You never lose steem, when you power up, it just converts your steem to steem power, which gives you higher rewards for voting on other peoples' posts. You're SP is not used up by voting, but the weighting of your votes will deplete and recharge as you vote and time passes. Your SP would only go down if you were to power down, in which case you are really just converting your SP back to Steem. You would receive installments when you power down, but in total you would never end up with less steem than you put into SP.