Led curcuit

[markus1234]

Hi,
i wathed eevblog 708 where dave debunks some free energy curcuit.
He says that it's actuall "piss poor" and only 50% efficient.

So for learning porpuse, what aould be the (nearly) ideal led curcuit to power 23 leds? ( in terms of efficiency)

thanks in advance,
markus