hpr-knowledge-base/hpr_transcripts/hpr3772.txt

Episode: 3772
Title: HPR3772: Adventures with a small solar panel
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr3772/hpr3772.mp3
Transcribed: 2025-10-25 05:13:24

---

This is Hacker Public Radio Episode 3,772 for Tuesday the 17th of January 2023.
Today's show is entitled, Adventures with a Small Solar Panel.
It is hosted by Andrew Conway and is about 28 minutes long.
It carries a clean flag.
The summary is I have a look at a cheap solar panel and learn a bit about how it works
and doesn't work.
Hello and welcome to another episode of Hacker Public Radio with me, McNalloo, also known
as Andrew.
And in this episode, I'm going to tell you a little bit about my adventures with solar
panels.
Not all about them, because I could probably go on for a long time.
Just actually literally about how I've discovered that solar panels work.
And most of this you can read by googling your own web searching, should I say, around
in the internet.
And but I found that two, well two things, which is quite usual when I'm trying to learn
something.
The first is there's a lot of rubbish in the internet, a lot of stuff that's playing
wrong in misleading, probably more than there used to be.
But secondly, something you just learn best by doing.
So this is all pretty much firsthand knowledge from me.
Okay.
So my first dalliance with solar panels was a small low voltage, almost a toy type solar
panel.
It was, I guess, this about the size of an A4 sheet of paper, you know, a side of a
normal notebook.
And it could claim, it could generate, I think, six to seven watts.
And maybe it could, if I took it inside the Earth's world, but to perhaps the distance
of mercury, mercury of Venus from the Sun, maybe then it could.
But here in the Earth, I couldn't get much more than five watts out of it.
And that was in a very good day with the panel pointed straight at direct sunlight.
So my first thing was, don't believe what's written on the label.
Now, actually, subsequently with more professional panels that are designed for generating
serious amounts of electricity for putting in your roof, for example, the specifications
are done according to much tighter standards.
But these smaller solar panels, not so much.
So don't necessarily believe that you're going to get the power that you think you will
out of them.
The other thing I discovered about solar panels very quickly was that they don't behave
at all like any power source that I've dealt with before.
Now the most common type of power source that I deal with, and I imagine most people
deal with, is mains voltage electricity.
And you have a socket, three pin here in the UK, maybe other countries have more usually
two pins, I don't know.
But generally, the point of this is that you plug your device into the wall, an outcome
is electricity.
And if you plug in an electric toothbrush, or if you plug in a 3 kilowatt oven, or some
kind of space heater, that's many kilowatts, you'll get the power, the amps will be delivered
and the voltage will hold up.
So in this country, I expect where I am 230 to 240 volts pretty constantly.
And when I plug something in, the voltage doesn't drop appreciably, in fact, if it did
drop by many volts, it would be time to investigate what the problem was.
So in other words, what I'm trying to say is you get the power within reason, the current
draw the power from your constant voltage source, and your voltage doesn't drop when you put
a load on it, or the drop is negligible.
So moving on to the next, most common source of electricity, which is batteries.
So batteries are a little bit more delicate to deal with, because when you draw anything,
but the small current from a battery, you will notice quite a sizeable drop in the voltage.
And of course, the difference of batteries, of course, is that they are DC direct current.
So there's no in mains voltages oscillating here, in the UK, it's 50 times per second,
50 hertz.
But a battery just delivers a straight constant voltage or a straight constant current with
no variation, unless the load is asking for a varying amount of current.
So the key thing here is that when you draw even fairly modest amounts of current from
a small battery, is voltage will drop.
Now the device, most devices are designed to expect that.
In fact, they're designed to handle the fact that the battery's voltage will drop.
So when you buy a, say, a AA battery or a AAA battery that's rated at 1.5 volts when
you get it, but by the time it goes flat and inverted commas, and I'll tell you why
I say inverted commas, it may be done to, say, 1.2 volts.
And the devices are designed to work down there, and then might give you a warning, or in
the case of a small torch or flashlight, the device may just quit working completely
with no warning.
Now the reason I said flat and inverted commas is because, just because it's a 1.2 volts,
doesn't mean it's got no energy left.
