Plutonium 238 would be your best bet for such devices, if only because it's halflife is 87.7 years. Strontium 90 might also work, with a halflife of 28.8 years.
However plutonium would not be easy to put in every house since you have to make it in nuclear reactors. You get about half a kilowatt per kilogram of it, I don't think production of it could scale that high.
Strontium is naturally occurring and relatively common. Sr 90 is a trace isotope though. Off the top of my head I would suspect you would be facing similar challenges that separation of Uranium isotopes face.
However plutonium would not be easy to put in every house since you have to make it in nuclear reactors. You get about half a kilowatt per kilogram of it, I don't think production of it could scale that high.
Strontium is naturally occurring and relatively common. Sr 90 is a trace isotope though. Off the top of my head I would suspect you would be facing similar challenges that separation of Uranium isotopes face.