The tests have been conducted in both New York City and in Chicago using the HTC One M7 and M8, and the Samsung Galaxy S4 and S5. Each test was done while the devices had "at least 3 bars of service", with Bluetooth, NFC, and Wi-Fi turned off, and brightness set to 150 nits. The results are pretty stark. Across the board, the T-Mobile variants of the devices have had better battery life, sometimes by quite a sizeable degree. The biggest difference was found between T-Mobile and Verizon on the Galaxy S5, where the Verizon model clocked in at 7:30 of battery life, while the T-Mo model scored nearly 11 hours. That's a three and a half hour difference.
Even looking at the average battery test results over the past 12 months and the past 36 months, the results held. T-Mobile devices had the highest average battery life, while Verizon models came in last. AT&T actually showed improvement, as it was in the bottom with Verizon over the 36 month span, but had climbed up close to even with Sprint in the past 12 months.
Unfortunately, it is unclear exactly why this is happening. Laptop Mag speculates that two of the most likely factors are network efficiency and possibly even carrier bloat. There is certainly a correlation between the amount of bloat and battery life (Verizon has the most bloat and the worst battery), but that doesn't necessarily indicate causation.
source: Laptop Mag