Can’t I backup without connecting to a power source

If you mean the automatic overnight iCloud back up, no, you need to be connected to a power source. Your phone needs to be plugged in, locked and connected to WiFi. But, as it’s best practice to plug your phone in overnight, it all works out.

Automatically back up with iCloud Backup

1. Make sure that iCloud Backup is turned on in Settings > [your name] > iCloud > iCloud Backup.
2. Connect your device to a power source.
3. Connect your device to a Wi-Fi network.
4. Make sure that your device's screen is locked.
5. If you get an alert says that you don't have enough iCloud storage space to complete the backup, follow the onscreen steps to buy more storage.

How to back up your iPhone or iPad with iCloud - Apple Support

With a Wi-Fi network connection, you can back up your device using iCloud. You don't need to plug your device into a computer to back up with iCloud. The only time you need to plug it in is if your battery is under 50%.

shiru96 wrote:

Can i backup without connecting to a power source

Yes assuming you have enough battery life.