Elementary solar questions

Am I correct that the optimal orientation for a solar panel is:
Perpendicular to Solar South
at Solar Noon
on the Winter Solstice
???
How about tilt? I suspect it also matters, how is the optimum for that determined?

The necessary components of a system are:
Solar panel(s)
Controller - to keep from frying the battery
Converter - to make the DC usable as AC
???

Any help greatly appreciated but be warned. If you bite on this I might have lots more questions.

Hi Mark,

Yes, normally, solar panels are oriented facing solar south. If you drive a stick into the ground at about 45 degrees (point the upper end at the sun) at solar noon so that its shadow falls on top of the stick, the stick will be aligned with solar south. Solar noon is the mid-point of your daylight hours that you can get from most weather web pages. For example, sunrise at 6:30am, sunset at 7:30pm, solar noon is at 6:30 + (13 hours of daylight / 2) = 6:30 + 6:30 = 1:00pm. Otherwise, a compass with adjustment for magnetic declination can be used. It doesn't have to be perfect, but keep in mind that you lose half the energy when the sun is 30 degrees away from perpendicular to the panel.

The tilt angle is usually set to your latitude + 15 degrees. You can get an estimate of solar production for your location (including different tilt angles) from the PVWatts calculator:

https://pvwatts.nrel.gov/

DIYSolarForum.com is a great place to find tons of info and ask questions about solar power. Here's a thread that talks about solar panel orientation:

https://diysolarforum.com/threads/how-to-correctly-measure-for-solar-angles-for-diy-design-and-build.8422/

You've got the main components. You also need some wire, panel mounting hardware and fuses or circuit breakers. It's good to figure out how much stuff you want to run on the system before you purchase anything. DIY Solar Forum has a whole Learning and Help Center section which is a good place to start.

Mark Reed wrote:Am I correct that the optimal orientation for a solar panel is:
Perpendicular to Solar South
at Solar Noon
on the Winter Solstice
???
How about tilt? I suspect it also matters, how is the optimum for that determined?

The necessary components of a system are:
Solar panel(s)
Controller - to keep from frying the battery
Converter - to make the DC usable as AC
???

Any help greatly appreciated but be warned. If you bite on this I might have lots more questions.

hi Mark,
So it depends...
Where abouts are you latitude wise? What size of array are you shooting for and what are your power uses? Her is why I ask. I'm at 45 latitude so we generally say add 15 degrees for winter, spring fall at latitude and summer at latitude minus 15. In the winter months I can expect an average useable sun total of 1,5 hours in November and December. So while it would be nice to maximize my array for winter production it would create a bad array for the summer if it was fixed. Also a winter array here unless it was huge would still require some generator supplementation so we set up a fixed array for the best spring fall compromise of 45 degrees...
The newer mppt controllers help a lot so don't buy anything but that for a new system.
To convert from DC to ac we refer to it as an inverter...
Cheers, David

Hi Mark

A couple of notes.

In response to these statements....

The necessary components of a system are:
Solar panel(s)
Controller - to keep from frying the battery
Converter - to make the DC usable as AC

You start with batteries.  To start with batteries, you need to decide on a system voltage.  Here are some rough guidelines....

12V: small system that can power lights, TV, computer, and other small home electronics
24V: medium system that can power all of the above, and also a refrigerator, freezer, microwave, some power tools, ect.
48V: large system that can be scaled to power anything, like 240V appliances, well-pumps, compressors, ect.

Golf-cart batteries are a good choice for starter systems.  They are usually 6V, so you wire them in series to get the desired system voltage.  So, to make a 24V system, you wire four 6V golf-carts in series to get 24V.  Lithium batteries are coming into vogue now.  They can be more deeply drained then lead-acid batteries, but do not handle cold well.  Charging a Li battery at freezing will destroy it.  You want to scale the battery size to your daily needs, plus accomodations for cloudy/rainy weather.  Batteries are usually rated in Amphours, or Ah.  You then multiply the Ah X your system voltage to get your Watthours.  Say you have a 210Ah golf-cart battery at 24V.  You've got 210Ah X 24V = 5040Wh of power.  You don't want to drain lead-acid batteries greater than 50%, so that 5040Wh is actually 2520Wh (2.52kWh) of USEABLE power. You scale the battery to your daily needs.  Don't get a 210Ah battery if you need 10kWh of power per day.

