Things are much more complicated and much more subtle than you think. First of all, the warming is not so much. Maybe 1 degree C over the last hundred years and just 1 or 2 more in the next 100. Yet many people believe this is a lot of warming depending on how it affects the world.
The problem is not so much the warming itself, but the believed cause of the warming which is thought to be caused by humans burning too much oil (as well as coal and natural gas). These fossil fuels were trapped in the Earth for hundreds of millions of years. It took hundreds of millions of years for the oil and coal to be formed from dead plants. Now it is all being burned in just 200 years. That seems a bit crazy to me personally. And also it is adding all this carbon to the atmosphere that was locked in the earth for about 300 million years. Somehow this feels like a bad idea.
I don't understand what you mean by "Why do we need the extra solar energy?". Do you mean: Why should we switch from burning coal and oil to using photovoltaic systems to create electricity? The reason for this is simple. Coal and oil are really stored solar energy. Such fossil fuels are created originally by plants growing 300 million years ago capturing the sun's energy to grow. They grow and die and more grow and die for millions of years and they are covered and compressed by rock and the changing shape of the Earth's crust to form deposits of coal and oil below the surface. This takes millions and millions of years. Try to imagine how long one million years is. Then try to imagine that times 300. When we burn coal and oil, we are burning 300 million year old plants, plants that caught the sun's energy back then and stored it.
Rather than burn all this excellent stored solar energy from millions of years ago, a smarter idea is to just catch the sun's energy today, more directly with solar panels. This way, we can save the coal and oil for more important uses, either for burning in the dark when there is no sun, or for pharmaceuticals and plastics and things that we cannot do very easily with the sun's direct energy.
Heat in the atmosphere from the sun also causes wind, which can be used to power windmills to create electricity. The sun's energy also heats the ocean and lakes causing evaporation, which then becomes rain, which then fills lakes and rivers, which can then power hydro-electric generators. So there are numerous ways we can convert the sun's heat to electricity more directly than burning oil and coal.
I certainly agree that global warming is happening, but I personally do not take the view that global warming is necessarily a bad thing. I think humans can handle a warmer planet and I think the changes will be slow enough, taking several generations to occur that we will be able to respond to the advent of a warmer planet. What bothers me much more is the VERY wasteful use of oil, gas and coal today by humans. We are simply burning it all up. These wonderful substances took hundreds of millions of years for nature to create. They are very precious because these fuels hold so much of the sun's stored energy from the past. Now humanity is poised to basically use it all up in just a couple hundred years. To me, that is the stupidest thing that humans have ever done.