Two identical systems can be installed in two houses on the same street in the same week, and one will cost noticeably more to run than the other.
Nothing is wrong with either of them. The difference is in how they were installed and how the households use hot water. Both of those are things you can do something about, and neither of them appears on a spec sheet.
Here is what actually moves the number.
Where the energy goes
Strip away the detail and a hot water system does exactly one thing: it takes water at the temperature it arrives at and lifts it to the temperature it is stored at.
Two variables set the bill. How much water you use, and how big that lift has to be.
Everything else in this article is a variation on those two. A heat pump changes how efficiently the lift is done, because it moves heat from the surrounding air rather than creating heat from scratch. That is why it uses meaningfully less energy than an element for the same job. But it does not change the physics of the job itself. Use more hot water, or start from colder water, and the system works harder.
This is also why the same system costs more to run in July than in January. The water arriving from the mains is colder, so the lift is longer. We covered that seasonal effect in detail in what actually changes in winter.
The lever most households never touch: when it runs
This is the big one, and most people do not know it exists.
A hot water system does not care what time of day it heats. The tank holds the result. So the question is not how much energy it uses, but when it buys that energy.
If you have solar panels, running the system in the middle of the day means it is largely heating with power you generated. Running it overnight means you are buying every unit of it. Same system, same tank, same shower — different bill, purely because of timing.
If you are on a tariff where power costs different amounts at different times of day, the same logic applies without solar.
Most systems can be timed, either through a controller or through the way they are wired. If you have solar and your hot water runs at four in the morning, that is worth a phone call to your installer. It is usually a settings change rather than a hardware one.
One caveat worth knowing. Shifting everything to the middle of the day works best when your hot water use is spread out or happens in the evening. If your household showers first thing in the morning, you want the tank already warm when the alarm goes off, so the timing needs a bit more thought than “just run it at noon”.
The lever that is invisible: where the unit sits
A heat pump gets its heat from the air around it. That single sentence explains a whole category of running cost problems.
Put the unit somewhere with plenty of air moving past it and it draws in fresh air, takes heat from it, and blows the cooled air away. Put it in a sealed cupboard, a tight alcove, or a narrow side passage with a fence a hand’s width away, and it starts breathing its own exhaust. The air it draws in is the cold air it just blew out. It works harder for the same result, every single cycle, for the life of the system.
This is not a fault and no alarm will go off. The system will simply cost more to run than the identical one next door, quietly, forever.
The fix is much cheaper before installation than after, which is why clearances are worth reading about before the plumber arrives rather than after. Our installation requirements guide covers what the unit needs around it.
If yours is already in a tight spot, it is often still improvable. Removing a gate, opening a vent, or repositioning by half a metre can change the airflow picture substantially.
The habits that matter more than people expect
Hot water use is not evenly distributed across a household. A few behaviours account for most of it.
Shower length is the obvious one, and it is worth knowing that back-to-back showers are harder on the system than the same total minutes spread across the day. A heat pump reheats steadily rather than instantly. Four showers in a row draw the tank down faster than it can recover, which is what triggers a boost element on systems that have one. That is the expensive way to heat water.
Washing clothes on a hot cycle when a cold one would do is the quiet one. Most modern detergent is formulated for cold water.
A dishwasher plumbed into the hot supply is worth checking. Some are, some are not, and some have a setting for it.
And the one nobody thinks of: a hot tap that drips, or a relief valve that weeps constantly. You are paying to heat water that goes straight down the drain, around the clock. It is a small enough leak to ignore and a large enough waste to notice on a bill. Our maintenance checklist covers what to look for.
Three things people worry about that matter less
A bigger tank does not mean a bigger bill. Running cost follows the hot water you actually draw, not the volume sitting in the cylinder. A larger tank loses slightly more heat standing still, but that is a small effect next to the water you use. An undersized tank that keeps running out and triggering a boost element will usually cost more than a comfortably sized one.
One very cold night does not matter. What shows up on a bill is the seasonal average, not the worst night of the year. A cold snap is visible in the daily data and invisible in the quarterly total.
Having a backup element is not the same as using one. Most systems include one for extreme conditions. Its presence costs nothing. It only matters if something is causing it to run regularly, which is usually a sizing, timing or airflow issue rather than a fault.
How to find out what yours is actually doing
You do not have to guess.
If you have a smart meter, your retailer’s app or portal will usually show usage by the hour. Hot water is one of the easier loads to spot, because it appears as a distinct block at a predictable time rather than the scattered pattern of general household use. Look for when that block sits and how long it runs.
Compare a summer week to a winter week. The difference tells you how much the seasonal lift is costing you, and whether it is in the range you would expect or something is making it worse.
If the block is sitting in the middle of the night and you have solar on the roof, you have found your single biggest saving without changing anything physical.
If the block is much longer than it used to be for the same household, that is worth investigating rather than accepting. Airflow blocked by a new fence or a grown shrub, a filter nobody has cleaned, and a slow leak are the usual three.
Frequently asked questions
What actually determines a heat pump hot water running cost?
Two things above all others: how much hot water the household uses, and how cold the incoming water is. After that, the timing of when the system heats, and how much free air can reach the unit, make the biggest practical difference.
Does a heat pump hot water system cost more to run in winter?
Yes. Water arriving from the mains is colder in winter, so the system has to lift it further to reach the same stored temperature. The effect is seasonal and predictable rather than a sign of a problem.
Should I run my hot water during the day or overnight?
If you have solar panels, during the day is usually far better, because you are heating with power you generated rather than power you bought. If you do not have solar, it depends on your tariff. Either way it is a settings question worth asking your installer about.
Does where the unit is installed change what it costs to run?
Yes, more than most people expect. A heat pump draws heat from the air around it. If it is boxed into a sealed cupboard or a tight passage, it ends up drawing in air it has already cooled, and works harder for the same result every cycle.
Is a bigger tank more expensive to run?
Not meaningfully. Running cost follows the hot water you use, not the volume stored. An undersized tank that repeatedly runs out is usually the more expensive choice, because running short triggers backup heating.
Why did my hot water cost go up without anything changing?
Common causes are a new fence, shed or plant restricting airflow to the unit, a filter or air path that has not been cleaned, a dripping hot tap or weeping valve, or simply a change in household routine. It is worth checking these before assuming a fault.
How can I see what my hot water is costing me?
If you have a smart meter, check your retailer’s usage data by the hour. Hot water usually appears as a distinct block at a consistent time. Comparing a summer week with a winter week shows you the seasonal effect on your own house rather than in general.
Not sure your system is set up to run cheaply?
If it is boxed into a tight spot, or heating at the wrong time of day, both are usually fixable. Call us on 1300 062 788 and we will talk it through.
Related reading: if running costs have crept up and none of the levers above explain it, that can itself be a warning. The signs worth acting on covers what else to check.
Related reading: the same levers behave differently when the house is full. Running out with a full house covers the trade-off between heating cheaply and not running short.