Hello everyone, I am at a loss on how to figure out my estimated taxes for 2015. I've never done this before.

I'm the sole income earner in a family of four (my husband is a full time student). My W2 salary is $45,000 a year and there is a possibility for summer work with my job, which could add up to $4200 more in W2 income.

I also started two 1099 positions. The amount of work I receive through these positions is variable. I didn't receive any income from these positions during Q1 so I did not file quarterly taxes. However, I am thinking that I will need to file quarterly now due to the amount I am earning. I'm up to $2000 so far, so I am thinking I'll likely reach $5000 or more.

My questions are:
1. Do I pay quarterly tax based on what I make from my 1099 position that quarter? Say I make $3000 in Q2, do I pay the 15.3% SE tax and then 15% income tax on the $3000, and then if I make $1000 in Q3, pay the calculated tax on the $1000? Or do I have to estimate for the whole year? I really have no way of knowing how much I will earn.

2. If I overpay quarterly taxes, will it be refunded after filing the annual tax return?

3. What type of penalty is there for not paying enough in quarterly taxes? I'm just curios as to how expensive of a mistake I could make.

I had more questions, but they are escaping me at the moment.