it isn't a true net metering...

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it isn't a true net metering...

Net metering customers are charged only for the “net” power that they consume from the electricity service provider that has accumulated over a designated period or, if their renewable energy-generating systems make more electricity than is consumed, they may be credited or paid for the excess electricity contributed to the grid over that same period.
SAM runs like a feed-in tariff and just does flat sell rate = flat buy rate . Is it correct?


Paul Gilman

Dear Elton,

SAM buys and sells power on an hourly basis. For hours when the renewable energy system generates more electricity than the load (specified on the Electric Load page), SAM sells the excess electricity at the sell rate that applies for that hour (as specified on the Utility Rate page). For hours when the system generates less electricity than the load, SAM buys the extra power necessary to meet the load at the buy rate that applies to that hour.

When you check Enable Net Metering on the Utility Rate page, SAM assumes that the buy and sell rates are the same. You can either model a flat rate that does not change with time, or TOU rates with different rates for different times of day and year. If you clear the Enable Net Metering checkbox, then you can model different buy and sell rates. SAM does not model the monthly or annual accounting that you typically see in actual net metering agreements.

UPDATE: As of SAM 2013.9.20, SAM's net metering model does monthly accounting with rollovers of excess kWh as is typical in most net metering agreements.

Best regards,



I am not sure I understand your final comment regarding the monthly or annual accounting. Are you referring to specific monthly or annual payouts by the utility depending on the dollar amount accrued and how often they will pay?

On another note, I have used both the Commercial and the IPP Model to simulate the same flat plate PV system using TOU rates in the Commercial model and TOD rates in the IPP model (I assume these 2 yield the same result in tariff rates). The annual revenues being projected are vastly different. I have enabled NEM in the commercial model because the buy/sell are the same, but it is not really a NEM deal. I have tried to remove all costs of money in both models to get that out of the way. Can you help me understand why there would be such a significant difference?

Thanks in advance, Jono

Paul Gilman

Dear Jono,

Regarding your first question, yes. SAM assumes that the utility buys all of the excess electricity generated by the system. It calculates monthly sales to the grid as the sum of hourly sales for each month.

The commercial and utility IPP financial models use different calculations to determine project revenues. One primary difference is that the utility IPP models include a margin to meet an internal rate of return requirement, while the commercial (and residential) models do not. Would you mind attaching a copy of your .zsam file to use to explain the difference in your particular comparison?

Best regards,

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