String system vs. Optimizer System

[Esther1989]

I am new to the solar world and new to this forum. I am looking for unbiased advice from experts/those more experienced (folks like you all!). For anyone willing to reply, would you recommend a 22.25kW, two-sided (bifacial) string system with three 7.7kW inverters or a 19.5 kW monofacial power optimizer system with two 10kW inverters? This will be a residential, ~30,000kWh ground mounted system, full sun all day with no shade excepting clouds, which we do get. Any advice? Thank you very much.

[SunEagle]

Hello Esther1989 and welcome to Solar Panel Talk

It would be hard for me to advise you which way to go. For me it would depend on the $/watt installation cost.

Although a lot of residential systems are limited to a specific kW of installed pv so I would make sure your POCO is ok with either the 19.5kW or 22.25kW systems before you purchase anything.

[heimdm]

Which bifacial panels? Is snow an issue where you are? Bifacial is done correctly, can be way to mitigate snow losses. LG has a decent bifacial design guide.

[Esther1989]

These would be 375W Canadian Solar bifacial panels. The other company (similar wattage and price) is offering Silfab 390s (with power optimizer and not bifacial). Both are good salesmen, which leaves me stumped. We do have snow on the ground frequently from mid-late December through February. Further thoughts? Thank you again.

[JonathanM]

Hi Esther, the first question for me is, why such a large system? Are you powering a multi-family residence, or an extremely large house? Typical residential solar systems are usually less than 10kW. Then, for me, I'd choose the optimizer system over the string system, especially with such a large array, because when a cloud (or a bird) shades one panel, it won't drop the output of all the others in the same string.

[J.P.M.]

These would be 375W Canadian Solar bifacial panels. The other company (similar wattage and price) is offering Silfab 390s (with power optimizer and not bifacial). Both are good salesmen, which leaves me stumped. We do have snow on the ground frequently from mid-late December through February. Further thoughts? Thank you again.

As others ask what's your climate like ? Lots of snow ? Latitude ? proposed array tilt angle ? Those are pretty big arrays. That makes me wonder what's your annual electric load as well as what conservation measures you've taken to reduce that load and so the array size.

If you are new to the world of solar energy as you say, I'd suggest you not commit to anything like a contract until you do three things:
1) Download a free (but slightly dated) PDF version of "Solar Power Your Home for Dummies" or buy a hardcopy of the revised version at bookstores/Amazon. Pay particular attention to the sections about reducing the load any PV array will offset.
2.) After the book, see something on the web called PVWatts. It's a pretty easy to learn/use PV modeling program. Then do your own preliminary sizing and design.
3.) Come back here and ask questions to fill in the gaps created by your own learning process.

You ask questions now, the good and thorough answers to which are a bit more involved than you may be able to understand at the point your current knowledge level seems to be at. Read the book and do your own preliminary sizing. You'll be better informed and the questions you then ask here will be a lot meatier. It'll also be a lot harder for the peddlers to B.S. you which I'd guess is happening now. Best of all, you'll have a better system for having made the effort to be informed

An example of why some/most questions don't have one word/one sentence answers: Opinion aside, whether or not your application can benefit from bifacials vs. non bicacial panels depends on, among other things besides price, your array's tilt angle as well as how much of the time you have snow ground cover, and also dependent on how much ground clearance the array will have or require for considerations not only of utilizing any albedo enhancing effects which are the crux of the bifacial benefit such as it may be, but also not blocking sunlight to the array from snow buildup. Reason(s): High ground clearance and/or high array tilt angles are usually necessary to get the best chance of any benefit such as it might be from the bifacial concept (and BTW, that performance enhancement by bifacials is usually a lot less in common applications than peddlers or their heavy on innuendo, but low on more than very specific adverts. would have us believe).
If the total array price per PV generated kWh were to be the same is the same using either panel, before such figures could be relied upon, the bifacial offerings and claims would then need some further consideration of how much the performance enhancement estimate for the bifacials was influenced or enhanced by the above mentioned sketchy and probably not realistic enhancement estimates.

Get the book, do the preliminary sizing, then come back here and ask questions before you consider spending dime one. As it is now, IMO only, and like most folks who drink the solar kool-aid, your solar ignorance will cost you money and result in a less than fit for purpose system. The way it looks now, you don't know what you don't know and that will cost you $$ and you'll have a not as good and array as possible. Some time and effort will pay big dividends.

Take what you may want of the above. Scrap the rest.

BTW, welcome to the neighborhood.

[solarix]

For what its worth, my experience is that optimizer type systems are markedly less reliable than string inverters. Lots more points of failure and in inhospitable locations. They claim 25 year warranties knowing that their MTBF is much less. However, you are talking a ground mounted system so this is not so bad as at least they are serviceable.
You are also talking a huge system for a residential application. Make sure the local power company will allow it before proceeding. Call them up and ask for an engineer to evaluate your service before signing anything.

[peakbagger]

IMHO String all the way on ground mount. Price is lower reliability is higher. Preferably run high voltage DC from the strings to a nice cool shady spot like a basement to "baby" the string inverters. The enemy of any electronics is heat, Optimizer are sitting on the back side of a hot panel, a string inverter in shady spot or a basement will be lot cooler. Many string inverters have multiple MPPT inputs so with a bit of creative thinking you can sectionalize the strings to deal with any shading events.

