the LEDs I had are supposed to be capable of handling a/c AND d/c. These just caused the LEDs to flash on/off .........
Finally I went 'old school' with some cheap 50va 12v a/c mains transformers, the toroidal type.
The array of 4 worked fine, but the 9 LEDs pulled so much current that the transformer became a little warm for my liking, so I doubled the transformers supplying the 9 LEDs, and all is fine.

These transformers: http://www.ibl.co.uk/?5002.00 cost about tenner or less, so coupled with the cost of the LEDs, I worked out a payback time of approx. 2 years, and we should be saving from then on...., coupled with the increased life expectancy of the LEDs ?

What I have yet to do is put a clamp meter on the transformers to see just how much current the LEDs are pulling in total, but I am sure it is a lot less than the 500+ watts of halogens were!

Cheers, and Happy New Year to you all

Be warned about claims for LED lifetime.

I have 4 GU10s on test and two have now blown. I doubt they have even done 1000 hours and they were supposed to do 30000 hours MTBF.

Just curious about the MR16s. Were they rated to work off12V DC so that they have built-in current limit inside the MR16 bulb? The GU10s I used worked off mains and I suspect it is the built-in mains switcher that failed not the LEDs themselves.

Interesting point about the heat, so I checked with an IR thermometer.

4 lights in the kitchen ceiling. The GU10 lights are protruding (i.e. not inset) and are made of metal coverings (so good heat conduction and dissipation). Two lights are halogen (because the misses complains about low light from the LEDs) and 2 are LED. Note the temperature measurement may be inaccurate because the IR is calibrated for a standard black body rather than shinny metal surface.

Measurements on the metal:
Halogen 41 deg C - ouch to hold
LED 23 deg C - easy to hold

So temperature is not the reason to fail in this case. It is a great pity as a 30000MTBF would make LEDs a much cheaper option to buy and run than halogen, but not if they do less than 1000 hours in practice.