Solar energy is possibly one of the most efficient energy producing methods for the money its worth. It practically pays for itself in the long run of owning it, and the money saved can go to bigger, more important things in your life. The downside is that it doesn't produce as much energy output as ordinary oil which, on the other had, is more costly anyway. It also is quite expensive to put in , but like I said earlier, it pays for itself in the long run.
I suppose it depends what energy sources you are considering, but off the top of my head at least I remember hearing that internal combustion energy has 20-30% efficiency. While oil consumption for power plants is likely higher than for automobiles, I assume that it is not by much, as energy production in general is not very efficient. I believe the solar cell efficiency is just above 30, so both are rather close, it is more the idea that solar power is cleaner and supposedly cheaper in the long term after initial capital investment of buying a cell panel. Energy efficiency is less relevant than availability in the short and long term.
On how one measures efficiency. Are we talking about energy efficiency? Cost efficiency? What? Now, it is definitely not the most energy efficient as the commercially available (the stuff that is actually worth the cost) is about 20% - 25% efficient. Most other energy forms are more efficient than that. However, with the continuously dropping costs, it is actually more cost efficient than most the alternatives.
If anyone would like to debate this, please shot me a PM.
I definitely support Solar power, and a lot more research needs to be done in that field. But to say it's efficient is a lie. It essentially powers itself yes, and it doesn't consume anything. But it's costly to install, produces little energy in good light, and isn't even guaranteed 24/7 energy source because of one simple fact. Nighttime! This source would still be viable except for the fact that there are limited places that a solar plant can be put in the world. Some of you will say you can put one anywhere there's light, but most places don't have intense enough light to produce the kind of energy that can stop burning of fossil fuels. It's a wonderful idea, and I wish it will work because the sun's energy will be with us forever, for we will not outlive the sun. But right now it is extremely inefficient for it's two values. Clean, and free energy source.
As this pointless debate continues, Id like to mention that none of you have provided proper scientific basis for your answers. This in terms simply means that both parties are not aware of what on earth is going on here. Please read about this area more. All the best, X.