With a VPN, your ISP may not know where packets are going, but they can still see packets moving. So, unless your VPN is injecting dummy data to mask all patterns (possible, but not common), your ISP is going to have a good idea if someone is home or not.
Creating useless traffic on a random schedule is pretty trivial.
So does your power company with real-time meter monitoring. Masking that is much harder and would be more expensive if it's even possible.