Our solar and wind power monitor needs the ability to save historical data so that we can track watt hours and amp hours daily and weekly. A SD card provides enough capacity, and allows us to easily download the data to our PC. There is a nice data logging shield available for under $20 that fits the application nicely.
Data logging shield for Arduino makes saving data to files on any FAT16 or FAT32 formatted SD card really easy. The included Real Time Clock timestamps all your data with the current time, so that you know precisely what happened and when!
For a power monitor idea, see http://www.mcqn.com/weblog/mazzini_monitors_its_first_appliance
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