Those numbers sound wrong.

I have 7.8kw in Canada, and if I paid out of pocket payback would be 6-7 years.

We pat $0.13 per kWh from the grid, get a one for one credit on anything we feed in. System makes 7.8Mwh in a year.

What are your friends numbers?

Grid price is also pre-approved to increase not less that 5% a year forever, so it will only go in my favour.

In spain he pays more than that on average, and only gets €0.04 for sold KWH. If it was one-to-one credit offset, as you have in Canada, it would certainly be very profitable.