Errors in excess credit calculation with net billing affecting Project NPV

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klucas@energy.s...
Errors in excess credit calculation with net billing affecting Project NPV

Hi,

I just downloaded the updated patch. However, there is still an error in the calculation for the excess credit in September, which in turn causes the sum of the first year monthly values to not match the annual first year values.

The attached files has 4 cases - TOU w/wo battery, and 4-part demand charge w/wo battery. In each case, the sum of the subparts of the bill (fixed, energy, demand TOU, demand fixed) match the total electricity bill for all months but September. In that month, the excess generation credit earned is greater than the excess generation credit cumulative, despite the cumulative credit having been zeroed out in August. The difference between these values also shows up in the difference between the sum of the monthly total electricity bill and the the sum of monthly subparts.

This error also carries over to the annual figures. The first year annual value for each subpart matches the sum of the monthly values, but the first year annual total bill does not match the sum of the first year annual subparts.

In this case, September is the last month in the summer TOU period. The excess credit for September is calculated correctly (in case N1, 145.05 kWh are exported at the sell rate of $0.377497 for a credit of $57.7572), but the cumulative excess credit is different ($43.1395). The implied rate of the cumulative excess credit does not match any of the rates.

This issue affects the final NPV calculation of the project, and does so for both volumetric and demand-based charges.

EDIT - it appears the difference between the cumulative excess credit and the credit earned is equal to the energy charge before credits were applied (in N1, $11.6177). So the cumulative appears to be used to offset this amount, properly showing a $0 in the energy charge for September, but the total electricity bill still shows $24.177, which is the fixed charge plus the original energy use. So the netting happens in the subpart but not the total.

Thanks,
Kevin

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Paul Gilman

Thanks, Kevin. We're investigating and I will get back to you when we learn more.

Best regards,
Paul.

Paul Gilman

Dear Kevin,

We've made some changes to the bill calculations as noted in the SAM repository on GitHub. This change will be included in the next release of SAM. Thank you for helping us find this problem.

https://github.com/NREL/ssc/issues/134

Best regards,
Paul.

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