Solar panels can be divided into two groups, 12V, and high-voltage grid-tie.  You'll pay the most money per watt for 12V panels, but can be coupled with cheaper PWM controllers to charge 12V batteries.  Panels must be quite close to the batteries they are charging to prevent voltage drop.  Grid-tie panels are cheaper per watt, but you need an MPPT controller to transform the higher panel voltage down to battery voltage.  Larger grid-tie panels can be wired in series to >100+V and then the current ran hundreds of feet through thin copper wire without power loss.  The very high raw solar then gets transformed down to battery voltage at the MPPT controller.

Controller: Two types, PWM and MPPT.  PWM are very cheap but can only work with panels close to battery voltage.  Can't increase the incoming amps.  MPPT controllers act as a transformer to transform raw high solar voltage down to battery voltage, converting the extra volts into extra amps.  They cost more though, sometimes a lot more.  As a general rule of thumb is a break-even point of 300W.  Below 300W, you spend the least money with PWM and 12V panels.  Above 300W you spend the least money with MPPT and grid-tie panels.

Inverter:  Do not say converter.  A converter is a component that changes DC current of one voltage to DC current of another voltage, say from 24VDC down to 12VDC.  An inverter changes low voltage battery DC into high voltage 120/240VAC.  Low budget inverters only produce square-wave or modified sine-wave power.  They are OK for things like lights, but will quickly burn out anything running an electric motor.  Sine-wave inverters can run any appliance, such as power tools, refrigerators, freezers, air-conditioners, or anything else powered with an electric motor.

For my own system, I built single-pole rotating array mounts that are adjustable for both direction and angle.  I rotate them by hand during the day.  I adjust the seasonal angle just two times per year.  I see much more power gain going east to west than I do summer to winter.  Below is a pic of one of my arrays that can hold as much as 1500W of panels.  What I do on the fly is to take a 12" piece of PVC pipe, and put it in front of the panel.  When pointed directly at the sun, I can see the sun shinning through the central hole.

Panels typically can be pointed due south, though there can be exceptions depending on your individual location.  To reduce the level of noon-time amps, some people orient one array SE and another array SW.  You get a lower noon-time peak, but a broader over-the-day power curve.  Maybe the easiest thing you can do is just go outside with sheet of plywood, and just prop it up till it's facing the sun.  Use the pipe-trick to optimize the angle, and then measure that angle with a protractor.  That makes the math easier to stomach.

I'd say a serious off-grid cabin system starts at 24V with ~1000W of panels.  Something like that can run your whole cabin with a 21'st century lifestyle with about a 2000$ investment.

Thank you all for the helpful responses. I am completely new to solar and am getting a "kit" for Christmas. I pretty sure it will include two 100 watts panels and an MPPT controller, plus some wires for connections. I'll be building a mount for it on my upstairs deck railing where it will get full sun all day long.

I'm still reading and learning about inverters and batteries which I will buy myself later. Haven't decided what I want to power with it yet except first some outdoor lights. I'm thinking I want the lithium battery over the lead acid. I don't really want the batteries to be inside the house so I'm thinking of building an insulated cabinet to hold it and other components and heating it with a thermostat controlled 12V heat pad such as I've read about being used on water tanks in RVs.

Like I said it's my first venture with solar and largely just for the learning experience but, if possible, I would like to start with quality stuff so instead of starting over after climbing the learning curve I can expand instead. After reading your posts I feel more confident I can do that. THANKS again!

I would try to dissuade you from buying a kit.  Especially if it includes 100W panels.  They are really very overpriced.  You'd get far more for your money buying components separately.  Buy a MPPT charge controller on-line, but buy the panels locally with local pickup.

Don't worry about having lead-acid batteries inside.  I kept my first system's batteries in the living room for years before I upgraded.  Shop at Costco, and you can get good-quality golf-cart batteries for 99$.

I am afraid I don't know enough about it yet to assemble all the components myself and this kit is going to be a gift. When asked all I knew from researching was to ask for the MPPT type controller. I'm not 100% sure what will show up. I might be wrong but thought that the battery might be the most expensive part so asked that it not be included, plus I didn't know what kind to get.

