How do I automatically log the battery percentage and time on my Retina Macbook Pro to an excel spreadsheet? (OSX 10.9.4)

Question

Hello! So I own a 13" rMBP, and I'm running OSX 10.9.4.
How do I log my battery percentage with the date, probably down to the hour, preferably to an Excel file?
I want to track the rate at which my battery discharges, so that in the future I can compare the present rate of discharge with the past and see exactly how much my battery has deteriorated by. I'll take averages over long periods of time to account for, e.g. the fact that watching Netflix drains my battery faster than editing an essay. I also want a solution that uses negligible power in the background.
I've spent hours looking for an app that both calculates the rate of discharge and keeps all data, but I haven't found any. Eventually I decided that hopefully it isn't too difficult to write an AppleScript or something that simply logs the system clock and battery percentage in tandem every hour or so.
Thank you for your time!
Herman

Answer 1

What Barney said.
Here is the script to create a CSV text file with
Date
Time
Current Battery Percentage as an integer
#!/bin/bash
batterycsv="$HOME/battery.csv"
export GREP_OPTIONS="--color=never"
# if you want the percentage sign too, exclude the second egrep
battpct=$(pmset -g ps | egrep -o "(\d+)%" | egrep -o "\d+")
datetime=$(date "+%Y-%m-%d %H:%M")
printf "%s,%s,%s\n" $datetime $battpct >> $batterycsv
Output on a fully charged MacBook Air to battery.csv will be individual lines of the following format:
2014-08-09,19:49,100