Try a Google search on [ LED white replacement MR16] or [ LED white
replacement GU10 ] for more details. The power consumption is about 2-5
watts per bulb and the light output appears to be equivalent to a 20W
incandescent.

As already pointed out, this is impossible -- no one has
invented a white LED anywhere near efficient enough.
What is most likely to have been achieved is a very narrow
beam which has the same intensity as a very small part of
the wider beam from a regular MR16. If you want a very narrow
beam, that's perfect, otherwise it's tough.

Furthermore, it would be impossible to produce an MR16 white
LED lamp with same output as a halogen one, because the heat
output and therefore temperature rise would be same as a
halogen one, fairly instantly destroying the LEDs.

The makers quote 50,000 hours life, but a recent R4 news
item mentioned that 150,000 hours is more typical for an LED.

Life depends on temperature. You can make an LED last for as
long as you like by keeping it cool enough (doesn't actually
matter much if it's being used or not although using it
obviously warms the chip). If you want reliable operation
beyond 20 years, you would be looking to keep it below room
temperature. However the white ones present a second problem --
deterioration of the phosphor and the plastic package by UV
from the chip. The phosphor efficiency (which is a theoretical
maximum of only 50% to start with and even that is not achieved
in practice) will be very significantly reduced by 50000 hours
use.

sob

Might there be something produced in the next few years which might even
last only, say, twenty years? That would do me :-)

And that was a rhetorical question, a merry quip ...

Mary

--
Andrew Gabriel