Off-Peak Hot Water: How It Works and What to Do When It Runs Out

Off-peak hot water is an electric storage tank wired to a separate economy tariff instead of the main household tariff. Energex, which manages Queensland's electricity network with Ergon Energy Network, describes economy tariffs as supply that is typically available outside peak times at a discounted rate, for appliances such as hot water systems and pool pumps that don't need power around the clock.

An off-peak tank only heats while that circuit is live. As energy.gov.au puts it, the water is only heated during the off-peak period, when energy prices are cheaper. That is why the tank stores the day's hot water rather than reheating whenever you use it: the hot water you run at 7pm was heated hours earlier.

Energex notes that, pool pumps aside, appliances on a controlled-load economy tariff have to be wired through a dedicated circuit back to the meter. That separate circuit, and a separate line on your power bill, is what makes a system off-peak. energy.gov.au notes that electric storage systems can be switched to an off-peak tariff, so the tank itself is often a standard electric storage unit.

Queensland has two economy tariffs, and the one your tank is on decides how long it gets to heat each day. Energex lists them by network name, and retailers may call them something else on your bill.

Tariff 31, which retailers may call Super Economy, Night Rate or Controlled Load 1 (CL1), supplies power for 8 or more hours a day, around 10pm to 7am. Tariff 33, which may be called Economy, Night Rate or Controlled Load 2 (CL2), supplies power for 16 or more hours a day, usually outside 4pm to 9pm.

The practical difference is recovery time. A tank on Tariff 31 gets one main heating window, mostly overnight, and has to last until the next one. A tank on Tariff 33 has power for most of the day and is usually off through the 4pm to 9pm evening peak, and at times through the morning peak as well, so it gets more chances to recover after heavy use. The comparison table further down sets the two side by side.

Off-peak hours are not fixed. Energex says the times power is available vary by location in Queensland and by tariff, and that availability times may change depending on the current needs of the network.

Rooftop solar is now part of that. Energex says that as the year moves from summer to autumn, the scheduled timing for economy tariffs has moved from managing evening peaks towards soaking up rooftop solar generation, so supply to these appliances is typically interrupted during the morning peak and resumed during the middle of the day. While that schedule applies, some Queensland tanks on an economy tariff heat partly in daylight, not only overnight.

Two things follow. A household that always had hot water ready for an early shower may notice the timing shift between seasons. And if a timer is fitted to the hot water circuit, Energex itself says its settings may need updating to avoid the morning and evening peaks. Ask the licensed electrician who fitted the timer to change it, rather than opening the switchboard yourself. Your retailer can confirm the hours that apply to your property.

The power bill is the quickest way to tell whether your hot water is on off-peak. Energex says that if your bill lists electricity charges separately from your main or daytime tariff, for example Economy Tariff 33 or Super Economy Tariff 31, you already have an economy tariff connected.

Retailers use their own names for these tariffs, so look for any second consumption line rather than one exact label. If the bill is unclear, Energex suggests asking your electricity retailer, and notes that an electrician can also check which appliances are connected to an economy tariff.

Knowing which tariff you are on matters before anything else. A tank on Tariff 31 that runs dry at 6pm is behaving as designed. The same symptom on a tank with power all day points somewhere else.

An off-peak tank runs out when the household uses more hot water between heating windows than the tank holds. Because its circuit is off for part of the day, an off-peak tank cannot top itself up the way a tank on a continuous tariff can.

The usual triggers are easy to spot once you know to look: visitors staying, longer showers, a load of washing on hot, or the first cold snap, when showers run hotter and longer. On Tariff 31 the tank may not get power again until late at night. On Tariff 33 it will usually start recovering once the evening peak ends.

Rheem's owner's guide for one of its heat pump models says the same about that model: where a timer is installed, enough time has to be allowed to reheat the tank, and heavy use can leave the water cooler than expected because there was not enough heating time. If running out has become a regular event rather than an occasional one, the tank size or the tariff may no longer suit the household. That is a planning question, not an emergency.

Work through these checks in order. None of them involves opening, adjusting or resetting anything.

1. Look at the clock and the day. If it is late afternoon or evening and the house has used a lot of hot water, the tank may simply be empty and waiting for its next window: after the evening peak on Tariff 33, possibly overnight on Tariff 31.

