Common solar PV questions/mistakes

bill von novakbill von novak Solar Expert Posts: 694 ✭✭✭✭
Below is a list of common solar mistakes. They come up often, in the form of questions on boards like this one. They are all reasonable questions for someone starting out in the solar field, but when you look more closely at the issue, there are some problems as discussed below.

Please don't take this message as "you're dumb if you think these things." I learned that most of these were mistakes by actually making those mistakes; I spent years underestimating costs, trying to install unapproved grid tie/grid interactive systems, and scratching my head when my systems didn't produce half the power I thought they would.

=======================

Mistake #1 - "I just got a bunch of solar panels! How do I hook then up? I spent most of my money on the panels so the rest of the stuff can't break the bank."

The Big Idea:
I came across a cheap site on the Net and I got as many panels as I could, because these deals can't last! Getting enough panels must be 99% of the battle; the rest is just nuts and bolts and wires.

Reason this is a bad idea:
The inverters, batteries (if off grid) and the other BOS (balance of system) hardware can cost as much - in fact often costs more - than the panels themselves. You need lots of wire, since solar panels are usually mounted at least vertically distant from the load they are driving. The wire you need will often be expensive; for battery wiring you could easily pay a dollar an INCH for terminated cables. You need fuses or circuit breakers, DC disconnects that can handle high voltages and currents, inverters to use the power, charge controllers/batteries if it's off grid. You need a mounting structure that will keep your roof on your house and prevent those panels from becoming spinning 40mph battering rams in a bad storm. And all those things are very important if your goal is installing a safe system. Wiring and circuit protection are two things you CANNOT skimp on - and this is even more important for people experimenting with solar, because you're going to make mistakes.

We see this question a lot because people see a good deal and buy a bunch of solar panels and want to make them work. Nothing wrong with that, but if you are money constrained, it's much better to buy half the number of panels and spend the rest on good inverters, batteries etc.



Mistake #2 - "I just want to feed a little power back to the grid. I saw this grid tie inverter on Ebay for $99 . . . ."

The Big Idea:
Getting grid tie approval sounds like it's a pain in the butt, and I just want to generate a LITTLE solar without all that paperwork.

Reason this is a bad idea:
It's not hard to design a grid tie inverter that feeds power back to the grid. In some ways it's easier than designing a standalone inverter, since the impedance of a grid tie connection is much more constant, and you don't need to provide a time base. But there's a reason that grid tie inverters tend to cost a fair amount - they meet NEC requirements for power factor, safety and anti-islanding. And again, these things are even MORE important for experimenters than they are for professional installers, because experimenters are going to make mistakes - and you'll want that extra level of protection when you accidentally island your loads.

Also, UL listed grid tied inverters are going to be part of any serious grid tie installation. You'll learn more about real installations by working with real hardware.



Mistake #3 - "I don't want to do grid tie, I just want to charge a small battery bank with solar, run off that, then use the grid only when they get low."

The Big Idea:
Getting grid tie approval sounds like it's a pain in the butt, and this way I can avoid all that hassle and still generate most of my power via solar; just add a few batteries to my system.

Reason this is a bad idea:
Batteries are the biggest cost in any off-grid power system. Not because they are the biggest cost up front - but because they don't last long. Golf cart batteries will give you around 500 cycles discharging to 50%, which is around a year and a half of solar off grid usage. People don't want to replace their batteries every year and a half; it's heavy lifting, they're full of acid and lead, you have to recycle them, the new ones cost money, it's the dirtiest part of the system and you ruin your pants etc etc. So they find ways to make them last longer. They use larger cells that a) last longer to begin with and b) only have to be discharged to 30% (or some other lower number.) This extends their life.

For people on the grid the answer is much easier - never cycle them. Use a hybrid inverter that keeps them float charged all the time while feeding excess power back to the grid. This allows batteries to last much longer. Telecom batteries in such applications (generally called UPS, or uninterruptible power system applications) can last twenty years because they are not cycled and kept at their "happy" state of charge. Only during the rare blackout do they see cycling.

The reason Mistake #3 is a mistake is that it attempts to do the exact opposite of all of the above - it attempts to use a small battery bank and cycle it hard every single day. This means you'll go about a year between battery changes if you're lucky, and you'll be assured of a lot of heavy lifting and buying of new batteries.



