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Author Topic: Building a small solar system  (Read 773 times)

KF7YED

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Building a small solar system
« on: February 28, 2022, 06:48:24 PM »

I currently have four 100W 12 volt Siemens solar panels. They each put out about 17VDC. I've been using a couple of them for camping and Field Days, one for the camper battery and one for the radio battery (with a Buddipole Mini charge controller which is really great, by the way), but I'm thinking of putting together a small system for emergencies using all four panels. Pack it all away in the basement for a rainy day (EMP). I've read articles until my head is spinning so I thought I start a conversation and soak in the ham knowledge here.

As usual, cost is everything and I don't intend to run a welding shop or the whole house on this system. Anything that will support some wall wart chargers for various electronics, maybe a laptop (which I never use except on AC because the battery isn't great). Running a fridge for a while during the day would be an extreme high-end load for this or maybe the microwave in the camper.

Some questions:

Is it even possible to run this 400W solar array directly to an inverter and skip batteries altogether, understanding it will only work in full sun?

If I have to use batteries, can I use a couple of plain old SLA 12v batteries, maybe 100 Ah each, just because they're cheap(er)?

How many batteries would make the most use of the 400 watts available or is that a function of the inverter output?

I have this bundle in my Wish list:

https://smile.amazon.com/Renogy-Converter-Controller-Parameter-Adjustable/dp/B09QKM28H9/ref=sr_1_10?crid=C7V6KSW8P2GY&keywords=mppt%2Bcharge%2Bcontroller%2Bmppt&qid=1646100641&sprefix=morningstar%2Bmppt%2Bcharge%2Bcontroller%2Bmppt%2Caps%2C337&sr=8-10&th=1

Couple of questions on that:

Is 2000 watts just a ridiculous expectation for my 400 watts of solar? Or does that just mean I need more batteries to make the most of the 2000W? i.e., fewer batteries would discharge quicker with that kind of load?

If I wire the panels as a 24V array, 2 in series and 2 in series and then the two pairs in parallel, do I use that current rating as the input for the charge controller? It says "40 AMP" so is that the max it could draw from the panels? I know they won't put out that much but is that a specification that's meant to stay under or to make sure I can deliver?

Wiring as 24V means I can get by with smaller wire, between and from the panels to the charge controller? 10ga? 4?

Thanks in advance. 73

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KA4DPO

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Re: Building a small solar system
« Reply #1 on: February 28, 2022, 08:31:14 PM »

In order to build a small solar system, you will need a small sun, and some even smaller planets. ;D ;D

Sorry, carry on.... :D :D :D
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N4UFO

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Re: Building a small solar system
« Reply #2 on: February 28, 2022, 08:47:13 PM »

In order to build a small solar system, you will need a small sun, and some even smaller planets. ;D ;D

Sorry, carry on.... :D :D :D

I know I thought the same thing when I saw the thread title.  ;D

I might suggest you look to a local solar installer for guidance... they run TV ads in my area constantly and even offer incentives like a free generator to sign up. I know you may not want a full house unit, but honestly with the tax breaks and savings, it might be the cheaper way to go. Most I ever did was I had a roll up panel and a small SLAB for a QRP rig. Donated that to an alternative energy education project in Vermont. (Never been to Vermont, I shipped it.)
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AF5CC

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Re: Building a small solar system
« Reply #3 on: February 28, 2022, 11:10:26 PM »

In order to build a small solar system, you will need a small sun, and some even smaller planets. ;D ;D

Sorry, carry on.... :D :D :D

And 9 of them-including Pluto!!

73 John AF5CC
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W9FIB

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Re: Building a small solar system
« Reply #4 on: March 01, 2022, 03:59:31 AM »

Typically you would want battery storage for 2 big reasons. You would have power when it is dark, and you could run heavier loads intermittently and then the panels recharge the storage when on light loading.

Probably would want to use 2 good sized batteries and keep them topped off before you deploy the system. Size would depend on how heavy is your biggest load and then your best charge rate between heavy loads. You want the ability to recharge what was used by the heavy load.
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73, Stan
Travelling the world one signal at a time.

