Will A Solar Hot Water Timer Save Or Lose You Money?

If you have solar, is adding a simple, cheap timer to your electric hot water system worthwhile?

Lately my life has been a lollapalloza of solar electric hot water heating.I’ve looked at more ways of heating water using solar electricity than you can poke a stick at.Or at least I have if your poking arm is weak and flabby and easily fatigued.

Recently I went into the details of using a relay with a Fronius inverter so it will turn on a hot water system when there is enough surplus solar electricity to power it.I also gave a run down on solar hot water diverters in general, which are devices that will take even small amounts of surplus solar power and send it to a hot water system in a form they can use.And I delved into the secrets of the Sun Flux hot water controller that uses it own dedicated solar panels to provide power for heating water.

Soon I will put several brands of diverters into a little Thunderdome and make them fight to see which one will come out on top. But for now I am going to describe the simplest and cheapest way to heat water using solar electricity, which is to use a timer so it switches on during the day, hopefully when there is enough surplus solar electricity to run it.

A timer may only set you back around $100 if you have it installed at the same time as your solar or hot water system.But if you need to get someone to come around just to install the timer it is likely to cost $200 or more.And you can always pay extra for something fancy, such as a timer you can easily adjust yourself.

A hot water system timer is a great way to save money provided it doesn’t end up costing you money.The drawback of a timer is it’s almost impossible to avoid using grid electricity at least some of the time.You will pay for that electricity at your standard rate and not at a lower controlled-load or economy-tariff rate.

But, under the right circumstances, it is possible for a timer to save money.These circumstances can include:

If enough of these duckies line up in the right way, then you may be able to save money instead of losing it by installing a hot water timer.

Why Not Use Solar Thermal Hot Water?

Recently, as I’ve been rabbiting on about using solar electricity to heat water, some people have said, “What’s wrong with traditional solar thermal hot water?You know, where water circulates through thermal panels on the roof, the way god intended!What’s wrong with that?”

Well, it’s not that anything wrong with solar thermal hot water, it’s just there is something right with solar PV, and that is, the price is right.Solar thermal is a mature technology that hasn’t come down much in cost over the past few years, while solar electricity has.As a result, many people have concluded PV is more cost-effective for heating water than solar thermal.

But if you want to install solar thermal hot water, please don’t let me stop you.It does have advantages:

Timers Are Dumb

In a world full of smart technology and intelligent systems, a simple timer that turns on a hot water system for a set period each day is pretty bloody dumb.All it can do is turn on the hot water system at a time when you hope there will be enough surplus solar electricity to run it and then turn it off later.It can’t decide to wait for the sun to come out from behind clouds and it can’t respond to household demand and shut itself off if someone turns on a hairdryer or otherwise increases electricity use.

Hot Water Diverters Are Smart

Solar hot water diverters are not geniuses, but they are very good at slurping up almost all the surplus electricity a solar system produces and sending it to an electric hot water system in a form it can use, so the result is pretty clever.Sure, it is possible during periods of cloudy weather and high hot water use that solar electricity alone isn’t enough and grid power is needed.But a diverter is the most effective way to make sure surplus solar electricity goes towards heating water.

Fronius systems are smart

A Fronius relay plus datamanager is a very smart system and capable of turning on a hot water system only when there is enough surplus solar electricity to run it and it can turn itself off if the sun goes behind clouds or household electricity use rises.It can’t do the diverter’s trick of getting a heating element to use less power than it is designed for.This means that, all else equal, more grid electricity will be required to heat water than with a diverter, but it is still quite clever.

Even if you use a Fronius inverter that doesn’t have a data manager², which is the part that is a clever clogs, and use a dumb relay by itself, because it will only turn on when the inverter is producing a set amount of electricity, it still ends up being smarter than just a timer.

Do You Feel Lucky?

So if you have a timer connected to your hot water system, every time it turns on, you are taking a gamble there will be enough surplus solar electricity and your hot water system won’t start consuming expensive grid power.

