Dear Stanley,
When you run the residential or commercial financial model with net metering, SAM calculates the electricity bill on a monthly basis, accounting for any reduction in the bill due to the renewable energy system. The renewable energy system's generation effectively reduces the total monthly bill, with excess credits rolled over to the next month on either a kWh or dollar basis.
Without net metering, electricity sales to and purchases from the grid happen on an hourly basis at prices determined by the electricity rate structure.
Most utilities in the United States use electricity rates with net metering, so in your case, I would expect SAM results with net metering to more closely match the actual bill than without net metering.
Best regards,
Paul.