Tenants haven't paid rent in over a month, and can't be reached by certified mail what's my next step?

Tenants haven't paid rent in over a month, in the lease it states they are responsible for paying their own electric. I had set the date of electric service to be taken out of my name the day they moved in. Upon moving in they told me they would not be able to get the service turned on in there name until a past do amount was paid. These are my first tenants and I felt bad so I agreed to keep it on in my name until they could pay off their past due amount, meanwhile they were paying me personally for their current charges. A little over a month ago they stopped paying everything all together. After 15 days I tried sending a demanding payment or eviction to follow notice by Certified mail. Over two weeks later Iv yet to receive confirmation of delivery and they live next door. What can be done if they won't answer the door to receive the letter? Since I kept the electric in my name yet it states in their lease they are responsible for it am I forced to keep in on as to not perform a self eviction? I'm trying to follow the law yet I can't get confirmation of them receiving notice which is making this process take longer costing me even more. Any help is greatly appreciated. Thanks.