Billing demand - potential discrepancy

Hello,

I am modeling a PV + Battery system in a market with demand charges structured as follows "billing demand will be the higher of the non-coincident peak or 80% of the highest demand in the past 11 months."  

From my reading of the support files, this should be straightforward to model; see inputs attempting to reflect this in the first figure below. 
However, when I look at the results of the simulation, I am noticing two potential discrepancies

1. After year 1, the billing demand in any given month is showing only 10kW (the indicated minimum billing demand)
2. Looking at monthly data for year 1, it is not clear to me that the lookback prior to year 1 is working correctly.  It appears that the billing demand for Apr - Dec is looking Dec of the prior year (at an 80% ratchet) as opposed to also considering the full prior 11 months, or the actual in month total.  As a specific example, shouldn't the billing demand for November be the greater of 80%*130.663 (March) or 68.68 (Nov)?   

Thank you in advance for any guidance.

Hello,

We will investigate this -- it would be helpful to have your .sam file to be sure we are looking at the same scenario. If you don't mind sharing it, could you post it as an attachment here, or if you prefer not to share it on a public forum email it to sam dot support at nrel dot gov?

Best regards,
Paul.

For future reference, you can reduce the size of a .sam file by clearing results from the case menu:

 

Best regards,
Paul.

Hello,

Thank you for emailing your file.

It took some investigation to figure what is happening here. It turns out that the billing demand calculations require that at least one period from the "Demand Rates by Time-of-use Period and/or Tiers" table be included in the "Billing Demand by Time-of-use Period for Demand Charges" table, even when the rates in the demand charge table are zero.

To fix the issue in your file, under "Billing Demand" on the Electricity Rates page set Included in Billing Demand (0/1) in the "Billing Demand by Time-of-use Period for Demand Charges" table to one for Period 1:

 

We will address this by adding a check to generate a simulation error when this happens: github.com/NREL/SAM/issues/1985

I've also attached a spreadsheet that shows how the lookback period works for a 11-month lookback period with 80% billing demand percentage for all months.

Thank you for helping us find this issue, and for your patience while we investigated.

Best regards,
Paul.