In fact, it could have the pendulum type of battery as much as, say, 50% of its energy
left, but it just can't deliver a voltage that's useful for the purpose that you bought
it for.
So you discard it and you don't think much of the fact that you've just thrown away some
potentially useful energy.
There is a way to get energy out of such low-voltage batteries.
It's called a dual thief, and maybe I'll do another HPR episode about that at some
future point.
Anyway, I digress.
The point is that a battery will not give you a constant voltage, it won't give you
the voltage, it's written on it, either because you're running it down over time, or because
you've placed a high current load in the battery.
So that means that you have to design your circuits with that in mind.
Now the big surprise with solar panels, I didn't really know much about them, is that
they're an extreme version of what happens with batteries.
So I could hook up this little plastic solar panel I bought, and the brand name was
Sunny Solar.
Good luck if you try and look at that one up.
It's almost certainly some meaningless brand name that's been pasted on, on some generic
product that's been turned out in a Chinese factory somewhere.
I'm sure there's many other identical products with different names.
But this particular solar panel, as I say, was rated at, I think it's claimed it could
do six volts at, I can deliver one amp, I think that's was the origin of why it claimed
it could do six watts.
Now when I plugged it in, and in full sunlight, I could, for a time, get about six volts
or even a shade more.
By notice that the two things, first of all the voltage would drop, not rapidly, but slowly
and then come to a stable value of 5.9 or 5.8 something, that'll be the first thing
I noticed.
The second thing, and this is the really dramatic thing, is that, oh, I've got, oh, I've
got just about six volts to play with here, okay, that's plenty for my applications, maybe
a bit too high for some of my devices, but here we go.
So the instant you plug in a device to it, nothing happens.
Or maybe you see a brief flash of an LED or something, and then it just dies, I think,
oh, what's going on here.
And when I connected an ammeter, the first thing that I noticed was, well, it actually,
in direct sunlight, it blew up the fuse in my ammeter because I had it in the low current
range.
One other lesson for you is, always go for the, if you've got one of these multimeters,
I'm saying ammeter, but it's a multimeter, especially the cheap ones, make sure, make sure
you put it on the 10 amp range, not the low range for looking at milliamps and microamps,
because if you don't, then you'll blow a fuse, which is maybe only rated to a few hundred
milliamps.
That was the first thing I learned.
But when I realized what was wrong with my multimeter and connected up in the 10 amp
range, so I was nice and safe, I saw that maybe very briefly I would get the high current,
like, I don't know, many hundreds of milliamps, maybe if it was full sunlight, but it would
quickly vanish, and the voltage would collapse, and I would be left with nothing.
I thought, oh, okay, what's going on here then?
And you have to really understand how a solar panel works.
So there are two key panel parameters on the specifications to the solar panel.
Well, there's three.
The one that most people describe a solar panel with is the power rating.
So in this case, it was claimed to be six watts, I think.
Maybe actually even seven, depending on the sales brochure you happen to look at.
But anyway, let's say it's six watts, but that actually isn't very interesting.
More interesting is the numbers that lie behind it.
So it's said voltage six volts and current one amp, okay.
Now I think what that really meant is that the open circuit voltage of the panel in full
sunlight, so that's a sunny day with the panel, suddenly fairly high in the sky, and the
panel pointed directly at the sun, so it's absorbing as much, you know, as much sunlight
as it can get on the panel.
What it was saying is at that point, the open circuit voltage of the panel should be six
volts.
Indeed it was.
I measured six volts.
And the reason the voltage I noticed dropped slightly is because as a solar panel heats
up, it becomes less efficient.
And in fact, a solar panel is more efficient at producing electricity, not hugely more efficient,
maybe one or two percentage points more efficient, depending how exactly how cool it is yet.
So what I was observing is as you put a panel which is dark in color and place it in the
sun, you'll heat up quite rapidly to temperatures above 50 degrees C, maybe it's operating temperature
could be as high as 70 degrees C in fact.
And so there's quite a sizable change in efficiency because of this.
And that's why I saw this voltage drop.
But anyway, it was still roughly, give or take a few hundred millivolts, it's still
you're talking about six volts, plus or minus a hundred millivolts, six volts.
