## Bill Calculations with Utility Block-Rate and Time-of-Use Tariffs

Consumer-oriented home energy calculators are only effective if they combine careful energy analysis with energy cost information in a fashion that yields meaningful energy bills. Energy tariffs (particularly those for electricity) are becoming increasingly complex, as they are redesigned to encourage efficient use of energy at the margin and management of peak demand. For example, the so-called “inverted block tariffs” present the user with increasingly high per-unit electricity prices as consumption rises. “Time-of-Use” tariffs present the user with high electricity prices at times when the utility system is likely to be facing peak demands (e.g. weekday afternoons), and correspondingly low prices at off-peak times.

Most energy calculators utilize highly stylized prices (e.g. a flat cents-per-kilowatt-hour value), which fail to capture the real-world conditions facing consumers. To address this void, the Home Energy Saver site includes actual electricity tariffs, which may be selected by the user instead of default electricity prices (described in Default Energy Prices).

An analysis of a home on Sacramento provides an illustration of the value of more realistic electricity price assumptions. Using an actual standard residential tariff from the local utility (SMUD) results in an annual cooling electricity bill that is 22% lower than that predicted using the statewide “default” average electricity price results. Conversely, using one of SMUD’s TOU tariffs results in a bill that is 32% higher than the basic SMUD residential tariff. A subsequent thermostat setback reduces the TOU bill by 18%. The results would be even more dramatic for more extreme cooling climates.

 Energy price Annual bill (\$/year) % change from previous Cooling bill (\$/year) % change from previous July monthly bill (\$) tariff details state average \$1,721 \$189 n/a 12.2 ¢/kWh SMUD res. tariff \$1,462 -15% \$148 -22% \$101 summer = 8 ¢/kWh up to 700 kWh, `then 14 ¢/kWh, then 15.7 ¢/kWh` SMUD RTG \$1,687 15% \$196 32% \$143 summer on-peak = 19.8 ¢/kWh, `off-peak = 8.5 ¢/kWh ` (summer peak = 2-8 pm) SMUD RTG w/ t-stat setback \$1,550 -8% \$160 -18% \$131 t-stat setback from 9 am to 7 pm (78 to 81 degF) SMUD RTG w/ t-stat setback \$1,525 -2% \$154 -4% \$123 t-stat setback from 2 pm to 9 pm (78 to 81 degF)

The purpose of this module is to allow users to compare their utility bills under alternative tariff scenarios, and to assess the potential bill savings from upgrades to the house or changes in behavior.

For TOU tariffs in particular there are a special set of consumer behaviors that can currently be modeled in HES to attain energy bill savings. Examples include:

• Use of the programmable thermostat module to represent setbacks/setups, etc.
• Technical measures to reduce air-conditioning demand (efficient equipment, roof color change, etc)
• Energy-efficiency measures in general (savings apportioned to end-use load shape)
• Shifting to a different tariff
• Shifting to a non-electric fuel

Underlying our method, utility tariff data are stored in the Tariff Analysis Project (TAP) database, and TAP is utilized to provide a web service for retrieving tariff data and calculating utility bills. Currently, 177 residential and agricultural tariffs from 87 utilities in 42 states are available in the TAP database. As new tariffs are entered into the database, they are automatically made accessible to HES users. We allow users to choose among residential as well as agricultural tariffs (as some homes located on farms will utilize an agricultural tariff).

With the exception of heating and cooling (which are currently modeled on an hourly basis), the end-use load shapes are static. Thus, users cannot currently define an alternate load shape and compute savings (e.g. to represent line-drying of clothes in summer), although this capability may be added in the future.