Bug in Cumulative hourly (subhourly) excess credited to next month option?

Hi,

I can't reconcile the results when switching between the two cumulative sub-hourly options. I set up a test file with one period, the same buy and sell rates, and a system that covers about 80% of load with a mix of surplus and consumption months.

When I run the model, the NPV of the system is identical in four of the five metering options, but the cumulative hourly (subhourly) excess credited to next month bill in $ at sell rates is way different ($704 for the 4 options, -$3,071 for the last). I can understand a small amount of difference if the buy and sell rates vary based on seasonal changes, but nothing of this magnitude. If I understand the help file, the only difference should be whether the credit is applied to this month or the next.

Thanks,
Kevin

Dear Kevin,
I tried to replicate the scenario you describe, but get different results. Here's a table showing monthly totals for the load and system generation:

 
And another showing results of a parametric simulation, NPV vs metering option:

Would you mind either attaching a .sam file that demonstrates the results you are seeing, or emailing it to me ?
Best regards,

Paul.

Paul,

I am attaching a file that has the same effect, although not exactly the same values as yesterday (i didn't save that one!). For this file, I kept all the default values of the "detailed residential" file, but modified the load scaling factor to 1.6 to eliminate annual excess energy. I zeroed out the fixed charge and have a simple 1 tier rate with a buy rate of $0.10 and a sell rate of $0.10.

I would expect all of the scenarios to produce the same result, as even though they might account for the kWh differently, all sales and purchase are at the same rate. But even as you have above, the values in option 4 are quite different from the others.

Seems the file can only be attached to the first post, so it is up there and called "Test metering options"

Hi Kevin,

Thank you for attaching the file. I need to investigate this further to see if there is a problem with the "Cumulative hourly (subhourly) excess credited to next month bill in $ at sell rate" option. We're a bit busy at the moment preparing to release a new version of SAM, so I'll need a little more time.

Are you trying to model a particular billing scenario?

Best regards,
Paul.

Paul,

Mostly I'm trying to compare the impact of different types of netting in various net metering scenarios. We are starting to see more utilities propose hourly or subhourly netting, so trying to understand how that impacts project financials. Thanks for looking into this. In the meantime, I assume the 'subhourly current month' option should produce similar results, just with the credits applied in the current month instead of the following month.

Also, I just download the new version, but the release notes are not updated yet.

Thanks,
Kevin