Hi this is Amy at the altE Store. We sell
a lot of solar panels for DIY off-grid solar
projects. Generally when we design a solar
system, we start with your loads, what you
are trying to power, and from there you figure
out what size solar panel you need. We’ve
got lots of videos walking you through the
calculations .
But now we are going to look at
it the other way around, what can you power
with a 100W solar panel? A solar panel is
rated by the amount of power it creates at
Standard Test Conditions, or STC. These conditions
include the intensity of the sun, 1000 watt
per square meter, the angle of the light hitting
the panel directly, the temperature, 25℃
or 77℉, and other criteria. So as they say,
actual mileage may vary, based on all of these
factors in the real world. So we generally
reduce the calculations based on the difference
between the lab setting and your actual installation.
When a 12V solar panel is rated at 100W, that
is an instantaneous rating, if all of the
test conditions are met, when you measure
the output, the voltage will be about 18 volts
and the current will be 5.55 amps. Since watts
equals volts times amps, 18 volts x 5.55 amps
= 100 watts. Watts is like the speed of a
car, miles per hour, how fast is it going
at that instant, 50 miles per hour. To figure
out how much power is generated over a period
of time, you can to multiply the watts times
the number of hours it is running. So in one
hour, 100W x 1 hour = 100 watt hours. Again,
with your car, 50 miles per hour x one hour
equals 50 miles. Now that we know the math
behind it, we need to figure out how many
hours to plug into the equation to determine
how much power the solar panel will generate
in a day. How many hours of sunlight that
is equal to the intensity of standard test
conditions, which is basically the sun at
noon, will the solar panel be exposed to during
the day? The number of hours of sunlight equal
to noon is called sun hours.
As you well know, even though the sun is up
at 8 in the morning, it is not as bright as
it is at noon. So you can’t just say that
the sun is shining for 10 hours, so I’ll
multiple 100W x 10 hours. The hour between
8 and 9 in the morning is probably only half
as strong as the sun from noon to 1 in the
afternoon, so the morning hour would probably
only be equal to ½ sun hour. But the days
are so much shorter in the winter than the
summer, the number of sun hours would be dramatically
different throughout the year. Also, the amount
of sunlight I’d get in Miami Florida would
be different than the amount of sun hours
I’d get in Portland Maine. Ugh, this can
get complicated. Luckily, some very smart
people have taken decades worth of weather
data and calculated out the number of sun
hours for all over the world, broken out by
month, and even the tilt angle that the panels
are mounted. So I can look at the charts to
see if I have a 100W solar panel, in Portland,
Maine, installed at about 45 degrees angle,
on annual average, I’d get 4.6 sun hours
a day. Likewise, if I took that same solar
panel in Miami Florida, installed it at a
25 degree tilt, I’d have an annual average
of 5.2 sun hours. Just as a little aside,
I want to make sure you see that during the
months of June and July, I’m going to get
more power out of that solar panel in Maine
than I will in Florida. With Miami being closer
to the equator and Maine being closer to the
north pole, the days are longer in the summer
in Maine, and so the sun shines on the solar
panels longer. Kind of cool, huh? OK, back
to the question at hand, what can I power
with a 100W solar panel? I need to figure
out my worst case scenario, what is the worst
performing month that I’ll be using the
panel? Since for this example I’m going
to be using it in Maine, during ski season,
I need to figure on December. So how can I
squeeze out as much power as I possibly can
in December? By tilting the solar panel steeper
so it points right at the low winter sun.
So I’m going to mount my 100W solar panel
at 60 degrees and figure on 3.2 sun hours.
I’ll now take 100W x 3.2 sun hours and get
320 watt hours a day in December. Now, as
you know, nothing in real life is perfect,
so I have to figure in losses that I’ll
likely incur, such as voltage drop across
the wire, dirt (or snow) accumulating on the
solar panel, losses through the charge controller,
etc. So I’m going to multiply the 320 watt
hours times .7. I know, that’s figuring
on losing about ⅓ of your power. Reality’s
a bummer. I now end up with 224 watt hours
of power that I have made with my 100W solar
panel on a December day. What can I do with
that power? Well, first of all I need to store
it in a battery so that I can use it later
when I need it. So I’m going to use at least
a 7 amp charge controller to manage putting
the power into a deep cycle battery that can
be charged and discharged on a regular basis.
What size battery do I need? Sorry, that calls
for more math. I have my 224 watt hours that
I’m making, and I’m putting it in a 12
volt battery. Because watts divided by volts
equals amps, 224 watt hours divided by 12
volts equals 18.6 amp hours. Even though I’m
putting it in a deep cycle battery, most batteries
still don’t like being drained down more
than half way, so I’m going to make sure
I get a battery that can hold at least twice
as much power I will be using, so I’ll only
use half of the power in it. 18.6 amp hours
x 2 = 37.2 amp hours. The amount of power
a battery can store changes depending on the
temperature of the room it’s in. If my battery
is going to be as cold as 60 degrees Fahrenheit,
I need to increase the size of my battery
by 11% to accommodate the cooler temps
37.2 amp hours x 1.11 = 41.3 amp hours.
I’m also
going to be converting the DC power from my
battery to AC using an inverter, and I’m
going to lose about 5% of my power through
that conversion, so 41.3 amp hours / .95 = 43.4
amp hours. Now I don’t know if you’ve
ever been in Maine in the winter. But trust
me on this one, the sun doesn’t shine every
day there in December. Not by a long shot.
So I need to figure out how many days without
sun that I need to store the power for to
get me through those sunless days. Let’s
say I need it to last me the weekend without
sun. 43.4 amp hours x 2 days = 86.9 amp hours.
Great, I’m going to get myself a group 27
deep cycle battery, that’s 89Ah 12V. OK,
now, I can finally figure out what I can do
with that power. I can run my laptop that
uses 45W for 5 hours. Because 224 Watt hours
/ 45W = 4.97 hours. Or I can power 3 of my
10W LED lights for 7 hours, and still have
a little power left over. Or I could make
myself a cup of coffee, listen to the radio
while reading a book with a 10W light on for
3 hours, and use my laptop for 2 hours. This
should give you enough information so that
you can figure out how to fit this into your
situation. You can change the numbers to fit
your area, and your power needs. If you need
a little help, you can go to our loads list
calculator to see how much power common appliances
use, and go to our reference page to see some
sun hours charts and maps. If you enjoyed
this video, give us a like and a share, and
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when we post more videos. Also go to our website
at altestore.com, where we’ve been making
renewable do-able since 1999.
