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Small-Scale Solar Experiments

While sheltering in place this week, I’ve been tinkering with a small-scale setup for solar power generation. I’ve got a 100 watt solar panel, and access to the sun. What fun things can I do with this? Is it actually useful? Let’s find out.

My first thought was to power some equipment directly from the panel, but that’s not practical for most situations. Even if I could tolerate only having power during daylight hours, the output voltage and available power from a solar panel fluctuates too much from moment to moment. I’d either need a DC-to-DC voltage converter with a wide input range, or some equipment like a pump that can tolerate a wide voltage range and doesn’t mind frequent stopping and starting.

For most purposes it’s better to charge a battery from a solar panel, and then use the battery to power other equipment. I already have a solar generator (a large battery with integrated charger, inverter, and other conveniences) that was ideal for this experiment. I only needed to connect the panel’s MC4 output to the solar generator’s MC4 input adapter cable, stick it in sunlight, and wait.

100 Watts? Not So Much

With a 100 watt panel and something close to 12 hours of daily sunlight, I expected to get something close to 1200 watt-hours of electric production daily. My solar generator has a 150 Wh battery, so it should only take about 1.5 or 2 hours to charge. So I confidently set up the equipment, and after an entire day in the sun I only managed to increase the battery level by about 40%. What?

Maybe 1200 Wh was a little unrealistic. Or a LOT unrealistic. After some reading, I concluded the panel would probably never output 100W unless it was noon on a bright sunny day somewhere near the equator. But I might hope to get about 70W at noon at my latitude, with lower power output during the morning and late afternoon. Factoring in shadows from trees and other buildings, I decided I might expect to get about 400 Wh of average total daily production, with more in summer and less in winter.

OK then, 400 Wh should still be enough to charge my solar generator’s battery almost three times during the course of a day. So why wasn’t I getting that result?

Measuring Solar Panel Output

It’s not so easy to measure the power generated by a solar panel. With a multimeter I could measure the open circuit voltage, and the short circuit current, but multiplying the two figures wouldn’t tell me the power. I need a load to get a useful measurement for power output. But a fixed resistive load won’t work, not even a 100W-rated resistor, because it likely won’t bring the solar panel to the correct voltage for optimum operation. That optimum voltage varies from moment to moment, based on the sunlight hitting the panel. To do this right, I needed a solar charger like the one integrated into my solar generator. Then I needed to measure the current and the voltage simultaneously. I could have built some wiring adapters and used two meters for the measurements, but instead I bought a cheap inline power meter and soldered MC4 connectors to it.

I measured 19.7W in full sun at noon. Huh?! No wonder the solar generator’s 150 Wh battery takes forever to charge. Is there something wrong with my panel? After several days of tinkering with the setup under different lighting conditions, I never saw a continuous output higher than 23W. Most of the time it hovered right around 20W. Hmm.

I began to suspect the solar generator was at fault. Sure enough, buried in the manual were the specs for the solar input: 13V-22V / 2A max. With my solar panel, that means I’m theoretically limited to about 40W max (2A at almost 20V). I’m not sure why I rarely saw more than 20W though, and never saw more than about 1.3A of current. Maybe the integrated solar charger is even more limited than the manual suggests? Maybe I have bad wiring, or another problem?

As an engineer, 20W from this panel is insulting! Even if I have no practical need for this solar panel, losing 80% of its output is unacceptable to me. To save my pride I’ve begun to research plan B, which will involve a stand-alone solar charge controller featuring a much higher maximum charging rate, and a separate battery. More about that soon. Maybe I’ll put together a solar-powered Mac Plus.

Finding the Parts

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Here’s the equipment I used.

Renogy 100W monocrystalline solar panel. You can find slightly cheaper panels, but the Renogy has an extra-sturdy aluminum frame and a strong reputation for quality. This particular panel is also more space-efficient than most other 100W panels, if minimizing area and weight are important to you.

