Claimed 30 mW may not seem a lot, but most of electronic sensors (even displays) are only used for short period of time. You continuously collect charge over ti

Claimed 30 mW may not seem a lot, but most of electronic sensors (even displays) are only used for short period of time. You continuously collect charge over time, often in a supercapacitor, while most of device circuitry is dormant. At some time the device wakes to take measurements, transmit data over some wireless technology, schedule next wake-up and then goes back to sleep. Even if you need measurements every minute, 30 mW is plenty for these kinds of devices to do their work.