Batteries are meant to degrade over time, which means after a while they won't hold the same battery capacity of which they used to.
Apple recommends that your battery health should reach 80% after two years.
You mention your phone is new. Is this a device you bought new or bought second-hand from someone? If it is a new phone, you should Contact Apple Support, as this isn't right, however, if it is an older device this could be common.
You might also find this helpful as it provides you with useful information on maximising your battery.