Like any other app, a VPN will impact your iPhone’s battery life. This is because a VPN constantly encrypts your internet connection, even when background processes like mail syncing are running.
However, I found that ExpressVPN didn’t drain my battery any more than a background app like Netflix or Spotify. If you really need to conserve battery, you can always switch off the VPN connection (although I don’t recommend this if you’re connected to a public WiFi network). Luckily, you can and see how it affects your own iPhone or iPad. It comes with a -day money-back guarantee, so you’re entitled to a full refund if you’re not satisfied.