A 240 Watt Solar Panel Can Provide Less than Ten
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,
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.
understand that the panel only receives optimal sunlight for less
than 6 hours per day on average, especially in winter months when the
day is short. This means that over a 24 hour period you can expect
that 6 hours of sunlight to provide 6/24 or 1/4 as much power as if
sunlight was continuous. One fourth of 240W is on average of 60 watts when spread
over the 24 hour period. This is not an efficiency loss but it is a
Second, over most of the US there is also a 40%
overcast, leading to yet another 40% reduction in power, we are now
down to 36 watts.
Third, and now comes the
tricky part, the actual efficiency losses. Most inverters, for
example, are less than 80% efficient, converting DC to AC, another
20% loss, now down to less than 30 watts.
in addition, battery losses, to
store the energy and play it back generally approach 50%. This means the
30 watts is suddenly degraded to a little more than 15 watts by the
battery so that less than 15 watts are left as continuous power. For over
a thousand watts of solar power striking a 2.4 meter square panel. Sunlight provides 1KW per square meter so we have taken
2.4KW and converted it into 15 W continuous.,
you could have that 15 watts continuous in the household... if it
held the 10% to 15% stated efficiency of the solar panel over it's lifetime.
but it doesn't! Efficiencies are often less than 5% after a few short
years, some panels even begin at that number. But because of weathering and photochemical destruction of the
panel, another 50% loss will occur meaning you are now down to around 7-10 watts,
or between 9 and 15 watts roughly speaking depending on age. Wow!
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.
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.
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
*Not a loss but a serious problem with availability
Watts 2400W ---> 240W ---> 60W ---> 36W---> 30 ---> 15 ---> 7W
% eff. 100% ---> 10%---> *2.5%---> 1.5%---> 1.2%---> 0.6%---> 0.3%
Rewritten for clarity 3-27-2017.