I need an equivalent Metal halide lamp in brightness and heat output to a 120W UHP lamp. Would I need a 120W MH lamp or are the UHP more brighter and output more lumens per wattage. Any help appreciated.

I would estimate a 100 watt MH would output the same amount of light as a 120 watt UHP.
But there's more to it than just that. A UHP lamp has a fairly short arc and is more suitable
for use in a parabolic or ellipsoidal reflector. The ballast is different also. And some UHP
lamps run on DC. You trying to retrofit a commercial PJ?

I have read it so far but never understood exactly.... I know some lamps do claim to work in DC but what do you understand by that?... I think it needs to be a switching circuit by force. DC or AC but with an electronically controled driver....

If you plug lets say a 100V DC suply to a DC lamp with no any control at all... then you will probably kill the power source...

I meant a ballast that provides DC, not just straight DC.