The watch does have a battery that is charged by solar power. The watch has a standby feature for the times that you do not wear the watch. I wear the watch three or four times a week and only have seen the level go to med. once. Only because i keep the light auto feature on where every time you look at your watch in dim lighting it will light up for you.
I wear a GW-300 that I have used for nearly 25 years and have not had to worry about either a battery nor a watch strap. As long as you feed it enough sunlight, you may get tired of it before you have to worry about it. If they are making them as well these days, these watches are AMAZING!