If you don't want to store online: Tape, blu-ray, usb harddisk, nas that only gets electricity when backup is runnning. Preferably a combination of those. Don't

If you don't want to store online: Tape, blu-ray, usb harddisk, nas that only gets electricity when backup is runnning. Preferably a combination of those. Don't use sd/ssd/nvram. Do test your backups though.