Mistake #4 - "I get like ten hours of sun a day so I shouldn't need a big system."

The Big Idea: Since it's nice and sunny ten hours a day up here I should be able to generate close to full power ten hours a day - so I don't need as many panels.

Reason this is a mistake:
Equivalent sun hours refers to the total full power sun the system sees. Most systems really see full power for only a few hours a day; the rest of the time the sun is at an angle to the panels and shining through too much atmosphere to get to full power. Even tracking arrays only help a little with this. Thus 12 hours of daylight might give you only 6 hours of _equivalent_ direct sun - and direct sun is what you have to base your total energy output on. The NREL website has good maps that show you how much equivalent direct sun you can expect in various locations, and it's almost always less than you think it will be.



Mistake #5 - "My neighbor has a two kilowatt system but he uses 600 kilowatts a month! That's not helping him at all."

No Big Idea here. This is a common misconception; it confuses power (kilowatts) with energy (kilowatt-hours.) A two kilowatt system, if exposed to four hours of equivalent direct sun a day, will generate around eight kilowatt-hours of energy. Actually somewhat less due to efficiency losses in the wiring and inverter.



Mistake #6 - "I don't want to run everything, just air conditioning and my efficient refrigerator."

The Big Idea - It might be tough to run a whole house, but just a few appliances should be that much easier.

Reason this is a mistake: Air conditioning, refrigeration, and pool pumps (if installed) are the biggest users of energy in a house. (Resistance heating can be an even bigger energy user, but it's madness to try to run resistance heating on solar electric systems.) A small standalone system is much better used to provide some LED lighting, backup power for laptops and radios etc. Even better, do a small grid tied system and offset some of the load from the bigger users.



Mistake #7 - "This system is real expensive if I get it installed. But I looked online and panels are really cheap, so I'll do it myself to save money."

The Big Idea - Greedy installers make solar way more expensive than it needs to be, so doing it myself will save me lots of money and make me energy independent from the bureaucratic power companies.

Reason this is a mistake: Much of the cost you see on an installer's estimate has to do with the BOS components he will have to supply - heavy gauge wire, DC disconnects, racking etc. These are the easy to overlook pieces that you will need to install the system. Once you add all those in you'll see the actual labor costs are fairly low compared to the costs of the parts. Thus you really aren't saving that much.

For people who really want to save money, AND have experience as an electrician, you may be able to find an installer who will let you do most of the work. They will take care of the parts that are dangerous (like wiring the DC disconnect) and annoying (like applying for permits and scheduling inspections) and leave you to drill holes, bend conduit and pull wires. In general you have to know someone very well to do this, because the installer won't want you to botch the rack installation then get sued when your roof leaks.



Mistake #8 - "Why can't you combine solar-PV and solar hot water panels?"

The Big Idea - If solar panels get hot anyway, why not cool them with water, then you can use the water for your hot water! That kills two birds with one stone.

This is a not a bad idea but does not work for domestic hot water. To get 120F water in your tank your outlet water will have to be close to 130F - and that means the panels themselves have to be 140-150F. It _sometimes_ works for pool heating since panel outlet temps on a well designed pool system rarely rise over 90F, due to low water temperatures and high flow rates. This can keep the panels cooler and give you some heat. The drawbacks of this are increased complexity (two connections per panel) and safety issues (high voltage DC and water don't mix well.)

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Common solar PV questions/mistakes

    Well said!
    I'm going to glue this down in hopes people will read it before they make any of those mistakes.
  • BB.BB. Super Moderators Posts: 26,768 admin
    Re: Common solar PV questions/mistakes

    Very nice post Bill. :D

    I will add, another one...

    Need to add more batteries--Do not have enough to power my system over night/couple days of cloudy weather.

    In general, a solar PV system needs to have its components balanced. Loads against battery capacity, Loads against solar array. Even batteries against solar array. Too large of battery bank or too small of battery bank creates their own sets of problems).

