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.
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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
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.
Matt
