I am confused as to what being a new house or not has to do with the guest complaint, but your description does state this is a new house. Your pictures are stunning! I agree with Letti (unable to tag for some reason) that you should not give a refund as you have a strict cancellation policy. It will not "buy" you a good review. When you get her review, provide a short response that the guest left early.
I did notice that you have stated there is a security deposit that is taken and refunded less the cost of power. That is NOT how Air BNB security deposit works, so you might edit that portion. If you are charging an additional amount for the usage of power, then you must disclose it on the listing. You would then use the send or request money function on the reservation to seek the fee.