KD6VXI

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Re: Building a small solar system
« Reply #5 on: March 01, 2022, 04:27:46 AM »

400 watts isn't a lot, and that 400 watts is high nooon, all panels at 100 pct illumination.  Expect 350 watts or so in your calculations.

In Bakersfield, CA we looked at 8 hours of continuous sun and sometimes as low as 6.  Yes, your panels will be illuminated more, but the sun will be lower in the sky so you won't see peak production.  Here in the Carribean they plan for 10 hours(ish)....  However that's a bit optimistic IMHO since we have more clouds that meander across the sky here

That gives you anywhere from 1800 to 2400  watts of charge a day.

Your calculations can be made from there.

Yes, 24 volts or even 48 volts would be better.   The inverter will have a higher conversion efficiency the higher in DC voltage you go.  We used 600 volts DC on homes and over a kilovolt on the commercial (utility grade solar fields).

When you size your batteries, you now have a size to go with.  You have 2400 watts.  If you want to be able to pull all that in a few minutes (microwave or fridge) then you need a good size battery bank to supply the peak current.  Then it will slow charge all day. If you are talking about running a few 15 watt light bulbs and a 95 watt laptop, smaller batteries in a bank will work.

Hope this helps.  Without more information it's hard to size anything.  Microwaves run from 600 to 1800 watts, etc.

--Shane
WP2ASS / ex KD6VXI
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W9IQ

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Re: Building a small solar system
« Reply #6 on: March 01, 2022, 04:29:07 AM »

I currently have four 100W 12 volt Siemens solar panels. They each put out about 17VDC.

That voltage will vary according to the amount of solar energy hitting the panels (called solar irradiance) and the load on the panel. An MPPT charge controller finds the optimum load for the solar array in order to transfer the maximum energy to the battery/load.

Running a fridge for a while during the day would be an extreme high-end load for this or maybe the microwave in the camper.

You will require a battery to store enough energy to run a small fridge or a microwave oven. Always look at the inverter specifications to see what inductive load it can handle during a motor starting scenario to make sure it won't burn out a motor or trip the inverter. Then remember that a fridge will take energy all day long so you need to look at your total energy requirements and make certain you can convert and store sufficient solar energy to supply to the inverter when needed.

Is it even possible to run this 400W solar array directly to an inverter and skip batteries altogether, understanding it will only work in full sun

Yes but then the total load will need to be far less than 400 watts as this is what it will likely put out on most sunny of days near noon time. Then you have losses in the charger controller/regulator. You will also quickly become aware of cloud cover and other weather issues as your solar system will drop out due to more energy being used than it can supply.

If I have to use batteries, can I use a couple of plain old SLA 12v batteries, maybe 100 Ah each, just because they're cheap(er)?

Just about any lead acid 12 volt battery will work on these systems. It is only a question of cost, lifetime and their ability to store and deliver the desired energy. The charge controller you reference appears to specify SLA batteries so do your research.

How many batteries would make the most use of the 400 watts available or is that a function of the inverter output?

It is time to start thinking in terms of energy (typically kw-hr in the US) instead of power. You need to consider how much energy your panel will generate on the average day. It will be less on rainy or partly cloudy days and it could be more on a sunny day. If you panel has 3 perfectly sunny hours in a particular day, you may generate 1.2 kw-hr (400 watts * 3 hours) but you will store less due to the inefficiency of the charge controller and the battery.

To calculate the average annual energy generation, you start with a map to show you the average solar irradiance for your area. You can then use this to calculate your overall infrastructure requirements. Monthly versions should then be checked to make sure it will meet your needs in winter when there is less solar irradiance:


Sengupta, M., Y. Xie, A. Lopez, A. Habte, G. Maclaurin, and J. Shelby. 2018. "The National Solar Radiation Data Base (NSRDB)."
Renewable and Sustainable Energy Reviews  89 (June): 51-60.


Is 2000 watts just a ridiculous expectation for my 400 watts of solar? Or does that just mean I need more batteries to make the most of the 2000W? i.e., fewer batteries would discharge quicker with that kind of load?

That is the general idea but again start thinking in terms of energy both from a charging and discharging perspective. Your energy into the batteries must exceed the energy drawn from the batteries in order to overcome inefficiencies in the battery charging and the converting of battery power to AC.

