Switch on efficiency

The Gen Less home energy savings calculator is designed to help New Zealanders understand the impact that efficient appliances can have on their energy bills.  

The calculator focuses on 5 high-impact energy uses/appliances:

• Heating
• Hot water
• Cooktops
• Driving
• Rooftop solar 

Every household is different, and while we have tried to include as many scenarios as possible, there will be some home energy set-ups that aren’t fully represented. We hope you find this a useful tool for considering the costs and benefits associated with major home energy purchases.

The data model that powers the calculator was last updated in November 2024. 

The full source code for our model is freely available: Home energy savings calculator source code(external link)

How savings are calculated

Your home

• The number of people in your household is used to inform hot water and cooking energy use.
• We use postcodes to estimate localised electricity prices, how much energy is used for heating and hot water, and how efficient heat pumps are in different climates. Energy used to heat hot water depends on how cold the cold water is, which varies around the country.
• Electricity and piped gas costs are based on the lowest-cost plan offered in each region by the four major electricity retailers. ‘Lowest cost’ is based on the annual cost for a fully electric house for electricity plans, and a mixed-energy house for piped gas plans.  
• This calculator does not currently cover piped LPG. The best approximation for this is to choose bottled gas. Piped natural gas is not available in the South Island.

Heating

• This calculator focuses on heating for a home’s main living area only in order to provide a simple comparison.
• Three levels of home insulation are modelled to capture the impact of heat retention on energy use: 
  • A ‘not well insulated’ house has no insulation in the floors and walls, a low level of ceiling insulation, and single glazed windows. It represents an old house that has received a very limited level of retrofitted insulation. 
  • A home that is ‘moderately insulated’ has a good level of retrofitted floor and ceiling insulation, but no insulation in the walls, and single glazing. This represents an old house that has received the best low-cost retrofits. 
  • A ‘well insulated’ house has fully insulated floors, walls and ceiling, and double glazing. This represents the new build standard between 2008 and 2023.
• For all scenarios, it is assumed that homes are heated in the morning and evening. Users can select how many days each week they use heating during the day in the main living area.
• Appliance efficiencies are: 70% for wood burners, 80% for gas heaters (piped and LPG), 100% for electric resistance heaters, and between 330% and 497% for heat pumps. The heat pump variation is based on climate data in each of the 18 climate zones defined by NIWA.  

Hot water

• Hot water use is based on how many people are in the household, and whether water use is characterised as ‘low’, ‘average’, or ‘high’.
• The average per-person hot water use is based on data from BRANZ (The Building Research Association of NZ) and The University of Auckland. Our low and high use variations are based on showering only. All scenarios assume average hot water use for other purposes.
• Appliance efficiencies are: 83% for gas instantaneous water heaters, 88% for gas storage water heaters, 100% for electric hot water cylinders, and between 276% and 415% for hot water heat pumps. Hot water heat pump variation is based on climate data in each of the 18 climate zones defined by NIWA. 
• Our calculations take into account storage losses for gas hot water cylinders, electric hot water cylinders, and hot water heat pumps.  These vary between the type of appliance, and its storage volume, which is based on the size of the household. Our modelling assumes that gas and electric hot water cylinders are located indoors, and hot water heat pumps are located outdoors.  

Cooktops

• Cooktop energy use is based on how many people are in the household. Our model assumes that a 2-person household uses 50% more energy for cooking than a 1-person household, and a 3-person household uses the same amount additional again (and so 2x a 1-person household). This trend continues for 4, 5, and 6-person households. Overall, this model is consistent with the national average.
• Appliance efficiencies are: 30% for gas, 71% for electric coil or ceramic, and 78% for electric induction. 

Driving

• Driving calculations take into account fuel efficiency and vehicle mass, based on data from the Fuel Saver database.
• Our vehicle size categories are based on information from the Motor Industry Association.
• Using this data, our model assumes an average fuel efficiency and vehicle mass for each size category and vehicle fuel type. 
• For plug-in hybrid vehicles (PHEVs), our calculator assumes a 100km drive cycle. In real life use patterns will vary. For PHEVs that are run mainly on the electric battery only, savings will be higher.