What It Is
Pella ThermaStar is a basic, inexpensive vinyl new-construction window available only at Lowe's or from qualified Pella dealers. Prices range from $84 for a 24"x24" slider to $240 for a 96"x48" slider window.
While the ThermaStar line includes sliders, single- and double-hung, casement, awning, and bay and bow windows, generally you will find only sliders and double-hung windows available on the shelves at Lowe's.
All ThermaStar windows are double-paned and have argon gas between the panes. White is the only available color.
New-Construction, Not Replacement
These are not replacement windows. Rather, they have an integrated nailing fin on the outside which allows for you to nail the window directly to the house siding. By contrast, replacement windows have no nailing fin and attach to the inner window frame.
If you want to DIY install replacement windows, ThermaStar off the shelf at Lowe's will not help you. Instead, you will need to special order replacements. Or you can go online to one of the limited number of outlets, such as Window E-Store.
ThermaStar tends to be a rather tepid offering from Pella. While it offers "window basics," such as double-glazing, Low-E coating, and insulating gas, consumers report issues such as problems with locking mechanisms, poor energy efficiency, and trouble sliding the window sashes.
The chief value of ThermaStar appears to be its availability (grab a window from Lowe's and install it the same day, rather than waiting 6 weeks for delivery); the opportunity for self-pick-up (saving delivery charges); and low prices.
Long ago, Pella could be found at The Home Depot. Eventually, Pella switched over to Lowe's.
ThermaStar still didn't take off right away. But lately, I have noticed Lowe's devoting more and more shelf space to the ThermaStar line.
In 2007, Consumer Reports rated ThermaStar 25 Series windows #16 on a list of the best replacement windows in the U.S.