The battery also needs to be able to deliver the peak power needed for the inverter and the load. A 2000 watt load translates to 95 to 100 amps at 24 volts out of the battery. Just like starting a car, that type of load will quickly drain a battery. You may only get 166 watt-hr of AC energy if the battery can only do this for 5 minutes.

If I wire the panels as a 24V array, 2 in series and 2 in series and then the two pairs in parallel, do I use that current rating as the input for the charge controller? Wiring as 24V means I can get by with smaller wire, between and from the panels to the charge controller? 10ga? 4?[

It generally works out best to put the panels in series up to the allowable voltage of the charge controller. The controller you reference appears to support 100 volt strings so you could put all four of your panels in series. Then check the amperage. If your entire string is in series, it will source less than 10 amps. Unless you have an unusually long run from the panels to the charge controller, 14 gauge wire would be more than sufficient. Use 12 gauge if your budget can support it. Check that the terminals on the charge controller can take the gauge your choose. Respect the potential voltage levels and wire it as if it were normal house wiring.

Don't forget to do your research on the RFI generated by the charge controller and the inverter if you have any hopes of using radios in the vicinity of the installation.

- Glenn W9IQ
« Last Edit: March 01, 2022, 04:35:06 AM by W9IQ »
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

KT4WO

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Re: Building a small solar system
« Reply #7 on: March 01, 2022, 04:59:00 AM »

Glenn posted alot of good info. Take advise from someone
who has "been there, doing that"   
What ever the size you decide you need----
DOUBLE IT!!!
I came up with a 400watt system just to run a few radios,(back-up)
and it did work but not as long as I thought it would.
All the numbers used to size a system are for "A good day"
Just when you need it....you will have 30 BAD days!!! and no/little
power when needed.
Really---- Double the size you think you need!
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KG4RUL

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Re: Building a small solar system
« Reply #8 on: March 01, 2022, 05:25:12 AM »

Something to Consider
+++++++++++++++++++++++++

SOLAR POWER IS “DIRTY”

Solar power is not “clean” power.  The manufacture and disposal of photovoltaic panels produces enormous amounts of toxic waste.  Among these are:  Cadmium compounds like Cadmium Indium Gallium (Di)selenide, Silicon Tetrachloride, Hexafluoroethane and Lead.

The production process of manufacturing solar panels is energy-intensive and polluting.  The process starts with mining of quartz sand. An oxidized form of silicon, non-crystallized silica, is the most common component of quartz sand.  Silicon is a perfect material for photovoltaic solar cells because of its ability to conduct electricity even at high temperatures, making it the most commonly used material in photovoltaic systems.  But in order to get silicon in the form required, it has to be treated in a process that pollutes the environment and releases greenhouse gases.

Solar panels do not use water to generate electricity, but the manufacture of solar components and their maintenance does require water.  In fact, the amount of water required for large-scale solar farms can reach as high as 396 million gallons during the installation process and 6.8 million gallons a year for cleaning.  Water is also used during the manufacturing process; water works as a medium to chemical reactions. The outcome of these reactions is water contaminated with chemicals like hydrofluoric acid.  Careless disposal of such wastewater poses risks to our fresh water sources.

Solar proponents claim that panels can be successfully recycled.  The reality is that if a recycling plant carries out every step, by the book, their products can end up being more expensive than new raw materials.

Approximately 90% of most photovoltaic solar modules are made up of glass that often cannot be recycled as float glass due to impurities including: plastics, lead, cadmium and antimony.  That glass is often disposed of in landfills where pollutants such as lead or carcinogenic cadmium can be almost completely washed out of the fragments of solar modules over a period of several months, for example by rainwater.

There are firms that may advertise themselves as "solar panel recyclers" but instead sell panels to secondary markets in nations with less developed waste disposal systems.  Governments of poor and developing nations are often not equipped to deal with an influx of toxic solar waste, leaving poor and developing nations are at higher risk of suffering the consequences.

