Google Maps API and calculations

I would need a calculator to calculate the cost of PV solar on residential homes.

First step is for the user to enter house value and adress.
Second step is to mark the edges of the roof in google maps to get the area automaticly to the calculation.
Third step is to show the user the full calculations
Forth step is to contact us for order

Here is a similar example
hemsol.se/solcellskalkyl

Attached is more details as image and excel (same)

The project will run on empty subdomain to main page on wix.com. So ftp, php, javascript and mysql-database is possible.

The end result must look good for the enduser both on computer and mobile.