Now let's choose an example with a provider who charges based on storage on your computer system.

Considering the example in part 4 - an 80 GB hard disk but you want to backup 2 GB.

In this case the provider simply charges you for 2 GB. Compression ratio etc doesn't really come into it for the user....although it does matter to the provider. If you backup 2 GB and the data compresses to 660 MB the provider has got a bargain as they only have costs for 660 MB while charging you for uncompressed storage of 2 GB!

Conversely, if the data doesn't compress then the provider has storage costs of 2 GB as well. In most cases, the most commonly backed up critical data consists of database files, office documents and spreadsheets etc. These documents normally compress very well indeed.

It is left to the reader to consider which type of provider to use based on charging model alone having explained the concepts involved. If you are backing up office documents etc as mentioned above that compress well, the provider who charges based on compressed storage - i.e. the provider in the first example - could end up cheaper.

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