Monthly minimum charges not enforced for net billing

Hi

I am modeling a net billing rate where a monthly minimum charge is in place. On SAM (v 2022.11.21) I observe the monthly minimum charges do not seem to be enforced for net billing.
How to reproduce the issue:
1) Start a new PVWatts residential model with default settings.
2) 'Electricity Rates': Set Monthly minimum charge to $10. Leave the default value of 'Metering and Billing' as 'Net energy metering'
3) 'System Design': Set system nameplate capacity to 100 kWdc (to model scenario with large exported energy)
4) Simulate. Annual bill with system is $120 (12x$10) as expected. Monthly minimum charge is being applied. Good.
5) Now change 'Metering and Billing' to 'Net billing'. In 'Rates for Energy Charges', set the 'Sell ($/kWh)' to 10 for all tiers (to model high value exports that will lead to a negative bill if the monthly minimum charge is not applied).
6) Simulate. The annual bill with system is a very large negative number, i.e. the monthly minimum charge has not been applied. 

I've uploaded a SAM file that shows the Net Billing case.


Is this a bug or am I missing something? If the latter, is there another way to enforce this minimum bill?

Thank you very much!

Hi Ricky,

For distributed (or behind-the-meter) projects, SAM treats the value of electricity exports as a reduction in the electricity bill. If the value of the exports exceeds the electricity bill amount for a given month, then that month's bill is a negative value. This ensures that the value is being accounted for in the project cash flow so that it affects the net present value and other financial metrics.

In the attached version of your file, I duplicated your case to make it easier to compare results for a rate with and without the minimum charge. The total annual bill for the rate with a $10 monthly minimum charge is -1,678,438, which is $120 more than the total annual bill with no minimum charge at -1,678,558.

SAM applies the monthly minimum charge to the electric bill when the monthly bill is less than the minimum charge amount, but it does not force the total bill to be the minimum charge. In this case the monthly bill is the sum of the energy charge, fixed monthly charge, and minimum charge:

 

The description in Help is a little misleading. For the monthly minimum charge, it says "If the monthly electricity bill with system for a given month is less than the monthly minimum, the bill for that month is the monthly minimum charge." This only true if the monthly bill is positive. I will revise the description to make this clear.

Best regards,
Paul.

Got it. Very clear. Thanks Paul.