Why did my power bill double in winter?

Almost always seasonal heating. NZ household electricity use roughly doubles from summer to winter because space heating and hot water both work much harder. A house using 400 kWh in February can use 800 kWh in July - at 42c/kWh that is the difference between roughly $220 and $390 a month, with nothing wrong. Compare your kWh to the same month last year before assuming there is a fault.

How do I tell if my hot water cylinder is faulty?

Feel the body of the cylinder - it should be warm, not hot, because modern cylinders are well lagged. If it is hot to the touch, if the element clicks on constantly, or if your hot water has become unreliable at the same time the bill jumped, get a plumber to check the thermostat and element. A stuck thermostat can add 200-400 kWh a month - $85-$170 - until it is fixed.

Can my retailer overcharge me with an estimated meter read?

Estimated reads are not overcharging, but they can produce a misleading bill. If a low estimate is followed by an actual read, the catch-up bill stacks the difference into one month and can look like a sudden spike. Check whether the read is marked estimated or actual; if it looks wrong, ask for a re-read, which is free.

Diagnostic

My power bill suddenly doubled. What changed?

Published 02 May 2026· Updated 8 July 2026· 8 min read

You opened your bill. It is double last month's. Maybe triple. You haven't bought a hot tub or moved into a glasshouse, so what happened?

Almost every sudden-jump bill in NZ traces back to one of six things. Work down this list in order and you will usually find it within ten minutes.

1. Heating just switched on for the season

The most common cause, and almost always the one. NZ household electricity use roughly doubles between February and July because heating goes from zero to 30-40% of total consumption. If your bill jumped between April and July, this is almost certainly it. Compare your kWh used (not dollars), not only to last month, but to the same month a year ago. If they line up, your bill is normal for the season and the only thing that changed is the calendar.

2. Your billing cycle changed length

A "monthly" bill in NZ is rarely exactly 30 days. Retailers commonly bill in cycles of 28-35 days. If last month's bill covered 27 days and this month's covers 35, you have 30% more consumption simply from a longer window. Check the "from" and "to" dates on the bill.

3. Your hot water cylinder element or thermostat failed

A failed thermostat that gets stuck "on" causes the cylinder to heat continuously instead of cycling. A failed element that has corroded can pull more than its rated wattage. Both can add 200-400 kWh/month ($85-$170 at current rates) without any visible symptom other than the bill.

The diagnostic: feel the cylinder pipework. The hot-out pipe should be very hot when you run a tap, and the cold-in cool. The body of the cylinder should be warm but not hot to touch (modern ones are well lagged). If the cylinder feels actively hot, or if you can hear the element clicking on every time you walk past, get a plumber to check.

4. A leaking hot tap or shower

A dripping hot tap is heating water continuously then sending it down the drain. A moderate drip wastes about 50 litres of hot water a day, which is around 2-3 kWh of cylinder reheat. A bad drip or running shower head can be 3x that. Walk every tap and check for drip and for the shower head dripping after the mixer is closed.

5. Something has been left on

The usual suspects, in order of how often they show up:

A 2 kW oil column heater in a spare room, left on after a guest stayed. 24/7 = $20/day, $600/month.
A heated towel rail with no timer. $20-25 a month if you only just noticed it, but probably already there for a while.
A spa pool in the back yard someone forgot to turn off after a party. $5-10/day to maintain temperature.
A second fridge in the garage someone plugged in for Christmas and nobody unplugged. $10-15/month, every month, indefinitely.
An EV charger that was set to "always charge to 100%" when it should have been on a schedule.

Walk the house with the bill in hand. Check every powerpoint. Reach behind the spare-room furniture.

6. Your retailer changed your rate

Under the Electricity Authority's Consumer Care Obligations (mandatory from 1 April 2025), retailers must give you timely written notice of price changes (typically 30 days). If you signed up on an introductory rate that has rolled off, or your retailer has done a seasonal repricing, your per-kWh cost may have jumped 10-25% without anything in your house changing. Check your most recent bill against last winter's bill, look at the c/kWh figure printed on each, and compare. If it has gone up, see the Day/Night plan article for what to do, or compare on Powerswitch.

A worked example: what a "normal" doubling looks like

It helps to see the arithmetic, because a doubled bill is often completely innocent. Take a household that uses about 400 kWh in February - fridge, hot water, cooking, lights, the usual background load. At 42c/kWh that is a summer bill of roughly $168 plus the daily fixed charge, call it $220 all up.

Now it is July. The same house runs a heat pump four hours an evening and a couple more on the weekend, plus the cylinder works harder against cold inlet water and the dryer comes out. That can easily add 350-450 kWh a month. The maths:

400 kWh (base) + 400 kWh (winter heating and hot water) = 800 kWh
800 kWh × 42c = 33,600 cents = $336 in energy
plus roughly $55 of daily fixed charge = about $390 for the month

That is a bill that has very nearly doubled, with nothing wrong and nothing broken - just winter. If your jump is in this ballpark and lines up with the same month last year, stop worrying: you have a normal NZ winter bill. It is only when the increase runs well beyond what heating can explain - say a tripling, or a big jump in a month that should be mild - that you should start hunting for a fault.

When to stop searching and call someone

Most of the six causes above you can diagnose and fix yourself in an afternoon. Two of them warrant a phone call:

Call a plumber or electrician if the cylinder feels hot to the touch, the element clicks on constantly, or your hot water has become unreliable at the same time the bill spiked. A stuck thermostat or failing element will not fix itself and can add $85-$170 a month every month until it is replaced.
Call your retailer if the kWh on the bill looks normal but the dollars do not, or if the meter read is estimated rather than actual (it will say so on the bill). An estimated read followed by a catch-up actual read can produce one alarming bill that is really two months of use stacked together. Ask for a re-read if the figure looks wrong - it is free, and under the Electricity Authority's Consumer Care Obligations the retailer has to help you understand the charge.

The five-minute audit

Run this in order:

Open the new bill and last winter's bill side by side. Compare kWh, then c/kWh, then days covered. You will know within 30 seconds whether it's seasonal, a tariff change, or something has changed at home.
If the kWh number is the surprise, walk the house. Spare bedrooms, garage, sleep-out, bathroom, outdoor lights.
Touch the hot water cylinder. If it's hotter than warm, get it checked.
Check every tap for drip.
Open the NZ Power Bill Calculator and tick what you actually have. If the calculator's estimate matches your bill, congrats, you have a normal NZ winter bill. If it doesn't, the gap is your mystery and you'll know roughly how big it is.

Most household bill mysteries are not mysteries for long.

Related guides

Average NZ power bill by region (2026) - check whether your bill is actually unusual for your region
How much does a heat pump cost to run in NZ? - heating is the most common cause of a surprise winter bill