How does steem power work ?

How do i sometimes earn 0.1 steem with 30 upvotes and other times i earn 2 steem with 3 upvotes.

This is because of the steem power of the users who upvote your post. You see , not every steem user has the same amout of steem power. Some users have the power to upvote for 1 steem and other users dont even give you 0.01 steem.

How do i get more power then ?

Posting high quality and unique content on steemit.com and voting for other high quality content that gets published, it takes time and effort, but the rewards you get in STEEM POWER directly increase your influence and the STEEM DOLLARS (SBD) rewards you can convert to STEEM and then use them to Power Up converting them to STEEM POWER.

Other ways

Buying Steem Power helps you raise your voting power. It is an investment in Steem for a longer term.

The most simple way to go about this is to use your build in Wallet at SteemIt dot com. There you will find a button that says: [BUY STEEM OR STEEM POWER] After that a pop-up will appear from where you can choose what to buy, Steem Power or Steem. By default this is set to Steem, so you need to click Steem Power first.

Then you have the choice how to buy the Steem Power. The build in exchange uses the @blocktrades service to pay with Bitcoin {BTC}, Ethereum {ETH} or Bitshares {BTS} to get Steem Power {SP}.
You can choose which one with the 'Deposit using...' dropdown menu, this one defaults to Bitcoin. The specific data will be shown where to send the payment to, with Bitcoin and Ethereum that will be an address.

NOTE: For Bitshares there will also be a Memo presented that you need to use.

There are more ways than this to buy Steem Power. It is possible to use other 'coins' also using ShapeShift dot IO, for example. But that would be a little more complex than this.

I hope this was helpful to some people , and don't forget to upvote this post so both our SP goes up.