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How does a day / night switch for a hot water service work?



We sometimes get asked how to get the benefit of different day and night power rates on heating water – generally called the hot water service. So here’s how it works, and also our take on whether or not it applies to you.


First up, we should just clarify the concept of day / night power rates. By far the most power is used during the day when people are out and about doing things. This is when the demand on the power supply system is the greatest and so power companies charge their usual or full rate during these hours. Fair enough.

But when things wind down at night, then the power companies offer a cheaper rate – it’s a bit like they’re trying to reward people that will use their power during the quiet times, which helps them cover their costs of the massive systems. But this cheaper rate can be considerably cheaper, so it’s a jolly good idea. The night tariff generally kicks in around 11pm, and reverts back to the full rate at about 7am. These times can vary, but it’s around the mark.


So there’s three sides to the story on how to get it set up to benefit from the cheaper rate, and these are –

– The hot water service itself
– What the electricians do
– Getting the power company to do their part


So let’s start with what the electricians need to do because they often set things up at the ‘rough-in’ stage before they even have the hot water unit in their hands. The main thing they have to do is put a different circuit breaker in the meterbox (or internal circuit board depending on your situation). This circuit breaker is called a ‘2 Pole’ breaker instead of a single pole. Nothing real fancy and only costs a few dollars extra, but without it you won’t get the benefit of that cheaper rate.


The next thing is the hot water service itself. This has to have 2 elements in it. An element is the item inside the unit that heats up to then warm the water. But if it’s only a single element unit, then it won’t work. A twin element unit means that one element is connected to one of the ‘poles’ in the circuit breaker and the other element to the other pole, and depending on whether it’s night or day depends on which element it’s using.


At the end of the day, either element can do the job, but it’s just the way that the system works. Because it means the power company can allocate a different price per kWh to a different pole in the circuit and control it. Which is the third point – getting the power co to do their bit.


Which of course infers that the hot water unit actually has an element! So a gas instant (sometimes called gas constant) isn’t able to be used for a day / night switch because it doesn’t have an element – it has a gas burner that heats the water on demand as it passes through pipes.


And the way this works is that the electrician has to notify them (the power company) that they want day / night rates on this property for the hot water service. If they don’t, then the power company won’t set it up. You might say, why doesn’t the power company do it automatically every time without asking? Because they can’t assume that there’s the 2 pole breaker there for it, and the hot water unit is relevant. So it’s important that the electrician gets that info onto the application form to the power company.


When those 3 ingredients are all mixed in, then it’s all good – at around 11pm, you’ll be charged a cheaper rate for the hot water service ticking away keeping that water hot for you.


But bear in mind that it will cost you a bit more to set it up like this. The hot water storage unit itself could be an extra couple of hundred dollars, and the electricians will charge extra for the little bit of additional work and inconvenience. But it’s still well worth it – the payback could even be a few months depending on the size of your unit and the kWh rate. See, a night tariff rate could be about half what the day rate is – quite a substantial saving for each hour that the unit is plugging away keeping that water hot.


Just a quick comment – if you ever get solar panels on your home – with you home still connected to mains power – then the day / night service automatically gets disconnected. You can’t have your cake and eat it too apparently!


And by the way, this day / night rate can also be applied to underslab or underfloor heating, or hydronic heating. Unfortunately you can’t use the special rate for every single time in your home – it has to be either the water heating or underfloor heating.


But please don’t assume your builder will do it for you. You’ll need to put your hand up and clearly state that you want this option included in your home.


So, while you’re sleeping your hot water service is heating at a much more efficient rate … not bad, not bad at all.

Disclaimer. This blog is our opinion only. The information provided in our blogs is accurate and true to the best of our knowledge, but there may be omissions, errors or mistakes. The information presented in our blogs is for informational purposes only and we are not professionals, so the content we provide shouldn’t be taken as legal advice. We strongly recommend consulting with a professional before taking any sort of action. We reserve the right to change how we manage our blog and we may change the focus or content at any time.

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