Combine Cases financial results are zero?

I have 4 separate PV Detailed cases which I am trying to combine using the "Combine Cases" macro.

The energy production numbers seem to make sense.  However, the payback period, net capital cost, and equity values are all zero.  The NPV value is non-zero, but is way too high (more than double what it should be).  I suspect that is because the simulation thinks the net capital cost is zero, but I can't be sure.

I'm using SAM 2018.11.11.

I followed the instructions given in the macro documentation.  I've gone over it again and again to make sure it is correct, and I've combined different combinations of the sub-cases to rule out the problem being with any one in particular.  Can anyone help me figure out why the combine case macro doesn't tell me the real financial analysis?

Hi Ron,

UPDATE: As of SAM 2021.12.02, the Combine Cases macro is replaced by the Calculate generation profiles and nameplate capacity from open cases option on the Power Plant page for the Generic System model and Generic Battery model. See the SAM YouTube channel for a short tutorial .

When you use the Combine Cases macro, you should make sure the inputs on the System Costs page are correct for each case. You can ignore the financial inputs in each case on the Financial Parameters, Incentives, Depreciation (PPA only), Electricity Rates and Electric Load (non-PPA) input pages, but be sure that they are correct in your Generic System case that represents the entire project. For the results, the financial metrics should be from the Generic System case, not from any of the cases that you are combining.

If you don't mind sharing your .sam file, please attach it to a reply and I will review it.

Best regards,
Paul.

Hi Paul.  I checked my file per your advice and couldn't find anything wrong.  My file is attached (I had to delete a few cases to make the file size < 10 MB).  Perhaps you can find the problem.

Thank you,
-Ron

Hi Ron,

Thanks for attaching the file, and sorry about that 10 MB limit.

There is a bug in the Combine Systems macro that you can see on the System Costs page of the Generic System case. The macro sets the value of the calculated plant cost (2 in the screenshot below) instead of the input field (1). You can work around the problem by typing the value in (1) -- in this case type 4,913.79 and then click the Simulate button in the Generic System case (don't run the macro again):



If you are modeling a PV system with up to four different array sections, you should be able to model them in a single case using subarrays on the System Design page. Is there a reason you are not using that approach?

Best regards,
Paul.

Paul,

Thanks, that appears to have worked once I manually put the price in that field.

The reason I'm simulating them separately is because I get different and I suspect more realistic results that way.  SAM doesn't seem to have native support for micro inverters, which is the configuration that I'm going to use in order to meet the latest NEC safety requirements.  The work-around suggested on this forum is to choose the micro inverter and then tell SAM that you have N of them and run the simulation that way.  Unfortunately, I think that leads to an overestimation of performance because then the power output of each panel is distributed across all of the inverters, when in reality each inverter is paired with a single panel.  My inverters are slightly undersized compared to my panel power, so I expected that there would be some clipping - but the single unified simulation didn't show that.  That's probably because I have arrays that face different directions, and thus they peak at different time of the day - but distributing the power from each panel across all of the inverters artificially makes it appear as thought there is no clipping.  When I run the simulation for each array separately, some clipping appears (although I suspect it's still not fully accurate, given that some panels are shaded while others are producing more power then their inverters could handle).

If you can suggest a better way to do it, please let me know.  I'd be happy to send you my entire simulation file if you can help me get around the 10MB limit.

Regards,
-Ron

Hi Ron,

Thank you for the explanation. Your approach with separate cases and the Combine Systems macro should be more accurate because the DC/AC ratio for each case in your file should be the same as the DC/AC ratio of each module-inverter pair that share the same orientation in your system.

The subarray option I suggested would not work as well for your application because you cannot specify a different DC/AC ratio for each subarray.

Best regards,
Paul.