If we talk about a series of molecules: ethene, prop-1-ene, but-2-ene, etc., we see both hyperconjugation (which increases stability) and steric hindrance (which decreases stability). So which one is more powerful i.e. which of these molecules will be most stable?
It's a contradict to explain the stability of substituted alkenes by using hyperconjugation and steric hinderance both. I think, it is more better to consider heat of hydrogenation factor in case for stability of substituted alkenes. The stability of substituted alkenes is explained by Hyperconjugation factor while the stability of geometrical alkenes (cis-trans) is explained by steric hinderance.
The stability of alkene can be determined by measuring the amount of energy associated with the hydrogenation of the molecule. Since the double bond is breaking in this reaction, the energy released in hydrogenation is proportional to the energy in the double bond of the molecule. This is a useful tool because heats of hydrogenation can be measured very accurately. The ΔHo is usually around -30 kcal/mol for alkenes. Stability is simply a measure of energy. Lower energy molecules are more stable than higher energy molecules. More substituted alkenes are more stable than less substituted ones due to hyperconjugation. They have a lower heat of hydrogenation. The following illustrates stability of alkenes with various substituents:
In disubstituted alkenes, trans isomers are more stable than cis isomers due to less steric hindrance because in trans bulky groups are in opposite sides so less repulsion between them as there is more repulsion in case of cis due same side of bulky groups. Also, internal alkenes are more stable than terminal ones. See the following isomers of butene:
Overall stability : trans-but-2-ene > cis-but-2-ene > propene > ethene