Okay, thank you very much. On Windows Duplicacy barely uses anything (less than 100MB). As for the battery concern, I was thinking of using an older phone that can be permanently on charger.
Actually everything started with the idea of a device that is permanently online with a backup connection (LAN/WiFi+4G/5G), that has a battery in case the power is out, enough storage, is low-powered (less that 10-15W), has sufficient RAM/CPU power, plus a way to manage it - to be my backup hub.
Initially I was looking into mini-PC, Raspberry Pi, NAS, so on… until I realized that what I actually need is a phone with a big SD card. My data from PC and actual phone can be synced with Syncthing to that phone and then it can also backup everything to the cloud with Duplicacy. So I have to create a Duplicacy APK and see how well it works.