Net Metering Function causes Electricity Cost Without System to change

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Paul Gilman
Net Metering Function causes Electricity Cost Without System to change
I am using SAM to model the economic viability of photovoltaic installations in several different regions and I have come across a puzzling outcome. I am modeling two scenarios. The first has a flat energy rate the second has a time-of-use rate. When I toggle "Enable net metering" for the flat energy rate, the "Electricity Cost Without System" remains the same as would be expected. However, when I do this for the same system with a time-of-use rate, the "Electricity Cost Without System" increases significantly with net metering enabled. Is this an error in the software or am I miss-interpreting how the Net Metering function works?
The answer to your question depends on some other assumptions you did not describe. For a scenario where the total monthly load is always more than the system's total monthly output, what you describe correctly describes how SAM models it. Because SAM applies the same rate structure to both the "with system" and "without system" case, switching between a TOU rate and a flat rate will cause the "without system" cost to change.
In other words, when you check Enable time-of-use and/or tiered energy rates on the Utility Rate page, SAM assumes that the TOU structure applies whether or not the photovoltaic system is installed.
For the flat rate scenario, net metering has no effect on the without system case, so switching net metering on and off has no effect on the electricity cost without system.
If you would like to model a scenario where two different rate structures apply, one with the system, and one without, you can use the SamUL script in the sample file named SamUL Cost Savings with Different Rate Structures. The notes in the file explain how it works. I also described it briefly in the "Analysis of Electricity Rate Structures for Residential and Commercial Projects" webinar earlier this year.
Best regards,

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