Solar PV Hot Water Timers Vs. Hot Water Diverters - Which One To Choose

Upon having a solar power system installed, most customers inevitably start to think about the ways in which they can use as much of their generated solar power as possible (solar power self consumption), rather than exporting power to the grid.  Customers can slash the cost of electricity bills when consuming self generated power, and if excess solar power remains,  excess power can be sold to the grid (selling at 8c - 12c per kWh).

Hot water cylinders are generally the largest electricity expense in homes, costing households between ¼ to ⅓ of their power bill.  Therefore it’s a good idea to ensure solar power is heating as much water as possible. To do this, solar diverters or hot water timers are needed. They automatically turn on appliances, generally during the day when the sun is generating electricity.

Solar diverters and hot water timers use solar power to heat hot water cylinders, utilising solar power before having to draw grid power.  Hot water timers aren’t as “smart” as diverters - therefore they won’t utilise your homes solar power as much, but they are not nearly as expensive. I will discuss the two options below to help to determine what the best choice is for your home. Otherwise simply ask a professional solar installer what they think is best for you.

How Does A Hot Water Timer Work?

A timer is installed into a buildings switchboard.  The timer can be set to turn on any time of day, but the time period will be dependent on how many hours the cylinder will need to heat, (normally between 2 and 4 hours a day). For instance, the timer might be set from 11am to 2pm.

A basic timer from a local hardware store won’t work.  An electrician will need to install the timer into a switchboard (It’s economical to get this installed at the same time as your solar power system).

If the cylinder is timed to run once a day and heat to 60 degrees, then legionnaires' disease is not a risk.  

You can set these timers to run multiple times during the day. For example, if you have a busy family you might want to set the timer at night/off peak rates to heat up the home again.  

Extended family over to stay? Customers can always turn off the timer, allowing the hot water to heat as often as needed.

By using a timer, grid power will occasionally be heating the hot water as the timer for the hot water cylinder might be programmed to turn on when solar power isn’t being generated. 

Does having a timer on a hot water cylinder work for you? Please consider the following questions:

• Do you have a small heating element in your hot water cylinder, or can easily reduce your element size, e.g. go down from a 3kW element to a 2kW element?
• Is your solar power system large enough? A larger solar power system is more likely to produce surplus solar power, justifying heating water during the day. It wouldn’t be viable having a 3kW hot water element trying to use power from a 1.5kW solar power system.
• Not using power on other appliances during the day?
• Do you have a discounted night tariff on power? Consider the value of off-setting electricity during the day with your hot water cylinder vs heating hot water with evening electricity prices (cheaper).

What Does A Solar Hot Water Timer Cost?

A timer will cost roughly $200 (installed), a fair price in the scheme of things; however, make sure that it will actually end up saving money, as it is not guaranteed.

How Does A Hot Water Diverter Work?

A hot water diverter (aka solar diverter), diverts excess solar electricity to heat an electric hot water cylinder.  By assessing the amount of electricity needed in the home, and by how much solar power is supplied, any left over power will go into heating the hot water cylinder (rather than exporting it to the grid).  A diverter aims to avoid the hot water being heated by grid power as much as possible.  

A solar power system delivers varying rates of electricity throughout the day (due to changes in the weather and cloud cover), a diverter is designed to cope with this variability.

What If The Hot Water Element Requirements Don’t Match The Solar Power System Output?

A 3kW hot water cylinder element typically wants to draw 3kW at a time, but the diverter allows for smaller amounts of electricity to feed in.  The smallest amount of surplus energy used by the hot water cylinder (with a diverter) is 100 watts. If smaller amounts of solar power are being fed to the hot water cylinder, the cylinder will take longer to heat during the day.  Should the hot water cylinder not get up to the set daily temperature threshold, then extra power will be drawn from the grid to make sure the cylinder is fully heated. 

The diverter has additional smart capabilities of measuring the temperature inside the hot water cylinder.  This ensures that if the cylinders temperature goes below the minimum, power can be drawn from the grid to heat the cylinder (only if required).

Not all solar diverters are equal, some have more features than others.  Look closely at the specs of a diverter before selecting one. Some varying features include the ability to power more than one heater, and some will provide electricity to appliances that don’t have an element.

A solar diverter can assist a household to consume solar power in the most efficient way possible, and enables the home owners to save money on electricity bills. Consider these questions to determine if the benefits apply to your situation;

• Does your household use a fair amount of hot water?
• What is the exported solar buyback rate?  If a low rate of 8 cents applies then a diverter is more likely to be worth your while.
• What is the off-peak electricity rate available for your household? Is ripple control available at an exceptional price? 
• Is your system large? Is there enough excess solar electricity available to justify a diverter?
• Can you divert all of your excess electricity in other ways, such as timing your dishwasher and washing machine?

What Is The Cost Of A Hot Water Diverter?

A hot water diverter will generally cost about $1000. As a fairly large investment it would be advisable to check with your solar installer to see if they think it makes sense to have one in your home.

Potential savings could be significant by heating your hot water with solar power, (with both technologies costing a lot less than battery storage), we highly recommend you look into both options.