The article below, written by Scott Young of CATCH Power, provides a look into the world of hot water diverters, which use excess solar energy to drive the element in electric storage-based water heaters. Hot water diverters are a great way to increase solar self-consumption and are significantly more affordable (albeit less versatile) than battery storage at this point in time. There are a range of hot water diverter options available in Australia; we have an (incomplete) list at the bottom of this article.
Please keep in mind that the example from this article may not be relevant to your situation, as water heating and solar setups differ from person to person. Also note that different diverter products have different functions and capabilities and other units may not operate as described below (CATCH Power, for example, uses an internet connection and predictive algorithm to optimise performance in some of its products.)
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We hear people talking about installing timers for hot water all the time. Do timers actually help solar homes to reduce their hot water usage charges? Although it may seem like a smart idea, the answer is unfortunately ‘no’ in most cases. We explain why this is the case in this article.
How heating water on a timer can actually cost you more
In most situations installing a timer for your hot water will cost you money. There are two simple reasons for this:
- When the timer turns on, you will be importing energy most of the time.
- For every 1 unit of electricity imported, your solar needs to create at least 2.
The elusive ‘perfect’ solar day
Lets start by looking at the perfect solar day. The graph below shows how a 5kW solar system is likely to perform on a perfect solar day. We have added in a typical hot water heating load being run on a timer (the orange stuff).
As you can see from the chart, if you have a 5kW solar system, and you have a perfect solar day the bulk of your hot water is in fact heated by the solar. But even with the perfect solar day you are buying some of the electricity from the grid.
Any of the hot water power that appears above the solar curve is energy purchase from the grid.
It is important to understand, the perfect solar day is like a Unicorn; they are exciting, some people have claimed to see them, but for most people they are always just out of reach. How do we know this? Because CATCH Power has hundreds of internet-connected, ‘smart’ hot water diverters installed, and we see the ‘real’ solar day playing out all over the country on a daily basis.
The typical solar day
So if we are claiming the perfect solar day is hard to come by, what does a typical solar day look like?
Again, we can look to the data from the CATCH hot water diverter units installed around the country, which shows what is actually happening in the home. The below graph shows you what a typical solar day looks like (in Summer).
The solar output looks a lot different when you start dealing with real weather and real solar yields. We have also added in the average daytime loads into the graph and sat the hot water load on top of it (because that’s how it often works with a timer).
You can see from this graph there is significantly more energy being purchased from the grid now. Also keep in mind this is an average day in the middle of summer. Things are even more diabolical in the winter months.
In order for you to save money putting your hot water on a timer, it is critical to make sure you are exporting two times more energy than you are buying from the grid. Using this average day as the test we can visually see this is not the case, in fact if the above graph was your house, you would have lost money on that day.
Here’s what we’ve observed from the data: if you have a 5kw solar array, there are only a handful of days in a year where putting a timer on your hot water equates to net savings. If your solar system capacity is less than 5kW, there are NO days in the year.
Taking a closer look
Where does this “export 2 for every 1 unit purchased” idea come from? It has to do with electricity tariff arrangements.
Lets make some assumptions:
- The cost of full tariff electricity is 26 cents
- The cost of Off Peak electricity is 10 cents
- The solar export rate is 8 cents
Now before you email us with disagreement about these numbers – I know there are some places in Australia where some of the electricity rates are significantly cheaper than our assumptions, and equally there are people choking on their Weetbix because they are paying a lot more, so I think these are fair assumptions.
Lets start by assuming we need to put 1kWh of energy into our hot water tank to get it hot.
Before there were solar panels and timers, and smart hot water diverters, off peak electricity was the only option to get that 1kWh of energy, which would have cost us 1 x 10 cents = 10 cents.
Now, along come our good friends the solar installation company, who say to you, “We can put your hot water on a timer. It will help improve the savings you get from your solar system.” So you decide to do it.
You now have a shiny new solar system, and your hot water is now running on a timer.
As it turns out, your first day of water heating turns out to be a rainy one (because – as everybody who owns solar knows – it usually rains for a week after you first get it installed). There is not enough solar being produced to cover all of your hot water load. That 1kWh of energy we used to get from off peak is now coming from the full tariff mains, so 1kWh costs 1 x 26 cents = 26 cents.
Now, before solar came along it used to cost 11 cents, but today it cost 26 cents, so we are in the hole by 26 – 10 = 16 cents.
But it is not all doom and gloom, there is time later in the day where we are exporting solar to the grid, so we can recoup our losses there. But in order to earn back that 16 cents we need to sell 16/8 cents = 2 kWh into the grid.
That is where the 2 for 1 comes in – and that is just to get you back to neutral a position! You need to keep selling back to the grid to actually make any real savings.
How a diverter fixes the problem
If timers don’t work, how does a smart hot water diverter fix the problem?
A solar diverter, such as CATCH, will not just identify what part of your solar is available to be used for hot water heating; it will also track it perfectly, all through the day, regardless of the solar output and the loads that come on in the house.
The above graph is taken from a real life working Blue CATCH installation. As you can see from the graph, January 11th was a fantastic solar day. But it was a hot day and there was a load (or series of loads) running in the house all day. At this time of the year, on a sunny day, it was most likely an air conditioner. The green area is the amount of energy that is surplus to the home through the day. If this hot water system had been running on a timer it would have been a very expensive hot water heating day.
As you can see from the graph, the energy delivered into the hot water tank (shown in orange) is matched perfectly to the amount of surplus solar available, as the surplus changes so does the rate of energy delivery to the hot water tank – until the thermostat cut-off was reached (orange dot). Clearly seen on the graph, the total amount of energy available (over a longer period of time) was enough to do the job of heating the water until cutoff. At no point was mains power required to do any of the heating job.
On a cloudy day
What about when the weather is cloudy or rainy? There will be little or no surplus solar to divert to the hot water tank.
If your diverter is smart, and connected to the internet (like Blue CATCH), then it ‘knows’ that tomorrow will be a poor solar day and make a decision to utilise the off-peak power to heat the water. This way, if a supplementary power source is required, it is the next cheapest power source, and not mains tariff power.
Here is an example:
In this example, there wasn’t enough solar to do the complete heating job on March 10th. CATCH anticipated this the night before and drew upon the off-peak to get the job done. A very small amount of off-peak (the orange spike late on the 10th) was calculated to be required to get the job started for the 11th.
In conclusion, installating a timer to heat your hot water with solar will very likely cost you money over the year. Only a smart hot water diverter is able provide the real savings you are expecting from your solar installation.
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- Water heating on a timer vs solar diverter: A case study - 22 June, 2017