Suaoki 150Wh portable power station. Despite its annoying 2A charge rate limitation, I love this thing and use it all the time. You can charge it from solar, from a car, or from a wall plug. It has a built-in 100W inverter for running small appliances, USB ports (including a Quickcharge 3.0 port) for charging phones and tablets, 12V ports for DC lights and other appliances, and an integrated high-brightness emergency lamp.

200A Inline Watt Meter. I’m not confident it will actually handle 200A, but it works nicely for lower currents involved in small-scale solar. It shows volts, amps, watts, accumulated amp-hours, watt-hours, max watts, and min volts.

MC4 Male/Female Solar Panel Cable Connectors. Solder these to the watt meter.

DROK 12V Battery Meter with adjustable limits. Other cheap battery meters typically have fixed voltages for the 100% and 0% charge state. This one makes it possible to set custom values for the upper and lower bounds. Measured from one of the Suaoki’s 12V ports, I measured 12.33V fully charged and 8.96V just before the low-voltage cutoff disconnected the battery. The discharge curve isn’t linear, so the meter won’t go smoothly from 100% to 0%, but this is still vastly better than the simple built-in 5-bar power gauge on the Suaoki.

Read 16 comments and join the conversation 

16 Comments so far

  1. Byte Knight - March 19th, 2020 8:04 pm

    I’ve been doing the same thing with two 100W solar panels wired together with a homemade Y-connector and a Honda/Jackery HLS 290Wh portable power station. I’ve got our bedroom off the grid with it – lamps, clocks, and various chargers all run off of it. When it’s really sunny out, I charge my Jackery 160 off of it too and use that to power my MacBook Pro. Fun with solar!

    Jackery just came out with a 1000Wh model that looks like it would really be fun to play with. Can’t justify the price though…

  2. Steve - March 20th, 2020 8:14 am

    Have you measured your daily electric production? Where and how did you mount your panels, and how did you run the cable indoors into your bedroom? I’m very reluctant to mount anything permanent on our shingle roof over fears of causing leaks. At the moment my panel is on the ground leaning against the house, but there’s a lot of shade there.

  3. Wesley - March 20th, 2020 10:55 am

    I got a 50w panel several years ago to use with our older travel trailer when camping at the beach without hookups. Shortly after, I got a 2nd matching 50w panel to go with it. I built a frame of PVC pipe with stainless screws and wing nuts to hold it together. The back “legs” that hold it up at an angle can be replaced with shorter or longer ones depending on the season and height of the sun in the sky. I got an inexpensive PWM type charge controller and it seems to do the job just fine. I actually put a deep cycle battery in an outdoor storage shed/workshop (mainly storage!) and would use the panel there when not camping. Ultimately got another charge controller that stays in the camper, and when home, one panel keeps the shed topped up, and the other keeps the camper battery topped up. Both batteries are group 27 deep cycle and don’t get much use. Like the other commenter, I would like to make use of some of that power in the house, but haven’t figured out how to properly and safely make that happen. I use the one in the shed to run an occasional power tool with a 1200w inverter, along with interior lights. When camping, we run a 12v fan at night, keep phones and tablets and such charged, along with a 200w inverter for occasional 110v needs. Works great for our limited needs. The charge controller has a light that starts blinking when it is fully charged, and it normally is blinking, so I think 100w is more than we really would need, but nice to have extra margin for cloudy days.

    I would like to have something to better monitor battery charge/discharge rate, load, etc. Problem is that so many you find have tiny wires and promise to monitor 200A, stuff like you are noting. I have thought about trying to create something with an Arduino and an amp meter module or something, but have never done so…and would require messing with the #6 cables to the battery. The other challenge I see is that when the sun in shining and charging the battery, the voltage is going to be artificially high…only after the sun has gone down and the surface charge dissipates will you know where you really stand. Lots to think about! And probably many solved by using lithium ion battery packs. I’d love to have one for the camper, but the prices are still very high.

