Easiest way to log data

Hi everybody,
I'm sure this question has been asked several times, but I didn't find a direct answer.

I want to measure the power consumption of a device over time. I have already programmed my ArduinoUno to read values from sensors, now I need to store data somewhere in order to read it later on a computer and perform some statistics.
I read sensors values about every 20ms across one month period. I'd like to produce one single file like a csv:

elapsedtime;sensor1value;sensor2value;sensor3value;...;sensorNvalue

Assuming that each line is about 50bytes length, the output file size will be about 6GB.

[Of course if needed I can set up to one measurement/s, so the output file size will be 124MB which is much more reasonable.]

Anyway, my question is: what is the cheapest and easiest (:D) way to do that in your experience?

Thanks a lot.
Bye

SD card module. Maybe something like this. For logging over such a long period a battery backed real time clock module might also be useful.
http://www.ebay.co.uk/itm/DS3231-AT24C32-IIC-module-precision-Real-time-clock-quare-memory-for-ArduinoT5-/400679939769?pt=UK_Computing_Laptops_EH&hash=item5d4a62aeb9

SD card is definitely the way to go. Also are you saying that file-size is an issue? If so you should not use a cvs then instead what you could do is use packed bits binary data. Are you using 10-bit ADC readings? If so instead of storing each reading as two bytes you can fit 4 readings in 5 bytes. There are multiple ways to do this however the easiest way in my opinion is to.
Store the LSBs (bits 7-0) for 4 readings then store the MSBs (9-8) for the 4 readings.
You could also
Store the MSBs (bits 9-2) for 4 readings then store the LSBs (1-0) for the 4 readings.
I think storing the LSBs first then the MSBs is the best option.
You could then convert it to a cvs on your computer using a simple script. If you can't do that for yourself I could write one. It would take only a few minutes to write.

Thanks a lot to both of you, I believe your solutions fit perfectly in what I need.
@Mr_arduino thanks for the trick, clever idea, and programming it is not an issue (the script to convert fileformat is quite straightforward).

Thanks again! :smiley: