2014.11.24 Net Metering Definition

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2014.11.24 Net Metering Definition

Was there an update to how net metering is modeled in this release. It seems SAM is still defining net metering different than the net metering available to us in Nebraska. When we sign up for net metering, it means excess produced in a month is credited at that month's retail rates for that month only (netting), and sale of un-utilized excess is done at a lower rate at the end of each billing period (month). We cannot bank at retail for future months, we can only bank/net for the month we are in.

The help indicates that if net metering is checked, excess kWh credits carry over to the next month. Credits are settled at the end of the year. Monthly carryover of credit at retail makes this option not valid for us, so I assume this should not be checked.

The help indicates that if net metering is not checked, excess generation is sold at a sell rate for that hour. This would somewhat work if sell rate was set to buy rate (retail), but would not account for month-end settling of net excess at below retail rate. Further, unchecking "net metering" on the rate page blacks out the hourly sell rate in the energy rate section.

Is there some other way to model how net metering is done in all of the state of Nebraska that I am missing?


Paul Gilman

Dear Jay,

Thanks for posting the first question about the new version of SAM!

Your observations are correct. There are a lot of net metering variations in the U.S., and SAM only models one general approach that we hope works for most scenarios.

Your description is very helpful, and we'd like to hear from anyone else trying to model different kinds net metering.

Best regards,


I would think that no net metering would mean no compensation for excess generation anywhere in the US - this is really the larger benefit of net metering over monthly credit carryover. Yet the model is indicated to compensate at "sell rate for that hour" under "without net metering"? And "sell rate for that hour" doesn't really have meaning in a tiered rate structure...all while the "flat sell rate" cell is active?

Hopefully future versions of the model can accommodate more than one approach to net metering like it does with other aspects of rate schedule.

Paul Gilman

Hi Jay,

I just re-read your first message, and see that you say "...unchecking 'net metering' on the rate page blacks out the hourly sell rate in the energy rate section." Could you double-check that? When Enable net metering is checked, the all of the sell rate inputs are disabled. But when the checkbox is not checked, the sell rate inputs should be enabled. Note that all of the Energy Charge inputs (buy rates and sell rates) are disabled when Enable time-of-use and/or tiered energy rates is not checked.

I think the only limitation in SAM that prevents it from not modeling the Nebraska net metering law is that SAM does not provide an input for a rate to apply to the monthly net excess generation, which the law stipulates be "compensated at the local distribution utility's avoided cost of electric supply over the billing period."

With net metering disabled, SAM does not compensate for net monthly excess generation. Instead, for time steps when the load is greater than the system's output, it applies the buy rate to purchase electricity, and for time steps when the load is less than the system's output it applies the sell rate to excess electricity in that time step.

Best regards,

Tom Durston
Tom Durston's picture

Paul, in California net metering accumulates excess production $ value credits to the end of the year (True Up Period). If the customer consumes more electricity $ value than produced in the early months of the year, the charges can be completely offset by producing more electricity $ value in later months.
To use the SAM model for a PG&E customer I use the Cash Flow Send to Excel with Equations and edit the Value of electricity savings.
Thanks for your explanation. I was wondering how the SAM developers came up with its net metering definition.

Paul Gilman

Hi Tom,

Thank you for that explanation. Given the different definitions of net metering rules in different states, it's tricky to create a model that works for all of them without being too complicated to use.

That's especially true for rate structures like the PG&E E-1 rate that has net metering with time-of-use and tiered rates, where the distinction between monthly rollover of dollars or kilowatt-hours is important. In California, the monthly rollover of credits is in dollars, but the annual true-up is based on a comparison of kilowatt-hours produced and consumed.

We're discussing this now, and hope to modify the implementation to make a little more flexible so it can model some of the subtleties of California net metering. Stay tuned for these changes in the next version of SAM, scheduled for release around the end of this month (June 2015).

Best regards,


Hi Paul,

My customers' net metering policies allow them to carry credits forward month to month and even into the next year. Using SAM, if I don't check enable net metering the end of year credits are lost and therefor the cost savings are lower than they will be in every year beyond year one. If I do check net metering and there is an excess of generation at the end of the year, then the reported cost savings will be too high, as we don't have a year end buy back option. So, is there a way to allow a bank of credits from previous years to carry forward to offset the first months consumption and more accurately reflect their multi-year savings?



I figured out a work around. I increased the load on December enough to use up any excess generation that would be consumed in subsequent months, but not high enough to consume the 12 month average excess generation of the system.

Paul Gilman

Dear Drew,

Thanks for the post and suggested workaround. As I've written elsewhere, we've tried to make SAM's net metering model flexible enough to reasonably model most comment net metering arrangements, but because of the large number of different variations and details, we do not expect to be able to represent all of the variations.

For the next release of SAM planned for the end of this month (June 2015), we are adding options for monthly accounting of excess generation in either dollars or kWh, and an annual minimum charge, so stay tuned for that.

Best regards,


Hello Paul,

I try to model a situation (Brazil) where there are taxes to be pais on the energy fed-in by net-metering. This means that energy fed-in in excess of the actual demand is losing value. How would I model this?
Situation 1) Monthly reading: The excess at the end of a month will be roled over but with re-purchasing it I will have to pay a tax on this energy
2)On medium voltage, same scenario but balancing within 15minutes
Any idea how to do that?
Sascha (again:-))

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