The truth is that solar panels create 300 times more toxic waste per unit of electricity generated than nuclear power plants.   If solar and nuclear produce the same amount of electricity over the next 25 years that nuclear produced in 2016, and the wastes are stacked on football fields, the nuclear waste would reach the height of the Leaning Tower of Pisa (52 meters), while the solar waste would reach the height of two Mt. Everests (16 km).

While radioactive waste is strictly monitored with precise steps for its disposal, the regulations of solar technology disposal are sorely lacking.

In the end, in addition to the trail of toxic waste that follows them, solar panels do not produce electricity at night, or as much when it is cloudy, and in northern climates, none when they are covered with snow and ice.  When they do produce power, there is no economical way to store excess power for later use.  And they make little economic sense, unless heavily subsidized.
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K5LXP

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Re: Building a small solar system
« Reply #9 on: March 01, 2022, 06:42:19 AM »

Some really good responses here out of the chute, including RUL's.  Just wait until spent lithium batteries from all the electric cars kicks in...

To the OP's question, all of this discussed so far is the "means", but in order for you to assembly a functional system you have to know what the "end" is.  What would the "ideal" system do for you, specifically?  Points to ponder:

What equipment do I want to run? What would be the minimum and maximum loads to expect?
How long does it need to run - hours, days, weeks?
How long, with no sun (clouds/snow)?

What you end up with is a load profile of minimum watts times x minutes per day and a maximum watts times x minutes per day.  You can then roll that up to what kind of inverter you might need if any, and the battery storage it will take to run the amps, and amp hours per the min/max/time requirement.  You can hand wave the efficiencies or deep dive into specific equipment profiles if you like, and you think it might matter.

The solar side can be done a few different ways, you've already zeroed in on MPPT which is fine.  Running a 24 or 48V battery as a system choice is OK, but it ties you into always running voltage conversion equipment to step up or step down the bank voltage to run your equipment.  That's fine, it's done all the time but there is a cost and efficiency tradeoff for that.  It may be "less" efficient to charge a 12V battery bank but "more" efficient to run your gear directly off the bank without an inverter or converter.  Then you consider your latitude, season and average insolation for your location, to see what you can reasonably expect for daily/weekly/monthly power (W9IQ's map) for different months of the year (winter is nominally ~1/2 of summer).  You compare that to your load profiles and see how close you are to net positive or deficit power for your operation.

None of this is guesswork, each section of the system can be modeled and most issues revealed with basic review.  I use a spreadsheet which can include as many or as few variables as I choose to include depending on the system goal.  A change to one parameter then gets rippled through the rest of the system and it's easy to play "what if" to dial the net result into a final configuration.

Now, with all this in mind you can also go with slapping a bunch of parts together and seeing how it works.  Most anything will "work", and maybe what you end up with is "good enough".  Don't let "ideal" get in your way, generally something is better than nothing and you'll learn a lot about this as you go.  If some part doesn't work out great then tweak it and see the result.

The only part of your plan I part with is the "rainy day".  Like any part of "emcomm" you have to train and practice or when "all else fails" you likely won't have the result you intend.  Put the stuff up, play with panel orientation, learn about batteries, experiment with different equipment and generally get comfortable running the system in it's deployed configuration.  Once a year for field day or whatever isn't enough practice to be able to set the thing up correctly and know what to do when there's an issue.  If it's in frequent use and you end up needing it, then it's just a matter of using it or relocating it to where it's needed, and it's ready to go.

Mark K5LXP
Albuquerque, NM
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W9IQ

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Re: Building a small solar system
« Reply #10 on: March 01, 2022, 08:02:27 AM »

Just to give you a starting point, let's calculate the energy that your panels might produce annually assuming you are using them near your home QTH. The average annual solar irradiation for that area will produce 4-5 kWhr/day per kW of panel. Your panels will produce 0.4 kW when illuminated with 1000 watts of solar power. Using conservative numbers:

     0.4 kW * 4 kWhr/day/kW of panel = 1.6 kWhr per day (annual average)

In the worst sun day in December and by titling the panels toward the lower sun, the solar production drops to 2 to 3 kWhr/day:

     0.4 kW * 2 kWhr/day/kW of panel = 0.7 kWhr per day (worst case sunny day)

Since the MPPT converter is not 100% efficient, multiply the above by 90% to the maximum energy available. So annually, your system could produce:

     1.6 kWhr/day * 365 days * 90% = 525 kWhr annually

That sets the absolute limit that your system will produce - the annual energy consumption of all of your loads cannot exceed this number. But as Mark and others have point out, you now need to consider the short term effects due to cloudy days, peak loads, etc.

