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Factors for selecting solar module for a given array size

Factors for selecting solar module for a given array size

Factors for selecting solar module for a given array size

I am fairly new to solar and have been doing some research to try to understand how a typical solar PV system is put together.  I have learned how to size an array based of factors such as avaliable sun hours in a day and the locations average daily consumtion however once you have an array size calculated how do you determine what is the optimal panel for use in the array?

There are several panel manufacturers out there each one offering several panel types.  What are some of the factors that go into selecting the optimal panel for a calculated array size?  Does it come down to cost, or is it more module specification driven?

Thanks for the help.

RE: Factors for selecting solar module for a given array size

Local climate and weather conditions, probable cleaning requirements, alignment tracking system, ... are all elements of obvious importance.
Average daily consumption can be a seriously misleading parameter.  The actual instantaneous daily loading profile with some good indication of weekly, monthly, or seasonal variations (or some reasonably good approximation) will provide a much better basis for estimating system requirements.  A very important consideration is the extent to which this system must be self sustaining.

Valuable advice from a professor many years ago:  First, design for graceful failure.  Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs.  Only then can practicality and economics be properly considered.

RE: Factors for selecting solar module for a given array size

Thanks ccfowler!

Thanks for poining out the actual instantaneous daily load profile vs the avearge daily consumption.  This makes sense.

So lets say I have not calculated the size of my array lets say 100kw for example.  How do I decide weather I want to use 230W panels, vs 240W panels or even 245W panels?  Would using aprox 409 panels at 245W vs using 435 panels at 230W be a better choice?

Also when looking at a grid tied system is there a certain panel range that is used for selecting panels in such a system?  For instance a range of 230-245W or some other range? Would you ever use lets say a 100W module on such a system?

Thanks for the help.

RE: Factors for selecting solar module for a given array size

Panel power is essentially arbitrary, and dictated solely by the specific technologies and circuits used.  All else being equal, it makes no difference.  The biggest question is indeed total power, and whether you are attempting to be completely offgrid, in which case, you would need to meet the peak demand, in addition to ensuring that EOL requirements are met.  EOL may require both allowance for degradation of the panel output over lifetime as well as potential increases in peak power demand.  I know that my power demand has significantly increased over time, and I'm probably consuming double the power that I used to consume, even within the same house.

Note also, that peak power needs to be correlated to the time of day at which that occurs.  If your peak power occurs at, say, 4pm, then you need to account for the fact that insolation will have dropped off at that time.  This may require substantial overcapacity, and may not be worth the investment.  That's a trade the owner must make.

In most residential applications, peak power may not be so critical, particularly as many homes' peak power consumption occurs near the end of the day, e.g., the A/C kicks on at 5 or 6 pm, and the insolation could not possibly support peak power without doubling or tripling the panel areas.  In such cases, the desire may be simply to ensure that the watt-hours supplied to the grid is just more than what's consumed, to minimize the utility bill.

Panel power is more of an issue with respect to total available roof area.  Ideally, you want to minimize the structural loading on the roof, so you want panels with the highest power per roof area.  This is often already limited by how much south-facing roof area you have.  There is at least one company that builds solar panels that are form-fit compatible with roofing tiles, ala EagleLite, i.e., to install the solar panels, you'd remove the existing Eaglelite roofing tiles and replace them with the solar panel shingles that cover something like 10 roofing tiles.  This would reduce the loading on the roof, and minimize the esthetic impact.

Another factor to consider is whether your panels come with micro-inverters, or whether the panels feed into a shared inverter.  There seems to be a divergence of opinion on that subject.  Seems to me that microinverters are more flexible, and more fault tolerant, assuming all else being equal.


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RE: Factors for selecting solar module for a given array size


The choice of panel really comes down to price and availability.  While a manufacturer may list 230-245W modules, you'll need to contact your distributor to find out what you can really get.  

In general, if you need a certain array power, higher power modules are better.  Fewer modules means less racking and less installation labor.  

Peak load is not really an issue. You can produce electricity in the day time and consume it at night.  If you are grid tied and net metered, the utility buys energy from you in the day time and sells it back at night.  If you are off grid, the energy is stored in your battery bank in the day time and drawn out at night.


RE: Factors for selecting solar module for a given array size


Very good point about higher power modules being better due to the overall reduced cost as a result of fewer modules, less racking etc....

So I guess ones aim should be to find a module that gives the best watt/$ output?

With so many manufacturers to choose from is it like anything else where you want to select a manufacturer who is reputible and you know will give you the best product?

You mention that you ideally always want to select the module that gives the largest output.  But are there ever cases where you may want to select a lower power module to the characteristics of the lower power modules such as Voc, Vmp, Isc, etc... that may fit better into the system design?

Thanks for the help!

RE: Factors for selecting solar module for a given array size

... I seem to arrive late to these conversations.

Yes, all factors can be important, depending on the system design (otherwise nobody would measure them).

For example, Voc is the peak voltage, with no load.  If the array open-circuits, and the string is 20 panels long, then a Voc of 34 Volts will cause over 700 Volts across the terminals.  That exceeds the capability of most equipment and a 600 Voc MAX (or less) is more typical.

Installed in a hot area, the temperature coefficients are also of importance in performance estimation, and in cold climates, the reverse is true:  the Voc can rise significantly and again get you into trouble.

You haven't mentioned if you are installing this on-grid or off, with battery back-up or if batteries have high priority in the system design, if there are multiple arrays or other RE sources...

Will there be a MPPT controller used?  (I hope so)

I'm not asking to explain the whole system design, but the economics of RE systems has a lot to do with the end-use, not just the equipment itself.  So there are a lot more details to consider.

Steven Fahey, CET

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