Sony include a 1910mAh capacity battery in with the handset, which is a decent-sized unit, but still 10-percent smaller than most of the big name phones in the same category.
In a continuous web browsing test on a Wi-Fi network we managed just short of four and a half hours of use before the battery went flat. Weirdly, the result for continuous 720p video playback came out at about the same, even though we'd expect this to be a few hours longer. Perhaps this comes down to Sony's Bravia Engine and the extra juice it needs to clean up the video images on the fly.
In normal, day-to-day use, the Acro S managed a decent work day on a single charge. Though it needed a charge each evening, we didn't feel worried about running out of power before we returned to the charger each day. That said, our 'normal' use doesn't include too many phone calls, and motor-mouths may find their mileage varies.