A still was an option too, so I won't know 100% for sure until after Christmas if I'm learning to make electricity or booze. I'm hoping for electricity, I'm pretty sure I can make my own still.  

With two hundred watts of power the lithiums won't give you any advantage. Usually the 12 volt kits come with a pwm controller. A lot of cheap controllers say mppt but can only take a Mac panel voltage of 20 volts ... Post when you get it I'd be curious to know.
Cheers,. David

Some of the best info I've seen so far. One of the key things here is no inverters. The way they did this is quite clever and economical:

https://livingenergyfarm.org/wp-content/uploads/2019/03/tech1.pdf

The OP was saying it was going to be a gift and they weren't that sure on how what to get or how to assemble and asked about kits:

https://livingenergylights.com

These folks are big fans of the Edison nickel iron batteries but I think lead acid batteries are still going to work better for many people, depending on the situation. Particularly on small scale.

One of the "catches" to the nickel iron batteries is that they have to be dumped out and refilled with fresh lye occasionally. Nasty process with somewhat dangerous  leftovers to dispose of. Getting lye pure enough is also a problem at the moment from what I'm reading. Those Edison batteries will go almost forever if you're game for the maintenance hassles though.

Lead acid batteries have a recycling system in place almost everywhere and are actually valuable as scrap metal goes.

Would someone please explain a comment made by Michael Qulek;  "Charging a Li battery at freezing will destroy it."  Does that mean that if I have a small solar light outside in freezing weather and it needs to charge up it in freezing weather it will be ruined??

Denise Cares wrote:Would someone please explain a comment made by Michael Qulek;  "Charging a Li battery at freezing will destroy it."  Does that mean that if I have a small solar light outside in freezing weather and it needs to charge up it in freezing weather it will be ruined??

Hi Denise,

This is slightly off topic, but yes, Li-ion batteries can be destroyed by charging with high currents at low temperatures (and high temperatures). That said, all is not lost! Many small outdoor lights use NiMH (nickel metal hydride) batteries instead of Li-ion. For example, the outdoor motion sensor LED light I have uses AA size NiMH batteries that can be easily replaced when necessary. These batteries are readily available and can be used to replace normal AA batteries in most devices.

Check your solar lights and see if they have replaceable AA or AAA size batteries. If so, they are probably NiMH and you are good to go.

Thank you M Rives for the clarification.  I will check on the batteries in my small solar lights.  

Denise Cares wrote:Would someone please explain a comment made by Michael Qulek;  "Charging a Li battery at freezing will destroy it."  Does that mean that if I have a small solar light outside in freezing weather and it needs to charge up it in freezing weather it will be ruined??

Most likely a light DESIGNED to work outside will not be damaged in cold weather.  The Li batteries I'm referring to are the Li batteries designed for off-grid solar.  The boxy shaped ones in the 100Ah range.  Those can NOT receive charge below freezing.  There is actually a chart, which shows the maximal charging rate as the temperature drops.  That charging rate drops to zero at about the freezing point (plus or minus).  Exceed those limits and the battery suffers permanent damage.

In terms of other Li battery based things, I see variable results that are temperature dependent.  I have a wireless weather station, powered by Li AA batteries, and I can see that the outside temperature sensor stops working when the outside temp drops below about 45F.  The next day, when the temperature warms back up, the sensor will start reading again.  Some other observations I've made are with Li battery powered bathroom stuff like an electric toothbrush and electric waterpic, both exposed to cold temperatures.  The toothbrush stopped working, but upon bringing it back to about 70F, it starting working again.  Keeping at 70F, and it has not again had any problems.  The waterpic however is dead.  Warming it back up did nothing.

Li batteries in general have circuitry attached called a battery management system, or BMS, that controls charging.  That's because Li batteries are known to have burst open and catch on fire because of improper charging.  The problem with BMS's is that they sometimes protect too well, and go into full shutdown mode.  Only a factory reset brings the battery back on line.  This is manufacturer dependent, and you have to contact your battery supplier to figure out how to do this.  For a big off-grid storage battery, this might be worth doing.  For a smaller, appliance battery, it's seems easier to just throw it away.

