General assumptions
For electricity we take the information you supply and work out which parts are for general power, hot water and heating. For gas the uses are cooking, hot water and heating. We adjust these proportions for where you live, your type of hot water system and how you heat your home in winter.
We also take into account the time of year covered in the information you give us – a bill for December is unlikely to include any power used for heating.
Household energy-use assumptions
In working out the amount of power each household uses we make the following assumptions:
- The amount of energy used in the period you supplied us with is typical of your average level of use.
- Winter months, when heating is used, are from April to September – except for the far north where the period is slightly shorter, and the central North Island and the deep south of the South Island where the period is longer.
- If you don’t have your bill we make an estimate of how much energy you use, based on what a typical household of the same size as yours would use. These amounts are then adjusted according to what you tell us about where you live and how your household uses energy.
- A house unoccupied during the day uses less energy for heating than one occupied all day.
- For water heating we take into account up to two forms of heating and share the energy use equally across these. We then adjust this for different methods of water heating. If you have a continuous supply system there is no standing loss like there is from a conventional cylinder, a hot water heat pump is assumed to be twice as efficient as a cylinder, and solar heating gives large savings in summer but also some savings in winter. A wet back reduces your winter use, but only if you use the fire as your main source of heating. We also take into account that households use more hot water in winter than in summer.
- We ask for your main way of heating your home and also up to two other types of heating you regularly use. We allocate the majority of your heating energy needs to the main source and the balance is split evenly between other sources.
- If you have a heat pump, we reduce the amount of electricity used for heating.
- Where you live matters. We use the average monthly temperature of different regions to adjust the amount of energy needed for heating.
Other assumptions
In calculating the cost of electricity we make the following assumptions:
- We split the electricity used into 3 groups: general usage – lighting, cooking, appliances etc; hot water heating; and space heating. We apply the appropriate rate of each pricing plan to each group.
- If a plan has day/night rates, 72 percent of the power is at the day rate and 28 percent at the night rate. Where the night rate covers the weekend, the split between day and night is 51/49.
- If a plan has summer/winter rates, the power is split between the rates and is also varied for different types of electricity use. The period for winter rates is May to September. This is one month shorter than the usual heating period of April to September and so a proportion of electricity use for heating is allocated to the summer rate.
- We have excluded any rebates from community-owned trusts, line companies, or any dividends paid by Contact Energy to its shareholders.