The one caveat is in one state in particular, Mass, (at some point possibly others) there are big incentives for home batteries but in order to get those incentives you need a system that can be controlled remotely. Solaredge has an optimizer based system that claims to be compatible as well as Tesla. So like many solar decisions it may come down to the details in what you want to do.may drive you to specific technology.

[Esther1989]

Thanks everyone. J.P.M., thank you for the recommendation. I have just downloaded the book and will get to it late this week. Peakbagger, the system will be very near a pole barn (barn directly north of where panels will be), so I will ask the companies about putting the inverters inside there (shade) - thanks. SunEagle, I will talk with a POCO rep, thank you. Solarix and JonathanM, you got it - big old farm house, not terribly well insulted with a large family. We have geothermal heat/air and are in a colder climate. So, while I am not proud of it, the reality is we have a very heavy load, ~November-March in particular. We are also accounting for the addition of two electric cars within the next 1.5 years. Added up, estimates to a bit over 30,000kWh/yr.

It sounds like optimizers = potentially better in clouds + potentially more to go wrong. String = More reliable with the risk of clouds impacting production more (I just don't know how to determine how much). I will check out the book and talk more with others. This has ben very helpful. Any further thoughts from you all?

[Mike90250]

> optimizers = potentially better in clouds
totally wrong, unless you have a climate where a small cloud parks itself over a single panel for more than a minute
Optimizers only shine, when you have difficult shade issues, like telephone pole that shades portions of your array for over half a day, or a neighbors palm tree casting shade that moves across the array.
Clouds block the sun, and there is nothing to optimize in that case.
Optimizers don't increase harvest, they prevent a terrible sag in power from the whole string, when one panel gets shaded.

[J.P.M.]

Thanks everyone. J.P.M., thank you for the recommendation. I have just downloaded the book and will get to it late this week. Peakbagger, the system will be very near a pole barn (barn directly north of where panels will be), so I will ask the companies about putting the inverters inside there (shade) - thanks. SunEagle, I will talk with a POCO rep, thank you. Solarix and JonathanM, you got it - big old farm house, not terribly well insulted with a large family. We have geothermal heat/air and are in a colder climate. So, while I am not proud of it, the reality is we have a very heavy load, ~November-March in particular. We are also accounting for the addition of two electric cars within the next 1.5 years. Added up, estimates to a bit over 30,000kWh/yr.

It sounds like optimizers = potentially better in clouds + potentially more to go wrong. String = More reliable with the risk of clouds impacting production more (I just don't know how to determine how much). I will check out the book and talk more with others. This has ben very helpful. Any further thoughts from you all?

Yea, agreeing w/Mike. If a ground mount, you'll have the best chance of optimum array orientation including avoiding shade. Therefore, if a mostly shade free location and orientation is chosen, optimizers will not yield much if any increase in efficiency or energy harvest, only increase the number of things to fail for more design complexity and probably more cost.

Not using energy by use reduction and cost effective conservation measures such as weather stripping and insulation are way more cost effective than offsetting the same amount of electricity saved by generating relatively more expensive energy with PV. As a bonus, any system can be smaller (read less $$) to meet a lower load. Turning off a light costs nothing for example - it's just not as sexy as PV.
Also, and to stress, knowledge is power. Don't make commitments and don't spend spend money without it. Caveat Emptor. Peddlers make money by putting their wares on your property, not by saving you energy or lowering your electric bills.

[khanh dam]

bifacial panels are better without a question. it is not even close! if they have a front and back glass that means they will last for 30 years easily. in fact most bifacial warranties are 30 yeras, not 25 years.
i do not trust any plastic back sheet panel to last that long. plus they look way cooler on a ground mount.

[bcroe]

bifacial panels are better without a question. it is not even close! if they have a front and back glass that means they will last for 30 years easily. in fact most bifacial warranties are 30 yeras, not 25 years.
i do not trust any plastic back sheet panel to last that long. plus they look way cooler on a ground mount.

I am waiting for a report on just how to use them to advantage. Is the sensitivity
under standard conditions THE SAME on either side? IF THEY WERE, they might
be candidates for replacing a pair of east-west panels I currently wire in parallel.

Just how I support them with no structure on either side is not obvious, guess some
sort of frame around them could do it. They would then need to be straight up to
not favor either direction, not really optimum, more needed, but good snow rejection
if the frame works. I am not sure if some were given to me, it would justify all the
problems they would would cause over back to back standard panels.

Even standard panels pick up light from the back, no numbers there either. Bruce Roe

[Hkwind1991]

+1 on bifacial panels and snow mitigation. I work on the project development side of utility-scale solar, and these are popular with most developers. If it's good for the utility-scale side, I would tend to believe it also good for the residential side in this case.

[SunEagle]

+1 on bifacial panels and snow mitigation. I work on the project development side of utility-scale solar, and these are popular with most developers. If it's good for the utility-scale side, I would tend to believe it also good for the residential side in this case.

That is pretty good news that utilities are using the bifacial panels to see how they work. Who is using them? I ask because here in Florida I have only seen basic panels arrays set up by POCO's. Since snow is not an issue here maybe the bifacial are not as efficient based on the cost per watt.