Terms & Concepts

Calculating utility bills and modeling energy profiles can be complicated. This section explains some of the more confusing topics to make working with the PowerBill API easier.

Solar Metering

The PowerBill API takes into account all the intricacies of the many styles of solar metering, including traditional net metering, Value of Solar Tariffs (VOSTs), Feed in Tariffs (FITs) and everything in between.

Net Metering
In traditional net metering, the utility compensates the customer for energy sent back to the grid at the same price they pay for energy used from the grid. There are three variables to consider for net metering: accounting units, true-up frequency and net surplus compensation value.
  • Accounting Units - Net metering can be accounted for with units of currency (dollars) or units of energy (kWh).
  • True-up Frequency - The frequency in which units accrued from net metering (dollars or energy) reset to zero and excess units are converted to actual payments. True-up frequency can be monthly (or by billing period), annually or perennially (i.e., carried over from one billing cycle to another indefinitely).
  • Net Surplus Compensation (NSC) - Payment customers receive for excess generation at the end of the true-up period. NSC is typically valued at the avoided cost of energy (around $0.04/kWh). Some utilities do not compensate for excess generation. NSC for a true-up period (this could be annual or monthly) is calculated as total production minus total consumption multiplied by the NSC rate. When total consumption exceeds total production, NSC is $0.
Detailed net metering accounting can be found in the ProductionCreditAccounting object.
Value of Export Metering
In value of export metering, the utility compensates customers for solar energy sent back to the grid at a specified “value of export” that may be different from the retail rate.  Examples of utilities that use this type of metering are the investor owned utilities in California (PG&E, SDG&E, SCE) and the Hawaiian Electric Companies (HECO, HELCO, MECO).
Value of Solar Tariffs (VOSTs)
With a VOST, the customer does not use the electricity generated by their own system.  Instead, the generation and consumption are metered separately and the utility buys all the electricity generated by the customer’s solar system at a certain rate, and bills the customer for their total consumption at their regular rate. An example of a utility using this type of metering is Austin Energy in Texas.

Feed in Tariffs (FITs)

FITs pay the customer for all the electricity generated by the solar system at a certain rate for a specified number of years.  FITs are handled as incentives in PowerBill, and a credit will show under the incentives section of the results each year the FIT is valid.  


Rate Elements

Minimum Bill
A minimum bill represents a threshold for the lowest bill that a utility customer must pay. If the sum of EnergyCharge, DemandCharge, and FixedCharge in BillTotals is less than MinimumBill, TotalCharge will be equal to MinimumBill and will not match the sum of other components. In California, the investor owned utilities (PG&E, SDG&E and SCE) have a minimum bill that only applies to delivery charges. Customers with renewable generation systems pay the “Minimum Delivery Charge” each month, and all other charges and credits are paid yearly on the true-up date. There are some exceptions for which the minimum bill does not apply. When net surplus compensation is issued to a customer it is considered payment from the utility (as opposed to a bill credit), similar to a direct deposit into the customer’s bank account, and is not subject to the minimum bill. As a result, TotalCharge may be less than MinimumBill if NetSurplusCompensation is included.
Demand Charges
Demand charges are based on the highest electrical use within an interval of time (commonly 15 minutes) during the billing period.  These charges are typically expressed in $/kW, and are sometimes called “capacity” charges.
Energy Charges
Energy charges are based on the total amount of power that a customer has used during the billing period.  These charges are typically expressed in $/kWh.
Fixed Charges
Fixed charges are billed to the customer per billing period or per day, regardless of how much energy is consumed.
Non-Bypassable Charges
Non-bypassable charges are charges that a utility has determined that everyone must pay on energy they consume.  These charges cannot be offset by credits for energy generated by the consumer, and are typically not included in credits given to the customer for generating their own electricity.


Rate Structures

Block Rates (also known as Tiered Rates)
Block or tiered rates charge customers different rates for energy used over certain thresholds within a billing period.  For example, a customer on a tiered rate may be charged $0.10/kWh for the first 500 kWh used in a month (Tier 1), $0.12/kWh for the next 500 kWh (Tier 2), and $0.15/kWh for additional kWh (Tier 3). The charge for 1200 kWh would then be: $ = (500*$0.10) + (500* $0.12) + (200*$0.15) This type of rate structure is not limited to just energy charges, but can used for demand charges as well.
Time of Use Rates

Time of use (TOU) rates charge customers different rates for energy used at specified times of the day (TouPeriods).  For example, a customer might be charged $0.05/kWh for electricity used during the middle of the night (Off-Peak), and $0.15/kWh for energy used during the middle of the day (On-Peak).

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