2. Look at the switchboard. Rheem's guide tells owners to check that the isolating switch marked HOT WATER or WATER HEATER, at the switchboard and at the water heater, is on, and to check the fuse or circuit breaker marked HOT WATER. If that breaker has tripped, or trips again after being switched back on, leave the circuit off and call. Resetting it repeatedly defeats the protection that is doing its job.

3. If a timer is fitted, note what it is set to. Do not touch the wiring.

4. Look at the tank for water coming from the body of the cylinder, not just a drip from a valve.

If the switch is on, nothing has tripped, and there is still no hot water the next morning, the tariff is not the cause. Call a licensed plumber.

An off-peak tank's supply is switched by load control equipment in the meter box, and the safe options for an owner are limited. Roughly in the order to try them:

1. Wait for the next heating window and spread hot water use out, with staggered showers and cold washes, until the tank recovers.

2. Ask your electricity retailer which economy tariff the tank is on and whether a move from Tariff 31 to Tariff 33 is available. More hours of supply mean more chances to recover.

3. Ask a licensed electrician about the options for your switchboard, such as a timer or moving the tank to the main tariff. Energex says moving equipment off an economy tariff needs an electrician, who should also lodge an Electrical Work Request if the tank is the only appliance on that tariff, so the load control hardware is removed.

4. If the tank is simply too small for the household now, talk to a plumber about size or a different type of system.

What you should not do is rewire the circuit, swap it to the day rate or bypass the meter box equipment yourself. Queensland's Electrical Safety Office says doing electrical work without a licence is illegal, and that hard-wired appliances must be connected by a licensed electrician.

A heat pump can work on an economy tariff, but the manufacturer's tariff recommendation for the model decides which one. energy.gov.au says heat pump water heaters use 30% of the energy of a conventional electric system, which is why people moving off an off-peak electric tank often look at them.

Rheem's owner's guide for its MPi 551325 heat pump recommends connecting it to either a 24 hour continuous tariff or an extended off-peak supply of at least 16 hours a day, depending on the model it replaces and the climate, and recommends a continuous tariff where it replaces an electric water heater larger than 250 litres. That recommendation leaves out a night-only 8 hour tariff such as Tariff 31. The same guide says a timer affects both the heat pump and its booster element, and that while a time-controlled supply is off, the indicator lights are off too, which is not a fault.

Solar hot water is different again. energy.gov.au notes that solar units may need a gas or electric booster in areas with less mid-year sun. When the booster is electric, how it is wired and timed affects when it heats, and the owner's guide for your model is the reference. Other brands set their own tariff rules, so check the installation instructions for the exact model before changing anything.

Off-peak can still lower the cost of running an electric tank, but rooftop solar changes the comparison. Energex says the saving depends on the difference in cost per kWh between your standard tariff and the economy tariff, and on how much the appliance uses. It adds that, in most cases, power used on an economy tariff does not count toward demand charges on a demand or time-of-use main tariff, which can make an economy tariff more useful if your main tariff has changed. Confirm how your retailer applies this.

On the other side, energy.gov.au says electric storage is generally more expensive to run unless powered by a solar PV system, and that moving it to off-peak cuts cost but not greenhouse gas emissions. If your panels generate well in the middle of the day, heating water then can use your own generation, and when Energex shifts economy supply towards the middle of the day, as it does moving from summer into autumn, the two can overlap.

There is no single answer. Ask your retailer for a comparison based on your actual usage, and if the tank is near the end of its life, compare running costs across system types before replacing like for like.

A tariff problem is about timing: the water is hot in the morning and runs short later. A system fault looks different, and waiting for the next heating window will not fix it. Treat these as faults and call a licensed plumber:

No hot water at all after a full heating window, with the hot water switch on and nothing tripped.

The breaker marked HOT WATER has tripped, or trips again after being switched back on.

Much less hot water than usual, or water that is not as hot as it used to be, even after a full heating window on a normal day of use.

Water coming from the tank body rather than a valve, or rust staining on the casing.

Finding which part has failed, whether an element, a thermostat or the cylinder itself, is licensed work, not an owner repair. The Electrical Safety Office says doing electrical work without a licence is illegal in Queensland. And under the QBCC's rules, installing or replacing a water heater, whether electric, gas, solar or heat pump, is notifiable plumbing work, which the plumber must register with the QBCC.