The HTC One M8 for Windows is visually identical to the Android powered HTC One M8. It even packs the same hardware including the 2600mAh battery. However it looks like the Windows Phone 8.1 model has a significantly longer battery life.
According to the specifications of both handsets on Verizon's website, the HTC One M8 for Windows is listed to offer 21 hours of usage, while the HTC One M8 with Android will last you 12 hours. That means the Windows Phone model gives nearly twice the battery life of its Android sibling. The standby time between the two is small though, with the HTC One M8 for Windows lasting 15.5 days, while the original HTC One M8 lasts 12.2 days.
So what this means is, that on paper, Windows Phone 8.1 is capable of offering a longer battery life on the same hardware specifications compared to Android. The Windows Phone platform is known for being well optimized, but we would be curious to put these numbers to the test in the real world. It would also be interesting to see how 'Project Volta' which debuts with the Android L release will affect the battery life performance on Android.