Sell Rates under “Energy Charges” vs. Flat Sell Rate

In modeling a project with SAM 2014.11.24, I keep getting curious results when specifying sell rates under “energy charges” vs. a flat sell rate. With the latter, a flat sell rate of $0.045/kWh returns a positive NPV. However, when I use the same value for the sell rate across all tiers and all periods instead under the “energy charges” section (with no flat rate specified), the model returns a much lower NPV. In attempting to troubleshoot, I found that I keep getting the same NPV no matter what value I use for the “energy charges” sell rate – using $0.000/kWh yields the same result as $0.045/kWh or any other values.

Can you help me figure out what I’m overlooking?

Thanks in advance.

Hello,

This is a bug in SAM 2014.11.24 that we have fixed for the next update, which we plan to release in the next few days.

For rate structures that do not involve net metering, SAM applies the electricity prices on an hourly basis. That means that for each hour of the year, SAM compares the system's output to the building load. If the output exceeds the load, it sells the excess at the sell rate for that hour. If the output is less than the load, it buys the difference at the buy rate for that hour. However, in SAM 2014.11.24, for the flat buy rate and flat sell rate under "Fixed Energy Rates," SAM was incorrectly applying the buy and sell rate to the difference between the monthly total output and load.

In the update version, SAM will use the same hourly approach for the flat buy and sell rates that it uses for the TOU-tiered energy charge rates.

Note that if you enable net metering, SAM does uses a monthly "true-up" period, so that the system output and building load are compared on the basis of monthly totals instead of hourly values.

Thank you for pointing out this issue.

Best regards,
Paul.