This has been the single-biggest surprise of 2015

Hulton Archive

Energy companies have astonished investors with their first quarter earnings.

Overall, companies have passed expectations so much that last week, earnings growth for the first quarter turned positive for the first time since January. 

And according to FactSet's Jonathan Butters, the energy sector is miles ahead, with an EPS surprise rate of 28.7%.

That's not to say that energy results have not been battered. Chevron, Halliburton, ExxonMobil, and Schlumberger are just a few of the companies that reported year-over-year earnings declines, largely because of the oil crash. The energy sector has reported the largest year-over-year declines in first quarter sales, according to Butters.

But what the EPS surprise clearly shows is that investors underestimated what these companies would earn by a huge margin after a jarring collapse in oil prices during the fourth quarter of 2014.

Butters notes that analysts lowered earnings estimates for the sector by 50.3%, the largest decline since FactSet started tracking the data in 2002.

Energy stocks generally had a huge April. Along with the earnings beats, West Texas Intermediate crude oil held above $50 through the month and rallied to $60 early in May. The Wall Street Journal notes that energy stocks on the S&P 500 rose 6.6% in April, the biggest monthly advance in over two years.

Here's the Factset chart that shows energy names were expected to be the worst performers and have ended up outpacing every other sector - at least relative to expectations:

Factset