It mainly covers your base consumption.
Profit you make is the savings you have because you have to purchase less electric energy from your provider.
Typically pay for themselves within 3-4years for such an installation on south facing balcony in my region.
The rest of the 20years of its lifetime is just free money after that.
I have a two panel installation, saving me ~120€ per year.

you know you are already on the internet, you could do a google search and learn about these plug in balcony solar kits yourself. your browsers search bar is literally right there.

the point of these balcony solar kits isn't to make fucking money, or to put electricity onto the grid. the point and how they work is your household appliances will use the electricity from the balcony solar first before going to the grid to get some when needed.

electricity is like water, it's going to take the path of least resistance and go from high pressure to low pressure. pressure in this case is voltage. that should sound familiar if you paid any sort of attention in highschool. these balcony solar kits push just enough higher voltage out than the grid so your appliances like your fridge, lights, tv, etc are going to get the electricity from your balcony first. it's only when your load is too much for the solar kit to provide (load as in the amount of appliances or devices wanting power) that it then takes from the grid. it's at that point you are now being charged for electricity.

if your house is not using any electricity or using less than what your plug in solar balcony kit provides, yes it feeds that back into the grid. and if your agreement with your power company allows it, you can get credits towards grid use electricity, which is then used when you get power from the grid instead of being charged money. so really depending on your usage your 'cost' is the installation of the balcony solar kit.

and it's that feeding back into the grid which is the dangerous part during a power outage. not for you, but for the power company crew who has to go work on the lines during either a scheduled power outage or an unscheduled one. if your balcony solar kit, or several from your neighbours is pushing electricity into the grid then crews can't work on them because they are live. THAT's why these kits have a kill switch and don't work in a power outage, because you can't stop the electricity from flowing from your home back into the grid, unless the grid is on.

the POINT of these balcony solar kits isn't to keep the lights on during a power outage. it's to lower your electricity bill. if you want to keep the lights on during a power outage then you get solar with a backup battery. it's that simple. but these solar kits don't come with a battery. that's the point. they are much cheaper than a full solar off grid kit. and allow for people who are renting where they can't install full solar. especially someone in say an apartment or a condo who want solar.