Isotope power supplies offer solutions for long-lived 100 years, low-power 100 We energy sources. The energy density of nuclear batteries uniquely serves applications for sensors or communications nodes that are required to last the lifetime of infrastructure. Efficiencies less than 10 are typical for either direct or indirect energy conversion of radiation to electric current. A tritium 3H beta-source 12.5-year half-life encapsulated in a phosphor-lined vial is coupled directly to a photovoltaic PV generating a trickle current into an electrical load. An inexpensive design approach is described consisting of commercially available components that generate 100 We for next-generation compact electronics and sensor applications. A total of 15 We electrical power is measured from individual 20-Ci tritium cassettes 80 cc using gallium arsenide GaAs PVs. A compact radiation sensor 400 cc has been designed and built to operate during long-lived missions. A low-power sensor architecture is described and implemented that uses microprocessor-controlled sleep modes, a judicious choice of low-power electronics, and a passive interrupt driven environmental wake-up. The low-power early-warning radiation detector network combined with a long-lived isotope power source enables no-maintenance mission lifetimes dependent only on the half-life of an isotope of choice.