The PV-3 is a three-channel Solar Irradiance DC Power logger.  Setup the Logger using the Electrosoft software, then fit it to the points that you're going to measure.
 
This logger is designed to be installed by a competent person (generally an electrician or engineer), and if isolated test points are available can be fitted without power interruption.  Once you're set up, just press the Start button to start logging!  The onboard indicator will flash Green when logging.
 
When the logger's finished recording (or you've captured enough data!) just disconnect from supply, hook it up to your PC and download the information.  Electrosoft will let you view the data immediately on-screen and save or export for later analysis.