Online electricity and gas rates calculator

Hi

I'm looking to make a consumer facing online calculator which will be embedded on another site. I'm flexible on which technologies will be used.

1. The calculator will show a number of fields where the user has to enter various values such as amount of electricity and gas used
2. Using an excel spreadsheet that I'll provide (I'm not able to share this until you're chosen for the task) it will then calculate the amount due. This is a fairly complex spreadsheet so this will be the trickiest part.
3. It will then display the amount that is due
4. I need a way to alter the way the calculation is done in part 2 - i.e. if the electricity rates change

I'm flexible on which web technology is used - the front end needs to look good on all devices and load quickly.

Thanks,

Andrew