  4. Byte Knight - March 20th, 2020 3:22 pm

    I can completely charge my 290Wh and 160Wh Jackeries in series from low levels to completely charged on a sunny day in MN right now. The panels are sitting on my lower roof outside the bedroom windows facing South. I’ve just got them sitting on some rubber feet so they don’t slide off, and the wires run in through the window for now. I’ll eventually drill a hole through the wall and run the cables though that.

  5. Steve - March 20th, 2020 3:57 pm

    I’m considering a similar solution of rubber feet sitting on the roof, but I’m worried what might happen during high winds. There aren’t many options for roof tie-downs without drilling into my roof. Even if I can tie down two of the feet, the panel could flip over in high winds, or swing back and forth like a wrecking ball. But finding four good tie-downs in the right directions will be tough. Maybe I can use the gutters, or vent pipes.

  6. Jerry Conrad - March 21st, 2020 2:51 pm

    I would try a 3 ohm resistance and see what output you get.

  7. SC - March 22nd, 2020 1:04 pm

    This guy does a lot of product testing.. cool stuff:

  8. Steve - March 22nd, 2020 2:49 pm

    Yeah Will’s videos are great.

  9. Jeff - March 26th, 2020 1:43 pm

    I built a small-scale solar system a few years ago after we had a freak snow storm one October that knocked out power here for almost a week. I built the entire thing from scratch-even the panels! They put out roughly 60 watts apiece, for a total of 120 watts.

    I built the solar stand from an old chair bottom that swivels so that you can track the sun during the day. I also welded extensions on it so that it would be more stable, and eventually put wheels on it so that I can move it around as needed.

    I have two different batteries/charging systems attached to it- one in my garage that consists of a used Trojan T1275 battery that I bought off of Craigslist 5 year ago, and a 55ah SLA battery mounted in a portable system inside my house that I just bought on eBay here:

    You can see a few pictures of it here:

  10. Steve - March 27th, 2020 7:21 am

    Neat! How did you build your own solar panels?

  11. Jeff - March 27th, 2020 8:28 pm

    Several years ago I bought a humongous lot of 3×6 inch mono-crystalline solar cells- of which I still have about 800 watts worth. I also bought two solar frame kits from eBay, complete with the UV encapsulant (Sylgard 184).

    I then solder 36 cells in series in 4 rows of 9, placed them into the frame/solar glass, poured in the encapsulant, and backed them with some Tedlar. 7+ years later and they are still working!

    Fun as it was building the panels, they are so cheap at this point (You can get an awesome Renogy 100 watt panel delivered to your door for about $120) it’s not really worth doing- unless you want the experience, that is! Man that encapsulant can be messy to work with.

  12. Ella from Cyanergy - December 23rd, 2020 6:57 pm

    You did something out of the box. Measuring exact solar power quite difficult because of the temperature, weather conditions, sunlight visibility etc. so there is a lot possibility of you are not going to get the theoretical result.

  13. Ecofin Solutions - April 20th, 2021 12:30 am

    It’s a fun experiment. 23W is too low for a 100W solar panel. You should check open circuit output of the panel or wiring.

  14. Byte Knight - April 20th, 2021 6:31 am

    I finally found a good use for my solar panels – powering the Mac Mini that runs my BBS. Using four 100W solar panels, two Jackery Power Station 300’s, and a 50 ft extension cord snaked down to my computer room, Captain’s Quarters II BBS (a Macintosh and Apple IIgs BBS) is now sending out electrons generated directly from the sun!

  15. Caliber Solar - November 25th, 2021 12:04 am

    Solar panel is useful to our life. It protect our environment. I believe that when a solar panel generates power during daylight hours, the output voltage and available wattage fluctuate far too much from moment to moment. I’d need either a wide-input DC-to-DC voltage converter or some equipment, such as a pump, that can tolerate a large voltage range and isn’t bothered by repeated stopping and resuming.

  16. Solar Emporium - December 19th, 2021 7:34 pm

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