But that's the open circuit voltage and that means that I have not completed a circuit
that was effectively infinite resistance between the positive and negative leads of the solar
panel.
That's what that parameter means, the open circuit voltage.
So you'll quite often see in solar panel specs V subscript O C voltage in the open circuit.
And that isn't the voltage you'll get because the moment that you connect a circuit to
it, the voltage will drop.
How far it drops, depending on how much current you try to draw.
So if I try to draw the one amp, it's in the label, it could do one amp.
If I try to draw one amp, I would not still have six volts.
In fact, I have something very close to zero volts and I wouldn't have one amp either
because the voltage wasn't there.
And at this point, it's probably a good idea to fall back on the analogy of voltage
current being the pressure and flow of water.
So let's forget electricity for a moment.
You have a hose pipe and you connect one into a tap or faucet, I guess, for my colleagues
or the side of the Atlantic.
And of course, you open and close the nozzle at this point, if you haven't opened the
tap, nothing happens.
So that's like the analogy there, the solar panel is in the dark, nothing is happening,
there's no voltage, no pressure, no current, there's nothing.
Then you open up the tap.
So this is a bit like taking a solar panel out and placing it in strong sunlight.
When you open up the tap, water pours in to the pipe and it's a fissing sound for a brief
time and then it stops.
And then all you're left with is a pipe full of water that's at pressure and it's the
means, in the case of the water pipe, it's whatever your means, pressure of your water
system is connected to the tap.
But that's the key thing, you've got a nice high pressure.
Now when you open this, so I should say this at this point, this is a bit like taking
a solar panel and leaving it in sunlight and you'll see this high voltage, in the case
of my small solar panel, a six volt across the plus and minus leads.
There's lots of voltage there, just as there's lots of pressure in the pipe, but nothing
else is happening.
Water is not flowing out, electricity is not flowing out, it's all quite static.
That's useless.
The moment you're trying to do something with the water, what happens is some of the
pressure that was built up in the pipe is dissipated as the water flows out in a jet
out in the nozzle and that pressure is turning itself into the flow of water.
So the pressure inside the pipe will drop slightly, but not to nothing, otherwise the water
wouldn't flow out of the pipe.
An amount of water that's flowing out of the pipe per second, let's say we measure it
in liters per second, that is analogous to the current flow in the solar panel.
At this point, the analogy isn't so good because the mains water is a bit like mains
electricity.
It will supply within reason whatever pressure you need for the nozzle of your pipe.
Now, if you close the nozzle at the end of your hose and then you went and closed the
tap, then there will be water inside the hose and it will be held at quite high pressure
because it can't go out through the closed tap or the closed nozzle.
So this is like analogy of a solar panel in the low sunlight.
There's some charge stored there.
There's next to nothing coming in, I know maybe you open the tap slightly, very slightly
electrical water in, but they're just really not enough pressure to sustain it.
So the moment you open up the nozzle or at the end of the hose, water will splurt out
and it will come on the splurt and then it will die down to trickle almost immediately
because there's just no pressure at the end.
Even if you have the tap open slightly to let some water in, there's very little pressure
from that, so it would take too long for the hose pipe to fill up again.
But if you did close the nozzle, the hose pipe would fill up again and you'd end up with
a pressure ice pipe again.
That's a bit like the idea of taking a solar panel, putting it in not very sunny conditions
but some light, holding it to open circuit voltage and then suddenly connecting something.
The voltage will collapse, you'll get a sudden burst of current which may be your
accumulator as I discovered, but it can't sustain that current for any length of time
and you'll just end up with a low voltage and an extremely low trickle of electricity,
a low current.
So that's how you should think of a solar panel.
Now the more sunlight that you have, the greater the voltage it can sustain and the greater
the current that you can draw off it without that voltage completely collapsing, didn't
I think.
So the game really is, is how much current can you pull out of a solar panel in the amount
of sunlight that you've got?
And that is a quite tricky problem, really.
But the answer is that you can tell from the voltage, roughly what level of sunlight
that you've got, the voltage will vary slightly in sunlight, but actually it's much better
to look at the other parameter that we printed in a solar panel and that will be not the
open circuit, but the short circuit current.
