California's NEM and NEM 2.0

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California's NEM and NEM 2.0

I am planning to model PV + optional battery systems in California and am therefore interested in the net energy metering (NEM) calculations that should be performed. In the SAM interface, there are generic NEM options (roll over kWh or $ or separate buy/sell meters, etc.). However, I want to make sure my calculations reflect the NEM rules in place in CA. Furthermore, we are on the verge of crossing the threshold to the recently passes NEM 2.0 rules, which include non-bypassable charges. In particular, SDG&E's territory is projected to cross into NEM 2.0 this month!

Here are my questions:
(1) Is there a good description of the calculations behind net metering in SAM? In particular how is net energy calculated for TOU rates (i.e. does each period have its own cumulative total or are the most expensive periods zeroed out first by excess generation)?

(2) Any comments about how the SAM net energy calculations parallel (or not) CA's NEM rules?

(3) How might I go about modeling CA's NEM 2.0 rules? Non-bypassable charges in particular seem to be a new kind of calculation charges that apply regardless of net demand. Are they the same for all kWh or can they also vary by season or TOU?


Paul Gilman


1. We are working on detailed descriptions of the electricity bill calculations. There is some description in the Help system.

Here's how kWh rollovers work for net metering when TOU periods apply:

  • Assume that you combine the Single meter with monthly rollover credits in kWh metering option with an energy rate table that has time-of-use periods.
  • For a month that the renewable energy system generates more electricity than the building load consumes, SAM caculates the net generation (generation minus load) for each time-of-use period and rolls that amount into the equivalent period in the next month.
  • If the next month has different TOU periods than the current month, SAM compares period numbers for the current month and next month at 12 am, 6 am, 12 pm and 6 pm to determine which TOU period to use for rollovers. For example, if October 12 pm is Period 1, and November 12 pm is Period 5, then SAM rolls the Period 1 credit from October to November's Period 5. We designed this approach to try to ensure that when the season changes from summer to winter and vice versa, credits between peak to off-peak periods roll over in a way that makes sense. Because SAM and the OpenEI utility rate database work with period numbers, SAM does not have information about whether a given Period number is for a peak, partial peak, or off-peak period.

2. We based SAM's net metering calculations on California's rules before the new "NEM 2.0" decision. In general, given the complexity of net metering rules and utility rate structures and the fact that we have designed SAM to model projects in any jurisdiction, SAM will only approximate actual net metering rules and rate structures.

3. We are studying the new California NEM 2.0 rules. SAM currently does not model the non-bypassable charges that are part of those rules. There may be a workaround for that using LK script that would run a simulation and then back-calculate the non-bypassable portion of the charges and then recalculate the electricity bill.

Best regards,

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