On the Scenario Report's Utility Bill Savings section, we occasionally field questions about how we calculate savings totals. This article details about each calculation and explains how some rate structures impact the results.
Total Energy Savings is the aggregation of the baseline energy charges for a year minus the aggregation of the projected energy charges before energy credits.
Annual PV energy savings is the sum of the PV generation used to to load at each interval multiplied by the total consumption rate applicable at that interval.
Saving =Sum( PV to load (15min kWh) * rate)
Annual storage savings is the difference between the cost of energy purchased from the utility offset by the battery discharge minus any cost to charge the batteries from the grid.
Starting in January 2021, Keystone Designer now supports bill calculation and modeled with tiered rates. The combination of a behind-the-meter microgrid and a tiered rate has interesting implications when comparing the cost of energy before and after installing the microgrid. In short, the addition of DERs results in the baseline and proposed rate structure after their installation being different. Because of this adjustment, it is possible to have what might appears to be a mismatch between the Total Annual Energy Savings and Total Annual PV Energy Savings plus Storage Savings.
A microgrid typically consists of one or more DERs that modify energy consumption from the utility. For example, solar panels may generate energy to offset utility imports, or an energy storage system may store excess generation for later use. In either case, the DERs modify utility imports and, as a result, tiers may change when compared to the billing in the absence of DERs. This discrepancy in tiers can, at times, cause differences between energy savings and the value of energy generated or stored by the DERs. Specifically, Keystone Designer defines energy savings as the difference between the cost of energy before and after the addition of DERs.
When Keystone Designer evaluates the energy generated or stored by a DER, the energy is multiplied by the current effective rate, not the baseline rate. For example, solar panels may generate enough energy to keep a billing period within the first tier pricing that would have entered the second tier without that solar generation, potentially modifying the value of energy for the entire billing period. While these issues above do not apply to time-of-use rates, where the cost of energy is unchanged before and after the addition of DERs, they are particularly relevant to understanding energy evaluation on a tiered rate.
If you would like more details about Tiered Rates, you can go here.