When you specify your utility, we find weather data, typical appliance annual energy consumption, and equipment rebates for your location. Your building type selection refines the appliances' annual energy consumption and provides a baseline for the hourly shape of that consumption. As you change the summer and winter thermostat temperatures, we reshape the hourly consumption profiles for cooling and heating appliances. The size of your home and its insulation and air tightness levels are used to scale heating and cooling consumption. We use information about the number of building occupants to scale the hot water consumption.

When you specify your current equipment types and energy sources, we use the equipment's efficiency ratings to calculate how much gas, propane, and/or electricity is consumed. Based on your gas, propane or electricity prices, we can estimate how much you're currently spending on energy, while accounting for time-of-use or tiered energy prices. We then calculate your energy bills using your selected heat pump equipment, and we consider how its efficiency changes in response to outdoor air temperatures. Lastly, we find the differences in how the selected heat pump systems perform relative to your current systems, and this allows us to estimate changes in energy consumption, utility bills, and carbon dioxide emissions.

We assume the heating season extends from October to end of May, with the remaining months being the cooling season.

Hourly load profiles are derived from NREL energy building simulation data and reduced to a functional form that responds to user-defined thermostat temperatures, building envelope characteristics, and occupancy levels.

Gas and electric rates are represented with all applicable time-of-use periods, seasonality, and consumption-based tiers. Future residential gas and electricity prices are escalated at annual growth rates estimated by the utilities, and users can modify these escalation rates.

Air-source heat pumps for space conditioning rely on performance curves from a review of manufacturer specifications. We assume that 'Standard' and 'High' efficiency space conditioning heat pumps will operate efficiently down to outdoor temperatures of 35F, while 'Cold Climate' versions can operate efficiently down to 5F. Below these temperatures, some backup heating -- from either the current heating system or electric resistance -- is assumed. Ground-source and hot water heat pumps are modeled with a constant operating efficiency.

We’ve included the utilities' current rebate offerings for space conditioning and hot water heat pumps, along with building envelope upgrades. Incentives vary as a function of heat pump efficiency and type, and also by envelope upgrade type.

Carbon dioxide emissions savings are estimated using utility-specific electric carbon intensities. "Green" or "renewable" rates asssume zero carbon intensity. For non-renewable rates, carbon intensity changes over time in alignment with the utilities' forecasts of generation sources.

We assume a 15-year lifetime for all the heat pump types.