Electricity Rates - Monthly Accounting of Excess Generation

I have just updated to the new version of SAM and the Monthly Accounting of Excess Generation seems to have new options than before (can't remember what they were), but when I toggle between

Option 1 - Cumulative hourly excess credited to current month bill in $ at sell rates ($0/kWh), and

Option 2 - All generation sold at sell rates and all load purchased at buy rates

The results change significantly - from an attractive return and 7yr payback to a negative return and NA payback.

Option 1 is for most types of net metering, where the system meets the load to reduce the monthly electricity bill, and any excess kWh at the end of the month is credited to the next month's bill. There is no sell rate because the kWh generated by the system are used to reduce the kWh billed to the customer.

Option 2 is for a special kind of net metering, where the excess generation at the end of the month is credited to the next month's bill in dollars instead of kWh. That dollar value is determined by the sell rate, so if you set the sell rate to zero for all time-of-use periods, then the dollar credit is zero.

Option 1 and Option 2 will give similar results if you set the sell rate in Option 2 equal to the buy rate. If the rates for the energy charge are flat, i.e., with no time-of-use rates, then the two options should be identical (as long as the Option 1 "Sell rate for kWh rolled over at end of year" is the same as the flat rate.

Please see the attached .sam file for an example ( SAM 2017.1.17 468 KB ).

You can use the values reported under "Monthly Data" on the Data tables tab of the Results page to explore the electricity bill calculations.

Please see the Help topic in SAM for the Electricity Rates page for more detailed descriptions of those options.

Best regards,
Paul.

Hello,

No. SAM's PPA Single Owner financial model assumes that the system is for a power generation project where all of the power generated by the system is sold at the PPA price (with optional time-of-day adjustment factors). For that model, there is no building load, and therefore no "excess" generation.

For your application, you would need to define a dispatch schedule and parameters to allow battery charging from the grid in the morning, and to allow discharging of the battery during the peak period to supplement the PV system's output. For that peak period, you would also need to disallow charging the battery from PV.

When you use the battery model with one of the PPA financial models, it does not explicitly account for the cost of purchasing power from the grid to charge the battery. To include that cost in the cash flow, you could follow these steps:

1. Run the model once, and then on the Data Tables tab of the Results page, display the "Annual energy imported from grid (kWh)" variable under "Single Values"
2. Use that value to estimate (by hand) a cost per MWh for purchases of grid electricity
3. Enter that value for "Variable cost by generation" input at the bottom System Costs page .
4. Run the model again to generate the financial metrics that account for the cost of purchasing grid power.

If the grid power purchase price varies by time, you could use the hourly (or subhourly) output "Electricity to battery from grid" to calculate the cost per MWh of grid power purchases.

Best regards,
Paul.