Possible bug with net billing

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Possible bug with net billing

I'm wondering if there's a bug in net billing.

I'm trying to verify that SAM is modeling my electricity bill correctly, but I haven't been successful yet. Particularly I'm seeing surprising results with the monthly "Excess generation credit $ earned" value. I thought I could verify that the credit $ earned is being calculated correctly by comparing it to "Excess generation kWh credit earned" x sell rate, and indeed it matches perfectly, for any month in which "Electricity to/from grid" is negative, but not if it's positive. See the attached screenshot.

Is my assumption correct that "Excess generation kWh credit earned" basically means how many kWh were exported, not necessarily an actual credit? My rate plan buy exported power at $0.1161 per kWh (see below for more rate plan specifics).

Also surprising is how "Excess generation kWh credit earned" sometimes exceeds "System AC energy." Look at June in my screenshot: System AC energy is 1487.32 but Excess generation kWh credit earned is 1925.2. I might expect that with the cumulative version, but not the non-cumulative version. What am I missing?

Am I thinking wrongly about the meaning of these values?

I'm using SAM 2018.11.11_r1, APS Saver Choice for the rate with $0.1161 plugged into the sell rate for all four tiers (to match their RCP rider for solar PV DG).

Thanks for your help in understanding how SAM is meant to work in this respect.


Regarding the choice to use net billing with carryover to next month:

I'm in APS land, and will be subject to the RCP rate, in which they "will meter this export power on an instantaneous basis and provide a monthly bill credit based on the purchase rate in this schedule...Any bill credit in excess of the Customer’s otherwise applicable monthly bill will be credited on the next monthly bill, or subsequent bills if necessary. After the Customer’s December bill, a Customer may request a check for any outstanding credits from the prior year; however, if the outstanding credits exceed $25, the Company will automatically issue a check to the Customer. Otherwise, the bill credits will carry forward to the following year." See https://www.aps.com/library/rates/RCP.pdf.

They lock us into the sell rate for 10 years; it does not escalate. I'm not sure how the model handles that, but if I can come to trust year 1 calculations, I can adjust for that 10-year lock-in manually.


Paul Gilman

Dear Matt,

The "Excess generation credit $ earned/applied" results variable are for the "Net energy metering with $ credits" option. The "Excess generation kWh credit earned/applied" results are for the "Net energy metering" option.

The monthly kWh credits are cumulative, so may exceed the system AC energy in a given month.

As for the APS RCP rate, I think you should use the "Net billing with carryover to next month" billing option. SAM will apply any remaining credits at the end of December to the December bill, so it will not exactly model the APS approach.

SAM applies the inflation rate from the Financial Parameters input page to the annual electricity bill to calculate the "Value of electricity savings" in the annual cash flow (see the Cash Flow tab on the Results page). To remove the effect of inflation on the bill calculation, you can set the "Electricity bill escalation rate" to the negative of the inflation rate. For example, if the annual inflation rate is 2.5%, set the escalation rate to -2.5. Note that if the degradation rate on the Lifetime input page is not zero, the value of savings will decrease annually due to the annual decline in the system's output.

Best regards,

Paul Gilman

Hi Matt,

Thank you for this feedback. We plan to make some updates to the electricity bill calculations over the next year, and this kind of information is very helpful.

As you know, SAM's bill calculations are designed to work for a wide range of rate structures developed by different companies, so it is not possible for it to exactly replicate the calculations for all rate structures.

Regarding applying the $ rollover credit to the total bill instead of only the energy charge portion, SAM assumes that the fixed charge cannot be reduced. Do you have documentation showing that APS treats fixed charges differently?

As you discovered, the "credits earned" and "credits applied" columns are intended to help you see the credit amounts. The "electricity blll" is the final bill amount.

Best regards,

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