It's hard to quantify affordable solar power exactly. That's like asking, how long is a piece of string... What is affordable to someone on an average wage in the US, the UK, Australia or New Zealand will not be possible for some poor fellow with no regular wage who's trying to put electric light in his wood and cardboard shack in some third world shantytown. But then I'm not writing this page for him. He's hardly likely to have a PC and internet access, but you never know. He might be able to make use of somebody else's internet someday.
Up until recently, the only affordable solar power available was that which you felt on your skin by lying out in the sun. Your cat knows about it, for cats have always known about it. Cats will always find a snug corner out of the wind and poor weather, where the sun can warm their fur and keep them cosy and warm.
Snakes and lizards rely on the free solar power from nature to speed up their metabolisms because they are cold-blooded animals. They sunbake and lie on sun-warmed rocks when it's cold, and they do this to get warm enough so they can then start moving and hunt for their food . And plants love the sun as well. It what helps them grow by a process called photosynthesis. We want just want to get that energy out of light and convert it to a form we can use — which is usually solar-electric power.
Yet affordable solar power has only really been available to us humans as technology that arrived in the last few decades. It's all thanks to the silicon chip and to space exploration and the solar panels used to power satellites and space stations. Solar-electronic technology for PV panels, charge regulators, inverters and battery storage banks that were only within the astronomical budgets of NASA and the military a few decades back, are now available to ordinary folks like you and me.
We can take that sunlight and get heat or electricity from it, easily and pretty cheaply too.
Affordable solar power for a home just means putting together a useful solar power system without a giant budget. It costs more to set up than mains lighting does, but once you're up and running, your electricity (and maybe solar hot water) is free, until you have to replace a storage battery or other electrical or mechanical component.
In any case, you can start small and add to your solar power system whenever you have a bit of spare cash to invest. The most basic solar electric system can start with one pv panel, a car battery and a couple of 12 volt lights — or very little more than that. It certainly won't break the bank. And I'll get into solar water heaters elsewhere.