So if you connect the plus and the minus leads of the panel, which you would normally never
do with mains electricity or even a battery, because a solar panel, at least a small solar
panel, a little wattage solar panel, isn't going to damage anything and blow up the wires,
you can do this quite safely.
You can connect plus and minus leads together.
And if you do so through an ammeter, a multimeter, set to measure current, then you can measure
what current is present when the resistance is negligible that we've got a short circuit.
What you will find then is the amount of current in that situation will be proportional to
the amount of solar power falling on the solar panel.
So if the sunlight goes from say, I don't know, let's say, 100 watts per square meter,
which is quite dim really, it was a very cloudy stormy day, to 200 watts per square meter.
So after a thunderstorm and things are brightening up again, you might see that.
You will see maybe the current jump from pulling numbers out of thinnier here, but let 100
milliamps to 200 milliamps, not my rubbish little solar panel, but a bigger one.
So you will see there's a proportionality between the short circuit current and the amount
of power from sunlight falling on your solar panel.
And that's, so that is a way that you can start to understand how the solar panels parameters
do relate to the sunlight. You can do it voltage, but I think it's quite a weak and nonlinear
relationship. But it still hasn't really answered the question that I was getting to is how much
power can you draw from that solar panel in given light conditions? Well, it doesn't add to,
I haven't got to that answering that question. But before I do, it makes sense of the stats
that are printed on my solar panel. It said six volts, that was the old Pensacret voltage.
So that's where I'm not drawing any load whatsoever from the panel, useless.
I can't really draw one amp from the panel by providing zero resistance by connecting the
plus and minus leads together. Again, yes, I get the current, but there's no power as we
associated with that because it's nearly at zero volts when I do that. So in both cases,
there's a negligible amount of power. Somewhere between those two extremes is where we want to be.
So we want to find a value of current draw from the solar panel. We're equivalently
a voltage that we want to keep the solar panel at that maximizes the amount of power.
Now, it turns out that for any given level of sunlight, there is such a point. It's got the
maximum power point. And I can't really describe a graph, I'm not going to try and describe a graph,
but if you look at maximum, such up in maximum power points, solar panels, you'll see these graphs
of current versus voltage, where the current is plotted in the vertical axis and voltage
along the horizontal axis. And there's a constant current up to some voltage and then the voltage
just disappears when you try to draw current. And sorry, sorry, other way around, the current
disappears when you try to hold the panel at a high voltage. And yeah, I said I wouldn't
try and describe the graphs and I have gone and tried to describe the graphs and now I've confused
myself and probably used what I apologize for that. Anyway, forget that confusing description
of the graph. The point is there's a current and a voltage for any given level of sunlight,
where you want to be at a maximum of the power draw. And if you're at full sunlight,
then in theory, you should get the wattage rating of the solar panel.
That's its full sunlight with the panel perpendicular to the sun's rays.
Now for a big professional panel, which I do have some of, those specifications are trustworthy
and the information is regulated, the solar panel companies are played by the rules by and large.
For cheaper ones, you get from hobbyists stores online, the lower wattage ones, a few watts,
up to maybe a dozen, a couple dozen watts. Yeah, watch out, you probably will get misleading
specifications. So what this panel should have said to me is that six volts was an open circuit voltage
and one amp was the short circuit current and it should never have quoted a power rating of six
watts or whatever, that's just wrong. And a good day, I can maybe poke four watts, maybe a little
bit more than for maybe four and a half watts out of it if I'm really on the ball. And what is
really on the ball mean? Well, it's finding, when one way you can do it is just connect
different values of resistors between the plus and minus leads until you find the one that gives you
the maximum power rating. You have to be a bit careful because most common resistors are rated at
quarter of a watt and you will literally see, even with a small solar panel, it does five watts,
you will literally see the resistor disappear in the puff of smoke if you managed to try and pass
several watts through it. You can buy bigger resistors. But actually, a much better idea is to
rapidly using pulse width modulation, open and close the circuit to try and regulate the voltage
in the current draw. And those devices are called pulse width modulation solar charge controllers.
