Today, renewables work only with fossil-fuel backup or Today, renewables work only as fossil-fuel backup

I was reading an article from nytimes and had two doubts in this passage:

Where will this gargantuan amount of carbon-free energy come from? The popular answer is renewables alone, but this is a fantasy. Wind and solar power are becoming cheaper, but they are not available around the clock, rain or shine, and batteries that could power entire cities for days or weeks show no sign of materializing any time soon. Today, renewables work only with fossil-fuel backup.

From the line but they are not available around the clock, rain or shine, and ..., I understand that wind and solar energy are not available around the clock, as it may rain or shine at any time of the day. Am I correct in my understanding?

And I feel last line should be Today, renewables work only as fossil-fuel backup. I can't interpret the line as Today, renewables work only with fossil-fuel backup.

Rain or shine is an idiom which means - under any circumstances. Here, it means that wind and solar power cannot be generated in every case i.e. there might not be enough wind to move the wind turbines and/or it might be a rainy day or just night and there's no sun to charge the solar cells.

In the line "Today, renewables work only with fossil-fuel backup", the author intends to mean that since the renewable forms of energy aren't always usable to generate electricity, they need to have a backup source, usually fossil fuel energy to save the day. For example, it could be a rainy week with no sun shining and as mentioned, batteries that could power an entire city for days don't exist, the said city will need a different power source during the time being, i.e. the fossil fuels.

What you are interpreting, i.e., "Today, renewables work only with fossil-fuel backup", has an entirely different meaning and not what the author has intended. (In the above paragraph, the fossils are being used as backup for renewable sources when the conditions are unfavorable and not the other way around where renewable energy is only being used as a backup for fossil fuel energy.