From the paper: &quot;For instance at a distance of 2 feet the battery charging system has 100 µW available compared to 10 µW at 10 feet.&quot; The Popular Science and UW Today articles both completely avoid numbers. But the numbers are vital, and if you do a little fermi math, you&#x27;ll discover that&#x27;s not enough to compete with watch batteries.<p>(The first watch battery I found with numbers on it had 240mAh*3V=720mWh, so one such battery could in theory provide 10 µW for 7.2e4 hours or 8 years.)