The Smaller The Hot Water Element The Better

A conventional electric hot water system consists of a storage tank with one, or sometimes two heating elements.The elements come in the following standard sizes:

The larger the element the more quickly it will heat water, but the more power it will draw.If there isn’t enough surplus solar electricity being produced when the timer turns it on, whatever is lacking will be drawn from the grid, so the smaller the element, the better when using a timer.

A 1.8 kilowatt element can heat 250 liters of water from 20 degrees to 60 degrees in 6.5 hours, which is plenty of heating power for a typical household of two or three people, but for a household that is a large user of hot water this may not be enough.

The Bigger The Solar System The Better

The larger your solar system, the more likely it is to produce enough surplus solar electricity during the day to run the hot water system without using grid power.And you will need a big solar system to be sure of having enough surplus electricity to power even a 1.8 or 2.4 kilowatt heating element.

Unfortunately, for a variety of reasons, rooftop solar rarely produces as much power as the total capacity of its solar panels.So if you have a north facing solar system with 5 kilowatts of panels, it may not produce more than 4 kilowatts of power even at noon on a clear day.From mid-morning to mid-afternoon on a cloudless day, such a system might produce from 2 to 4 kilowatts.Assuming household electricity consumption at that time is minimal, this is only enough to power a 1.8 kilowatt element without using grid electricity.

The situation is improved with a 6.5 kilowatt system, which is around the maximum size that is practical for many households to install.This could be expected to produce from around 2.6 to 5.2 kilowatts from mid-morning to mid-afternoon — enough for a 2.4 kilowatt element, provided there is no one at home using power.

Larger Solar Systems Help With Cloud Cover

Unless a home’s rooftop solar system is at the edge of being unfeasibly large, it is impossible to avoid a hot water system on a timer using grid power if it switches on during cloudy weather.While light cloud may cut solar output by 75% or less, very dark cloud can cut it by 90% or more.

If overcast conditions cut output by 80%, then a 5 kilowatt system may only produce from 0.4 to 0.8 kilowatts from mid-morning to mid-afternoon. That’s only enough to supply around one-third the power consumption of a 1.8 kilowatt element, provided the home has no other electricity consumption at that time at all.

The More You Are Away During The Day The Better

If you are not at home during the day then you are not going to be consuming electricity and reducing or eliminating the amount of surplus solar electricity available when your hot water timer switches on.So if no one is at home during weekdays that is good, and if you are also usually out during the day on weekends too, that’s even better.

The Higher Solar Production In Winter The Better

The colder it is, the more hot water people use, so the higher solar production in winter the better.Sydney, Brisbane, Perth, and Darwin all have reasonably good winter output and all average 3.25 kilowatt-hours or more per kilowatt of solar panels in June, which is the worst month of the year for solar power.This is shown below on graphs of monthly output for Sydney and Darwin:

Canberra, Adelaide, Melbourne, and Hobart all do much worse.Here is a graph of monthly average solar output for Canberra:

As you can see, Canberra only averages 3 kilowatt-hours a day of solar electricity per kilowatt of panels in June, but with its more southerly location and cloudier winters, Adelaide’s June output is even less:

In Melbourne the average for June is almost as high as in Adelaide, but Hobart averages under 2 kilowatt-hours a day per kilowatt of solar panels.

Unfortunately, though not unexpectedly, Australian capitals that have the coldest winters and the highest hot water use, have the worst winter solar production.

Controlled Load Or Economy Tariffs Often Can’t Be Used

Most people with an electric hot water system have it on a controlled load or economy tariff.This allows people to pay less for grid electricity used by their hot water systems, but the power is only available for a limited amount of time each day³.

Hot water systems on a controlled load tariff are not permitted to receive grid electricity from another source.As a result, most hot water systems cannot be put on a timer to receive solar electricity during the day and be on a controlled load tariff.An exception to this is double element heaters, which can have one element on a timer and the other on a controlled load tariff.The element on the controlled load can be set to a lower temperature so most of the heating gets done during the day.

