Google has started selling upgrade packages for its already generous free storage allowances.

The web giant began rolling out extra storage late last week that users can purchase to use across several Google products.

Initially the storage will be available across Picasa Web Albums and Gmail, but will be usable by other applications like Google Docs & Spreadsheets.

There is currently no way for users to use their free storage space from one product to store content from another.

But with the paid-for extra storage from Google, that space will be shared by both products as needed when users upload or receive more data.

The new storage plans start at $20 a year for 6GB, $75 for 25GB, $250 for 100GB and $500 for 250GB.