Michael Qulek TY for your observations. I checked my little solar lights and they indeed have Li-ion battery a rather large/oversized AA almost a double one in a plastic wrap (but it is not double AA), so not sure what size to call it. I have charged new lights outside and it is 30deg now and they seem to work. I think you're right they are designed to be used outdoors. So temperature extremes cold/hot may shorten the life of batteries but it may be a variable thing with some types of batteries more sensitive than others (maybe a manufacturing flaw in the short lived/malfunctioning ones?).  I had another outdoor solar light wired for use inside the shed which was rather short-lived with a 5-pack AA battery Ni-Cd. It would not charge after a couple years.  Florescent bulbs also will not light up quickly in cold weather (I use them in the garage). I read in the manual for my Li-ion battery powered tool set that the charger and batteries should not be charged in conditions below 50 deg or more than 84 deg. Also the battery can be used in hot/cold conditions but must "be in a suitable temp range" before it will charge in the charger. So that must be to protect the Li-ion from damage like was said.  I don't know about Ni-Cd or if temperature applies to them.

Denise Cares wrote:I checked my little solar lights and they indeed have Li-ion battery a rather large/oversized AA almost a double one in a plastic wrap (but it is not double AA), so not sure what size to call it. I have charged new lights outside and it is 30deg now and they seem to work. I think you're right they are designed to be used outdoors. So temperature extremes cold/hot may shorten the life of batteries but it may be a variable thing with some types of batteries more sensitive than others (maybe a manufacturing flaw in the short lived/malfunctioning ones?).

Hi Denise,

Thanks for confirming that you have Li-ion batteries in the small solar lights. They may be 18650 size, 18mm diameter (just under 3/4") x 650mm long (about 2 9/16"). These can be ordered in styles with a small button top or a flat top so keep that in mind if you need to replace them.

For more info on the temperature issue, the following forum entry discusses a paper that tested Li-ion batteries for 100 cycles at various temperatures. The summary seems to be that the batteries have lower capacity at lower temperature but that the capacity recovers after the batteries warm up. They are charging at a low current like your solar lights with a small solar panel which probably helps prevent damage.

https://diysolarforum.com/threads/why-you-cannot-charge-lifepo4-below-0-degrees-celsius.2912/post-370147

As Michael said, this is a much bigger concern in a larger solar power system. Large LiFePO4 cells can be pretty spendy so erring on the side of caution is a good idea. The small 18650 cells can be found for about $3 so replacing them after a few years is doable.

Michael Qulek wrote:In terms of other Li battery based things, I see variable results that are temperature dependent.  I have a wireless weather station, powered by Li AA batteries, and I can see that the outside temperature sensor stops working when the outside temp drops below about 45F.  The next day, when the temperature warms back up, the sensor will start reading again.

Hi Michael,

The Davis weather station I have showed the same problem. The Lithium AA battery (which is not a rechargeable Li-ion in my case) provides backup. The main power comes from a super capacitor that is charged by a solar panel. After some time, the super capacitor wore out. Replacing it allowed the weather station to transmit all the time again. If you've tried replacing the battery and it still stops reporting when cold this might help your problem, too.

Now we're totally off the original topic. Is that OK here?

Here's a different method to consider ...

http://greenerenergy.ca/PDFs/Tilt%20and%20Angle%20Orientation%20of%20Solar%20Panels.pdf

I took a look at that "Green Energy" post and found fault with it immediately.  I disagree with the following statement....

[b]The winter season has the least sun, so you want to make the most of it. The tilt should be
designed so that the panel points directly at the sun at noon. To calculate, multiply your
latitude by 0.9, and add 30 degrees.[/b]

I would say no, you don't want to do that.  Pointed exactly perpendicular at noon means that the panel is only at an optimum angle for a few minutes each day.  For the rest of the day, it's less than optimal.

What I recommend is orienting the panel such at it's optimal about about 11am instead of noon.  That way the sun is oriented within a few degrees of perfect for several hours instead of several minutes.  Since my rotating arrays are almost infinately adjustable, I've fiddled constantly with array orientation to get the absolute maximal output, mostly for running my 240VAC well-pump.

Just build adjustable array mounts like I did, and you can experiment with this yourself to your heart's content.