[Bassel]

Haloalkanes undergo SN1,E1 and E2 when the yard tertiary.
Tertiary means more steric effect and thus more potential energy leading to decrease in stability of a compound. So in the carbocations of SN1 and E1 how come the tertiary carbocation is more stable ?

[Schrödinger]

Tertiary also means more stability of the carbocation through charge stabilisation. S_N1 and E1 both being mechanisms where the carbocation is the intermediate in the rate determining step, are favoured by extra stable carbocations.

[TheOrganic]

Haloalkanes undergo SN1,E1 and E2 when the yard tertiary.
Tertiary means more steric effect and thus more potential energy leading to decrease in stability of a compound. So in the carbocations of SN1 and E1 how come the tertiary carbocation is more stable ?

Remember, a carbocation is planar. A carbocation has a full positive charge. SN1 and E1 reactions occur in polar solvents that can stabilize the charge on the carbocation.

I hope that should help you answer your question by yourself.