Just to put these numbers in perspective, a small 4.8 cu ft, energy efficient refrigerator consumes over 200 kWhr annually. That is nearly 40% of the total energy generated by the solar panels without accounting for inverter inefficiency. So on an annual average basis, this should work. Keep in mind, however, that your daily energy production in December drops to less than half of the annual daily average. And if you had to keep the refrigerator running for a cloudy week with little or no solar energy generation, you then need to look at your battery and inverter system to determine if it could supply all of the needed energy.

- Glenn W9IQ
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

KF7YED

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Re: Building a small solar (POWER!) system
« Reply #11 on: March 01, 2022, 09:50:22 AM »

Some great info here to digest. QRM aside, it's been very illuminating, pun intended.

So, a little clarification on the intended purpose: I know it's very limited in capability but the use will be more as a "better-then-nothing" power source for occasional recharging of a few items, I forgot to mention cordless tools before. The reference to the 'fridge was dreaming and not intended to run 24 hrs. 4 - 6 hrs would be nice but the 'fridge would probably be toast anyway so forget that. Same for the microwave: probably toast.

I agree with the point that after the grid falls is the wrong time to learn how to do this, and yes, I'd be playing with it quite a but to see what works and how far. But at some point the electronics, including the panels, will be stored underground in between uses. I have all my surplus gear and test equipment down there now in ammo cans, except when I use them for camping/FD, etc.

So all in all, I'm hearing that I'm kinda on the right track. MPPT, 24V for thinner wire but not crazy thin, as many batteries as I can afford... And nobody really ripped my choice of charge controller and inverter so that might be good to go.

As for location, my best sun spot is beside the south side of my garage and I can put the batteries inside the garage just inside that wall, so that would only be about 10 - 12 ft of cable from the panels to the charge controller. If I mount the controller and the inverter all on one hunk of wood with the attendant fuses, etc., it can be easily disconnected for storage. Same with the panels: wood frame remains in place but the panels come off for storage.

Thanks all. More thoughts welcome. :)

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KT4WO

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Re: Building a small solar system
« Reply #12 on: March 02, 2022, 02:32:56 AM »


"SOLAR POWER IS “DIRTY”"

Brought to you by BP and Shell.

Solar also caused AIDS and COVID.   Ohhh wait,,,  5G Cell towers caused COVID--my bad.

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KT4WO

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Re: Building a small solar system
« Reply #13 on: March 02, 2022, 02:59:15 AM »

"And they make little economic sense, unless heavily subsidized."

Oil and Gas are VERY "heavily subsidized"

Tell ya what RUL--  You fire up your "jenny" in your bedroom and take a nap
and I will do the same with my panels. Lets see who wakes up first. 

Also ---that radio you use is full of stuff that was toxic to make and will be
after it's disposed of.  So I guess you should not have a radio.  HAHA
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KG4RUL

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Re: Building a small solar system
« Reply #14 on: March 02, 2022, 03:48:22 AM »

"And they make little economic sense, unless heavily subsidized."

Oil and Gas are VERY "heavily subsidized"

Tell ya what RUL--  You fire up your "jenny" in your bedroom and take a nap
and I will do the same with my panels. Lets see who wakes up first. 

Also ---that radio you use is full of stuff that was toxic to make and will be
after it's disposed of.  So I guess you should not have a radio.  HAHA

Yes, oil and gas are heavily subsidized.  Never said they were not.  Solar power ONLY makes sense with plans like "net metering" in place.  The equivalent of a retailer being legally forced to buy a product from a wholesaler and reselling it at the same price.

My "jenny" (which is a female Donkey not an electrical power generating device) is outside my home with 250 gallons of propane to fuel it.

AND, I assume that you will be responsibly disposing of all your "toxic" radios also?
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