Do you know what power rating your lamps are?
Halogen lights are typically; 20watt, 35watt or 50watt each
So guessing at a few calculations...
If you have 20 lamps you could have around 400watts to 1000watts of power consumed.
Or for 1 unit of electricity you will get between; One to Two & half hours worth of use.
And some common LED power ratings could be around 4, 5 or 6 watts each...
So with 20 LEDs you could have around 80watts to 120watts of power consumed.
Or for 1 unit of electricity you will get between; Eight hours twenty minutes to Twelve and half hours worth use.
So once you have accounted for the supply and fitting of replacement lamps...
the running costs can give significant electrical consumption savings...
BUT another factor that can cause issues if if the lamps are controlled by Dimmer switches..?
LED's and dimming can be a lot more hassle than just normal off/on switch..
As you need to replace switches as well!
:coffee