Setting The Timer

Setting a hot water system timer involves striking a balance between minimizing grid electricity use and not running out of hot water.If your hot water system is on a controlled load tariff and you occasionally run out of hot water, then unless you found these events to be exciting opportunities to have invigorating cold showers, then you will probably want to set your timer so your hot water system will run long enough to heat a full tank of water.

In winter, the water entering a hot water tank might be around 15 degrees⁴.To heat a 250 liter tank of 15 degree water to 60 degrees, which is the minimum temperature electric hot water systems must be set to⁵, will take the following amounts of time:

For a 315 liter tank increase the times by 25% and for a 400 liter tank increase the times by 60%.

So if you have a 250 liter hot water tank with a 1.8 kilowatt element, you may want to set the timer so it will run for over 7 hours a day.On most days the hot water system will reach its maximum temperature and switch off well before the 7 hours are up, but on days of high hot water use it may run for the full 7 or more hours.If the timer was set to switch on at 10:00 am then it should usually reach its maximum temperature by 2:00 pm, but if that is not long enough it will be able to keep heating until past 5:00 pm.

The longer a hot water system is left on, the more likely it is to use some grid electricity to maintain its temperature later in the afternoon.But that’s a price most people are willing to pay to not have a cold shower.

If you have spent the money to get a timer you can easily adjust yourself, then changing the settings if you need to is not a problem.But if the timer has to be set by an electrician you’ll want to get it right the first time.

Estimated Savings From Installing A Timer

As you can probably guess, it is pretty difficult to wring savings out of a hot water timer.I thought about making a 3 dimensional, rotating graph, that would show all the possible permutations of hot water timer phase space and allow people to see exactly under what conditions they could save money.I thought about it, but I never had any intention of making it.Instead, I’ll give a variety of examples and use them to estimate the possible savings that may result from using a timer.

I freely admit that in creating the estimates I used some intelligent guesswork.But don’t worry, if you don’t like that, I’m more than happy to give you unintelligent guess-work.

All the figures for electricity prices and feed-in tariffs are based on actual figures at the locations.

Brisbane Example

If a household in Brisbane has the following characteristics:

Then under these conditions a hot water timer would save $18 a year.

If the portion of solar electricity used to heat water fell to 62% or less getting a timer would lose the household money.

Sydney Example

In Sydney, a household with the following characteristics:

Would only lose money, as the feed-in tariff is higher than the cost of the controlled load tariff.It would make more economic sense to send leave the hot water system on the controlled load tariff and send surplus solar electricity into the grid.

If the cost of the controlled load was twice as high and the household had the following characteristics:

Then the household would save $23 a year.

Adelaide Example

If an Adelaide household had the following characteristics:

Under these conditions the putting the hot water system on a timer would save $107 a year.

Perth Example

In Perth controlled loads for hot water systems are no longer available.So for a household with the following characteristics:

Under these circumstances installing a timer would save $210 a year.

Melbourne, Hobart, And Darwin

Due to poor solar output and the existence of controlled load tariffs, a hot water timer is very unlikely to save money in either Melbourne or Hobart.In Darwin the feed-in tariff is equal to the cost of grid electricity so a timer can’t save money.

A Hot Water Timer Is Unlikely To Pay For Most Households

While it seems possible for a hot water timer to save money in a some locations under favorable circumstances — such as having a small hot water element and not being home on weekdays — it seems likely that putting hot water systems on a timer will lose the majority of Australian households money.

The exception is Perth.Unlike most of Australia, controlled load tariffs are not available in Western Australia.This means there is no danger of losing money by putting a hot water system on a timer.I would say it is worthwhile for anyone in Perth who is getting rooftop solar installed to have their electric hot water system put on a timer at the same time.But very few people have electric hot water systems in Perth, as most homes there use natural gas hot water.