Investors might want to bet on TransAlta (TAC) , as it has been recently upgraded to a Zacks Rank #1 (Strong Buy). This upgrade primarily reflects an upward trend in earnings estimates, which is one of the most powerful forces impacting stock prices.The Zacks rating relies solely on a company's changing earnings picture. It tracks EPS estimates for the current and following years from the sell-side analysts covering the stock through a consensus measure -- the Zacks Consensus Estimate.Since a changing earni ...