  Advances in cloud computing have simplified the way that both software
development and testing are performed. This is not true for battery testing for
which state of the art test-beds simply consist of one phone attached to a
power meter. These test-beds have limited resources, access, and are overall
hard to maintain; for these reasons, they often sit idle with no experiment to
run. In this paper, we propose to share existing battery testbeds and transform
them into vantage points of BatteryLab, a power monitoring platform offering
heterogeneous devices and testing conditions. We have achieved this vision with
a combination of hardware and software which allow to augment existing battery
test-beds with remote capabilities. BatteryLab currently counts three vantage
points, one in Europe and two in the US, hosting three Android devices and one
iPhone 7. We benchmark BatteryLab with respect to the accuracy of its battery
readings, system performance, and platform heterogeneity. Next, we demonstrate
how measurements can be run atop of BatteryLab by developing the "Web Power
Monitor" (WPM), a tool which can measure website power consumption at scale. We
released WPM and used it to report on the energy consumption of Alexa's top
1,000 websites across 3 locations and 4 devices (both Android and iOS).
