How we estimate the amount of power you use in a year.

We take the information you provide us, work out which parts are for general power, hot water and heating.

Where power for more than one use is on the same meter we split the use assuming 35 percent general, 35 percent hot water and 30 percent heating. We adjust these proportions for where you live and for if you use gas or alternative means for hot water or heating.

We also take into account the time of year the information you gave us covers - a bill for December is unlikely to include any power used for heating, for example.

General assumptions

Some of the specific assumptions we make in estimating the annual usage:

• The amount of power used in the period you supply us is typical of your average level of use.
• Winter months are from May to September and power is not typically used for heating outside these months. 

Household energy-use assumptions

In working out the amount of power each household uses we make the following assumptions:

• For houses which are usually occupied and only use electricity, we allowed the following annual electricity usage levels: small (7,700 units); medium (11,000 units); large (16,000 units).
• A house unoccupied during the day uses 20 percent less electricity for heating than one occupied all day.
• A house that primarily uses wood, coal or gas for heating would still use a small amount of electricity for heating (10 percent of the amount for an all-electric house). This covers the use of small electric bar heaters, etc.
• If you have a heat pump we reduce the amount of power used for heating by 50 percent.
• Where you live matters. We use the average monthly temperature of different regions.

Other assumptions

In calculating the cost of power we make the following assumptions:

• We split the electricity used into three groups: general usage - lighting, cooking and so on; 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, 70 percent of the power is at the day rate and 30 percent at the night rate. Where the night rate covers the weekend, the split between day and night is 50/50.
• If a plan has Summer/Winter rates, the power is split 50/50 between the rates, except for heating which is all charged at the winter rate.
• If you are on a Day/Night plan and have a night-store heater it is attached to the night meter.
• We have excluded any rebates from community-owned trusts, line companies or any dividends paid by Contact Energy to its shareholders.