And if you want to get a bit more juice and make sure you find that maximum power point,
then you can use these so-called MPPT solar charge controllers. They also use pulse width modulation,
but they, rather more intelligently, will hunt down that maximum power point that I was talking
about. And they will adapt to, as a level of sunlight changes, they'll adjust the current draw to keep
the maximum amount of power flowing out. Now, of course, this has a problem. If you've got
electronics, let's say, for example, like an ESP32 microcontroller, it really only wants 3.3 volts,
and it's tolerance, it's not going to like it if you give it five volts because the sun's come out.
So, the solar charge controller's other job is not just to find the maximum power point,
but it's to output a reliable level voltage. Generally, they will output five volts for these
hobbyist ones for larger ones for charging battery storage and households. You'll typically see
the output voltage of the solar charge controllers at something that will charge either a 12 volt,
a 24 volt, or even a 48 volt battery. So, my house, this is for a future HPR episode, I've got a 24 volt
system that I put it again on myself, but that uses a rather chunkier and more expensive charge
controller. But, for the one for the small solar panel, I got this, I think I bought it from
Pimironi or Pihut, I can't remember which way I bought it exactly, but it was called, the brand
name was DF robot, and it was just ideal for taking the power delivered by the small solar panel
and outputting it at five volts, literally through USB, and it also allowed me to charge
connected battery, in fact, you really had to connect the battery different to work properly,
and you could plug in USB from another source and charge the battery without solar power, so you could
do both. So, it was quite a clever little device. The thing is that if you only had solar power,
you couldn't, and the sun went behind the cloud, then the five volts output would collapse, so you
really need a battery and the solar panel together in order to smooth out the tremendous variation
in power delivery, that the solar panel is going to give you, because of a cloud going front the sun,
or somebody walking in front of the sun, or the sun moving behind the tree, or all these kind of
things. So, the other lesson that I learned was really, solar panels by themselves are not that great,
you need to have another power source to work with them, either they offset what you're drawing
from the grid in some way, or, and this has been my preference, is that you have a battery,
and the battery stores the power, so that when the sun is shining, you're charging up the battery
and powering the load, and when the sun isn't shining, then the battery takes over and can deliver
charge. And so, in this way, you can run pretty much indefinitely low power electronics, with a small
solar panel, you could probably, and not probably, you can definitely run an ESP 32 night and day,
pretty much indefinitely, even through a Scottish winter, I've discovered, it's quite possible.
If your battery can take in enough charge, when it is sunny, probably a small five watts
solar panel is going to do the job and keep your device topped up and powered through long nights
here in Scotland. So, yeah, that's really all I've got to say about solar panels.
I might do future episodes, if people are interested, and I'm interested, which I probably will be,
to be honest, on my larger solar panels, I've got two sets on the go, one for my house, and one
observatory that I've been involved in building with Astronomical Society of Glasgow.
And so, that's a different game. You're dealing with mains voltage coming out of inverters,
you're dealing with solar panels that can output kilowatt, and that will arc and spark,
and DC circuit breakers that can burst into flames, you know, and all kinds of exciting things.
And you can electrocute yourself and blow up multimeters, and oh yes, it's a whole level of new
fun to be had with those. So, I'll just end that with the warning, is it's very safe to play
around with these load voltage solar panels. So, if you want to play with them, be very careful
playing with the big boys and girls. When you're dealing with hundreds of watts, kilowatts,
solar panel array and inverters, which output means electricity, you have to be much more careful.
And also, you're dealing with DC. So, that's, again, as I said, different from dealing with AC.
So, if you do want to play with bigger solar panels, I do advise some caution, as they can be
surprisingly fun, shall we say. Anyway, I'll leave it there. If I've got anything wrong,
or could explain better, please do leave comments, and or do a show of your own. If you've got
solar panels, I'd love to hear your experiences. I've got certainly plenty more to learn myself,
so I'd love to hear other views on how people have gone with their solar panels out there,
and hook up Hacker Public Radio Land. Okay, thanks very much listening. Bye-bye.
You have been listening to Hacker Public Radio, and Hacker Public Radio does a walk.
Today's show was contributed by a HBR listener like yourself. If you ever thought of recording
or classed, you click on our contribute link to find out how easy it really is. Hosting for HBR
has been kindly provided by Anonsthost.com, the Internet Archive, and R-Sync.net.
On the satellite status, today's show is released under Creative Commons,
Attribution 4.0 International License.