Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of the second largest residential solar installer in the country Vivint Solar (NYSE:VSLR) jumped as much as 22% today after announcing fourth-quarter earnings.

So what: Quarterly revenue came to $6.9 million and net loss was $38.1 million. After losses attributable to non-controlling interests, the loss to common stockholders was $6.1 million, or $0.06 per share.  

The numbers were OK, but they don't tell the whole story for Vivint. Fourth-quarter bookings were up 150% from a year ago to 52 MW, retained value grew 153% to $481 million, and cost per watt fell to $2.96. That's significant because it's only slightly higher than SolarCity's (NASDAQ:SCTY.DL) cost of $2.86 per watt last quarter and is coming down faster than SolarCity's costs.

Now what: What's impressive is the growth rate and the fact that Vivint Solar executed so well in a quarter when winter was setting in on the Northeast, where Vivint still has most of its exposure. The cost cuts also show that Vivint is competing well with the industry's leader and could potential match SolarCity in 2015 if cost cuts continue.

At the very least, Vivint Solar's market cap of $1.1 billion (even after today's pop) looks inexpensive compared to SolarCity's $5.0 billion market cap. Vivint's installations of 155 MW compares to 502 MW for SolarCity and Vivint is actually growing faster. On a value basis, I think Vivint Solar is the better of the two stocks and the cost reductions and growth show just how quickly Vivint is closing the gap with its largest rival.