A Close Election: In order to have power, a voter needs to have a good chance of being the deciding vote in that election. This is what polls are good at tracking- the closer an election is, the more power voters have in it.

Influence: The person being elected needs to have the ability to influence policy in a meaningful way. Executive branch folks (IE: A President or Governor) always have significant influence in policy implementation. Legislators have more influence when a legislative chamber is balanced, and less when they are part of a big majority or tiny minority. The more influential the elected person, the more power the voter electing them has.

Impact: There are some elections that impact more people than others. A vote for a governor can impact everyone in that state- a vote for a senator can impact the whole country. This can give certain elections, and the voters in them, more power due to their high impact and smaller voting population. A good example would be the Senate race in Montana this year- the balance of the US Senate (impacting over 300 million people) may rest in the hands of the just over 1 million residents of Montana.