    Many times, people have too large of battery bank, draw too many kWH per day, and don't have enough solar panels to recharge the bank quickly and completely. Just adding batteries usually makes the situation worse (more battery capacity takes even longer to recharge with same solar array, have higher losses, may not last as many years because of chronic "deficit charging", and cost more to replace the next time).

    In the "olden days", batteries were relatively inexpensive and solar panels were horribly expensive. Today--It batteries are "not cheap", and solar panels are relatively inexpensive--So for many people, their days of nursing battery banks and cutting loads can be addressed by adding to the solar PV array. Batteries will last longer, recharge quicker, and during sunny summer weather you may even have the ability to run an A/C system (modern, high efficiency, "inverter" type compressor) in your off grid home (some folks are even able to use heat pumps for winter heating and lower fuel costs/wood usage).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bill von novakbill von novak Solar Expert Posts: 694 ✭✭✭✭
    Re: Common solar PV questions/mistakes
    BB. wrote: »
    Very nice post Bill.

    Thanks, and thanks for the addition. I'll keep track of the additions in case I publish it somewhere else if that's OK with you. (Might also add a caution about mixing old and new batteries, and having a lot of paralleled strings.)
  • BB.BB. Super Moderators Posts: 26,768 admin
    Re: Common solar PV questions/mistakes

    We are here to spread information.

    Always nice to have a link back to whomever supplies information--But not going to worry about it.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Common solar PV questions/mistakes

    You left out what may be the most asked question, which is "How can I get my grid tied system to keep working when the grid goes down? Can't I fool the inverter somehow into thinking that the grid is still up?"
  • bill von novakbill von novak Solar Expert Posts: 694 ✭✭✭✭
    Re: Common solar PV questions/mistakes

    "How can I get my grid tied system to keep working when the grid goes down?"

    Another good one; I'll add that.
  • icarusicarus Solar Expert Posts: 5,090 ✭✭✭✭
    Re: Common solar PV questions/mistakes

    Mistake number 1B. I overestimate my solar input. Mistake number 1B. I under estimate my loads(and have little idea on how to reliably estimate either)!

    Mistake number 2. I have x watts of pv, I should be able to. Ue the same X watts of power when the sun is shining
    Mistake 2 B. See 2 above,, and add in more loss for a batter based off grid system.

    Mistake Number 3. Deep cycle batteries can deliver power to 0% SoC and fully recharge without damage

    Mistake 3A I can recharge a large battery with a small solar panel.

    Enough for now,, do you suppose anyone is going to actually read these?

    Tony
  • bill von novakbill von novak Solar Expert Posts: 694 ✭✭✭✭
    Re: Common solar PV questions/mistakes
    icarus wrote: »
    Enough for now,, do you suppose anyone is going to actually read these?

    Tony

    Probably not. I'm doing it more so I can just copy and paste (or direct people to such a post) when I get asked the same question yet again.
  • BB.BB. Super Moderators Posts: 26,768 admin
    Re: Common solar PV questions/mistakes

    Bill,

    When you are "done", will post a copy in the working thread section.

    -Bill "without the cool last name" B.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bill von novakbill von novak Solar Expert Posts: 694 ✭✭✭✭
    Re: Common solar PV questions/mistakes

    Posted, although in two chunks due to message length limits.
  • nsaspooknsaspook Solar Expert Posts: 396 ✭✭✭
    Re: Common solar PV questions/mistakes

    Mistakes is being too judgmental, "an expensive misadventure" is the proper PC term to use to explain all the "junk" in the shed. :D
    http://farm9.staticflickr.com/8424/7701587722_9e3e30a317_z.jpg
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Common solar PV questions/mistakes

    another one is mixing different pvs with differing values. series pvs need the current to be similar with current staying the same and the voltages will add. parallel pvs need similar voltages with the voltage staying the same and the currents will add. we agree that the farther out of tolerance the more mismatched it will be and some feel 10% is tolerable, but i like the 5% figure. some out there have just bought random pvs that don't mix well with systems that will not work for their requirements.

    batteries are a similar, but a more critical case as one should use the same exact battery for all as close in age as possible up to about a year differential without abuse to them with wiring that is not just ample for the rated current, but low enough in resistance to help keep a balance between the batteries preventing the over or under working of batteries in a bank. bill von novak mentioned the batteries, but this goes beyond just the mixing of old and new batteries.

