A 240 Watt Solar Panel Can Provide Less than Ten Watts Continuous
Here's the Math. Usage is part of the problem.

         When I originally wrote this article, it seemed straightforward. But then I began to realize the losses were so great, so numerous, so nebulous, that the article became confusing. So bear with me, I'm going to assume a 240W panel with 10% solar efficiency to simplify things. This makes the "effective" surface area about 2.4 square meters. And I will summarize at the end for those of you, like me, that get lost in this maze.

But let's go back,

Assuming a new panel, and multiply by 4, full gonzo on a bright day, and again and granting a generous 15W continuous, we might say 60 watts were salvaged out of 2400 watts of sunlight, about 2.5% efficient, all the rest has been turned into heat, talk about your global warming, being down wind of a solar electric farm can get mighty toasty, killing animals and plants living below that expensive expansive umbrella from hell (even more toasty than a solar thermal farm where some losses are visual but efficiencies can approach 50% instead of the less than 3% of solar electric). Remember those 50% battery, 40% overcast, and 20% inverter losses. Not good. Compared to worrying about the efficiencies of the tungsten light bulb where all the heat is captured as winter heating fuel, solar electric starts to sound pretty darn lame. And note, as I mentioned at the beginning you are now down to maybe 9 watts, barely enough to light a night light 24-7.
For humor,
suppose you wanted to heat a home. With overcast and inverter losses you might expect 6 hrs at 144watts or 860 watt-hours divided into 300,000 watts-hours for a large home (that's 24 hrs x 12,000watts), you would need between 300-500 solar electric panels just to do the job, no battery losses but you would have to rely on great insulation and a fantastic thermal reserve system, like a swimming pool, to absorb and administer the load. Solar electric panels won't do it but you can do it with evacuated tube solar thermal panels where actual efficiencies approach 80%. Now, of that 2000 watts of sunlight, 1600 watts become available for use. With a 2:1 reflector, 3200 watts. Now if you run that for 6 hours you have 18,000 watt hours and you can do the whole job with 20-40 panels, still a lot of panels but not so much that it breaks your pocket book or brings about Armageddon due to heating. Sadly, when winter really sets in, it often will deny you even that meager amount of sun power and an alternate form of power like wind starts to sound much better.

So what ARE they good for?

     If you live in the desert southwest or on the side of a mountain in Pakistan, and you only use the panels to run a machine shop or supplement line power then this scenario does not hold. This is why people usually employ solar panels. you gain back almost 50% by not using batteries, another 40-50% by avoiding overcast locations, and another 4 fold by not distributing over a 24 hour period (144+ watts continuous), but you still have the weathering and the inverter losses. In some applications, this might be appropriate but you are probably not going to heat a 12kw house in winter with solar electric, that's a whole different horse and buggy! You might keep your animals or greenhouse watered in winter, provide light and water to that cottage on an Island in Rainey Lake. And there are more efficient way to store the energy like selling it back to the power companies, reverse metering.

        Initial--->panel eff.--->6/24 dark  --->overcast--->inverter ---> battery loss--->aging
Watts 2400W ---> 240W ---> 60W ---> 36W---> 30 ---> 15 ---> 7W
% eff. 100% ---> 10%---> *2.5%---> 1.5%---> 1.2%---> 0.6%---> 0.3%
*Not a loss but a serious problem with availability
Rewritten for clarity 3-27-2017.