Specify System Size

Specify desired array size seems to yield a less than optimal nameplate capacity. If I choose Specify Array Size to be 4 kw with 300 watt panels SAM gives me an Actual Layout at 3.329 kWdc and 11 modules. If you use specify modules and inverters you can very closely bound the expected nameplate capacity. I would expect SAM to land far closer than .7 kWdc off the desired value. I have noted this in other cases as well. In some cases SAM lands very close to the expected value in others it is not at all close. Can you shed some light on this?

Hello,

When you use the Flat Plate PV model's "Specify Array Size" option to size the system, SAM does its best to find the number of modules per string, strings in parallel and inverters that results in the system size in DC kW and DC-to-AC ratio that you specify. How close it can get depends on the characteristics of the module and inverter that you choose. Because the module and inverter capacities and ratings are discrete, the sizing algorithm does not always result in a system capacity as close to the desired capacity as you might like.

For a detailed description of the sizing algorithm, see the "Sizing the Flat Plate PV System" topic in Help -- scroll down to the "PV Array Sizing Calculator Algorithm" section. Here is a link to that topic in the copy of Help on the SAM website: Sizing the Flat Plate PV System .

We recommend only using the "Specify Array Size" option to get a preliminary sense of the number of modules and inverters. To design the system to meet your requirements, you should use the "Specify Modules and Inverters" option.

Best regards,
Paul.

Is there a way to input the number of modules and get the outputs that way?

Thank you!

{I'm on a mission to cover all those darned parking lots (2329+ large chain store ones in NC) with pv.}

Dear Joy,
Yes. On the Flat Plate PV model's Array input page, choose the "Specify modules and inverters" option. Then you can specify the number of modules per string, number of strings in parallel, and number of inverters.
We recommend running the model in this way, rather than relying on SAM to size the system for you. The automatic sizing option is intended to help you get started. Specifying the number of modules and inverters yourself gives you more control over the system's design.
For more on this, see the "Sizing the Flat Plate PV System" topic in SAM's Help system, which you can open from the Help menu. Here's a link to the topic in the copy of the Help system that we've posted on the SAM website:
SAM Help - Sizing the Flat Plate PV System
Best regards,

Paul.

Paul,

I am looking for the SAM Help - Sizing the Flat Plate PV System link that you provided in this post, but it appears to have moved/disappeared. Can you provide an updated link?

Thanks,
Jeff