If you do not want to rent/buy/borrow a generator use a star drill to roughly outline the opening and then start to work with a chisel and drilling hammer.

An impact drill and some carbide bits would make shorter work of a large number of holes, reducing the chisel work needed.

Thank you so much Brickeyee!
Some good hints here,...think I should invest in a generator, can you recommend any?

Or you can rent one of these along with the drilling advise given for the corners and the bottom where the saw can't get to.

Here is a link that might be useful: saw

i have used the chisel and drilling hammer approach when I was younger and it works well, but it can make for a long day. At this point in life, I value my time more, so a gas-powered rental saw would be my tool of choice, particularly if I knew I would encounter rebar. Below is a link to an article on cutting concrete which has some interesting observations.

Here is a link that might be useful: Cutting concrete

Thank you all!
I will take a visit to the rental and see.

Will this unit do for the job, running a hammer drill?
I know...have not much to spare, it should run up to a 10 amp motor.

Here is a link that might be useful: 1200W Generator

"it should run up to a 10 amp motor."

Motors have a large current draw at start-up, but if you get the drill turning before starting cutting it might handle 8 amps.

most of the generator power ratings are like Sear's "maximum developed horsepower."

A pure fantasy.