The Internet of Things (IoT), while still in its infancy, is shaping the future of many industries and will also impact daily life in significant ways. One of the key challenges of moving IoT devices from concept to reality is to have long-lasting operation with tightly constrained energy sources, and thus extreme power efficiency. IoT devices such as sensors are often deployed on a massive scale and in places that are usually remote and difficult to service regularly, thus making their self-sufficiency essential.