We are sorry to inform you that, despite what you might find when running a quick Google search, there is no portable version of the spreadsheet software. If there is any, it is most likely to be made by a user on its own, so it has neither the approval of Microsoft nor official support from them. Either that or someone with bad intentions is trying to trick you into installing something on your PC that will bring them profit. We insist: no matter what you find in Google, these portable versions, in the vast majority of cases, are not legitimate.
You can indeed use the mobile versions of Excel, or Office 365. The latter, being cloud-based, you can use it on any machine without installing the software, although you need to pay a subscription to use it. In that sense, you could also turn to the cloud version of Google Docs and its office suite. However, if you want to stay within the Windows environment, you will have no choice but to use LibreOffice or OpenOffice, which do have portable versions.
Portable versions are much lighter than full versions, can be taken anywhere on a flash drive, and like cloud-based versions of office software, do not require installation for use on any machine. While a large number of programs do offer the user the option of a portable version, Microsoft programs do not. We are sorry to be the bearers of bad news.