My power bill suddenly doubled. What changed?

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 June, this is almost certainly it. Compare your kWh used (not dollars), not just 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 ($75-$150 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 classic 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.

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