    another is the notion that they can build a quality pv cheaper than those commercially made to save money. even if they could it won't pass inspection as only certified equipment is suitable for passing an inspection.

    there are more i'm not thinking of right now, but i'm sure they too will eventually get added.
  • MarkCMarkC Solar Expert Posts: 150 ✭✭✭
    Re: Common solar PV questions/mistakes

    Just joined - but I do have a few months of actual experience with a 4000 watt grid-tied system. I'd like to add a couple of MY misconceptions:

    1. I should get close to the "nameplate" capacity of the panels down here in the Gulf Coast during the height of summer sunshine! Well apparently, when the ambient climbs to 95 plus degrees F (often 100 +) for several hours during the best sun of the day, the panels get hot enough to reduce their productivity - by up to 15 - 20%. This WILL happen for most days in July and August in the location of my system. Note that this could also affect the inverter if installed in an uncontrolled environment as it can potentially overheat and begin to cut back on its capacity.
    2. The highest instantaneous watts should be produced on the brightest, sunniest days!. Appears to me that bright, sunny days WITH scattered clouds actual produce significantly more instantaneous watts and as long as the clouds are widely spaced, you might even get better daily production??
    3850 watts - 14 - 275SW SolarWorld Panels, 4000 TL-US SMA Sunny Boy Grid tied inverter.  2760 Watts - 8 - 345XL Solar World Panels, 3000 TL-US SMA Sunny Boy GT inverter.   3000 watts SMA/SPS power.  PV "switchable" to MidNite Classic 250ks based charging of Golf cart + spare battery array of 8 - 155 AH 12V Trojans with an  APC SMT3000 - 48 volt DC=>120 Volt AC inverter for emergency off-grid.   Also, "PriUPS" backup generator with APC SURT6000/SURT003  => 192 volt DC/240 volt split phase AC inverter.  
  • jcheiljcheil Solar Expert Posts: 719 ✭✭
    Re: Common solar PV questions/mistakes
    MarkC wrote: »
    Just joined - but I do have a few months of actual experience with a 4000 watt grid-tied system. I'd like to add a couple of MY misconceptions:

    1. I should get close to the "nameplate" capacity of the panels down here in the Gulf Coast during the height of summer sunshine! Well apparently, when the ambient climbs to 95 plus degrees F (often 100 +) for several hours during the best sun of the day, the panels get hot enough to reduce their productivity - by up to 15 - 20%. This WILL happen for most days in July and August in the location of my system. Note that this could also affect the inverter if installed in an uncontrolled environment as it can potentially overheat and begin to cut back on its capacity.
    2. The highest instantaneous watts should be produced on the brightest, sunniest days!. Appears to me that bright, sunny days WITH scattered clouds actual produce significantly more instantaneous watts and as long as the clouds are widely spaced, you might even get better daily production??

    Actually you will get the most wattage when it is bright and COLD. The heat really kills production from the panels. In the winter here in central Florida my 2460w (rated) array sometimes puts out 2650+ watts at noontime in January when it is 40-50 degrees out. In the summer when it is 90+ degrees, it never goes much over 2150-2200.
    Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html
  • RyanRichardsonRyanRichardson Registered Users Posts: 3
    Excellent post with tons of good advice! You’re right, lots of people seem to think that they can buy a bunch of solar panels and the rest will just take care of itself. It’s really important to remember that getting them properly installed and wired up is no simple task that involves expensive hardware.
  • terrycrunkterrycrunk Registered Users Posts: 1
    Question: should charge controller be grounded to earth ?
    My panels are grounded to earth a long with the disconnect box, the inverter also to same ground, can't get the charge controller and i tried two different ones to either stop charging or start charging with limit settings, is my grounds causing this?
  • BB.BB. Super Moderators Posts: 26,768 admin
    Terry,

    Please create a new discussion (thread) with your questions. Then we can focus on your Q&A vs the above topic. And you can tell us more about your setup (what charge controller(s), battery bank setup, AC inverter if any, etc.).

    In general, earth grounding should not affect the day to day operation of your charge controller--Earth grounding is more for Lightning and Static Energy discharging.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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