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Catch Power – A Better Solar PV Hot Water Timer

Solar hot water timers have been spruiked as a must-have to heat your water with free solar power. They’re fairly cheap to install (about $300) and they make you think you are saving lots of money. The truth is they are unreliable, may cost you money, and are overall a dumb solution. In this post, I’ll show you why. I’ll discuss hot water diverters, and then introduce Catch Power: a superior solution that meets my three requirements: accurate, reliable, and affordable.  (This blog also comes in video format below!)

Hot Water Timer – Fails the First Test

The main reason I prefer the Catchpower Relay over Hot Water timers is that Solar Hot Water Timers fail the first test – they’re not accurate. Let’s say it’s raining in the morning and then sunny in the afternoon. The timer doesn’t care. It will force your water to heat up with grid power during pre-determined set hours regardless of what the weather is doing. So it could heat before the sun comes out, or continue heating through extended cloud cover. It doesn’t even have to be about the weather. Say you’re cooking up a Sunday roast and your aircon is pumping. The water heating will turn on anyway and you’ll be paying the power company for that extra power.  

Timer Poor Solar Usage

In many cases, customers would have saved more money if they stayed on an off-peak tariff (T31 or T33). This is why timers potentially cost more money. If a timer doesn’t even pass the first test, then we won’t discuss if it’s affordable and reliable. Before I discuss the Catchpower Relay, let’s review the next solution: hot water diverters.

Hot Water Diverters : 2 out of 3

Fronius Ohmpilot – an example of a Hot Water Diverter

Accuracy and Reliability

Instead of being an on-and-off switch like a timer, a hot water diverter acts like a dimmer switch. Say it’s 8 am in the morning and you only have 200 watts of excess solar power. Instead of sending that power back to the grid, the diverter turns on and dims the power it sends to your hot water to 200 watts. This is a more efficient use of your solar production as it’s taking into account all of your solar energy flow.

If you get a good quality diverter like the Fronius Ohmpilot, it will be very reliable and accurate. It’s likely to heat almost 100% of your water with excess solar. So a quality diverter meets two out of my three requirements: accurate and reliable.

Affordability – Fail

In fact, I use the Fronius Ohmpilot hot water diverter at home. For me it was a perk of testing stuff. For you, it’s about $2000 installed. Unless you use lots of hot water, diverters such as the Fronius Ohmpilot usually fail the affordability test. There are other diverters, but they usually meet the same two out of three scores. Or if they’re more affordable, they will likely suffer on the reliability or accuracy front.

So for a long time, we’ve been stuck with an inaccurate and unreliable solution that is cheap – or a really awesome solution that is expensive.

So here’s what I think is by far the best solution for most homeowners… it’s called the Catch Solar Relay. 

The Catch Solar Relay : 3 out of 3

Catchpower Relay
Catchpower Relay – Optimal Solar Usage


This device is like a timer as it’s designed to turn your hot water on when you have excess power – but instead of programming it to a set time, the Catch Solar Relay measures the energy flow much like a diverter. That means it still requires the full kW draw capacity of the tank like a timer, but it reads the solar data and draws only when your system meets the power usage of the hot water tank. If it is raining or you are using the oven for a roast, then the water heating won’t turn on. 

So, if the Catch Relay looks for the full kW capacity of the tank, does that mean you’ll run out of hot water on a rainy day? No. Regardless of the weather, we can set the Catch Relay to have a minimum runtime ensuring your tank still heats. It looks for the best times during the day, but if those times are never met, you can still force whatever heating you need. There are a whole bunch of other things that the Catch Relay does that I’ll explain soon, but first, let’s tick the box, yes the Catch Solar Relay is accurate – it will generally heat your water only when you have enough excess power. 


There’s not much going on in the Catch Solar Relay, and that’s a good thing. The only device you need to add is an automatic switch, a simple off-the-shelf contactor. If you choose the right one (we use Voltex), they are reliable devices that have been used by electricians for eons. Reliability is an easy pass for the Catch Relay, so we’re down to the final test.

Typical Solar Consumption Graph


We have accuracy and reliability, but is it affordable? This is what I like about the Catch Solar Relay. On most of our installations, we install a consumption monitor. This device reads your solar data and your household consumption data. It graphs this data to show you how much excess solar power you’re sending back to the grid, and how much power you’re buying from the grid. Handy information and a must-have feature for all solar installations.

Now for the cool part. The Catch Solar Relay takes the place of most consumption monitors on the market. So let’s say you have a Fronius inverter, the Catch Solar Relay communicates with the inverter and displays consumption monitoring on This costs $200 more than a Fronius consumption meter, but it also controls your water heating. I’d also contend that even if you weren’t purchasing a consumption monitor with your system, the total price is still an affordable solution. If you want consumption monitoring and you have a hot water tank, this is a no-brainer.

The Catch Solar Relay is better

So we have the affordability of a timer, the reliability of a contactor, and accuracy close to a diverter. For such a simple yet effective solution, why hasn’t this been done before? The truth is, it has, but it’s never been done this well before. Previous products such as the Fronius Relay ticked most of my requirements, but fallen short in key aspects. The Catch Solar Relay can read and react to your home’s energy flow, plus it has neat programmable features in their Monocle app that takes it to the next level. There’s a long list such as voltage reacting, boost times, overriding, etc, but here’s some common key examples:

  • It’s raining on Christmas day and the whole family is over with their own shower habits at all hours of the day. No problem, jump into the Monocle app and override all the maximum self-consumption parameters. Force it to run and have your hot water heating as it needs. Cranky cold showers are not worth the savings for a day or two.
  • Your hot water fails. Is it the hot water timer/diverter you have? Or is it the hot water unit itself? You can log into the Monocle and with a fair degree of accuracy diagnose what has failed.
  • Hot water boosting! No other relays can send a night-time boost to get you going in the morning. This often comes up in winter when heat retention is lower, or you’re simply taking longer hot showers than normal. No problem, maintain the optimal heating hours for the day, then jump into the app and program a 1-2 hour boost at night for that cold winter spike.

All in all, Catch Power are a great Aussie company who have proved themselves in the Australian market. Where timers and diverters have failed to meet all three requirements, they have a reliable, affordable and accurate product that manages a large part of your electricity costs – water heating. If you want to know more about Catch Power, watch this video interview with one of the founders, Scott Young. 

8 Responses

  1. I thoroughly agree that the catch power relay is a great idea. So much so that I wanted to employ two of them in my new solar system (install completed 16/10/23). I wanted one for the hot water system and one for the swimming pool pump. I was really disappointed when I discovered that the catch power three-phase version had not yet been released. Great concept, just hope that the 3-phase version becomes available very soon.

  2. Hi, could you please comment how the Catch Solar Relay interacts with the latest Smart Meters which measure energy usage multiple times per second. With a the subversive move by power suppliers to move to time of day charging, a solar diverter that frequently switches between grid power and solar during peak periods would import power at max. rate approx. 60c/kW/hour. To me, I think it would be more economical to use off peak rates for top-up heating of the hot water and allowing home solar to do its best during day to heat the water – albeit with variable results. I look to your feedback regards, Tim C.

  3. We have a 10kw system with a Fronius 8kw Inverter installed about 4 yrs ago. We receive 8c FIT. I use CL1 for our HWS and our bill is around $220 annually. At the time when we purchased our system we looked at installing a Timer to allow connection to direct solar to HWS but opted out. Now I want to re-examine my options. I have looked at the what has progressed since then – timers, relays and diverters. I also are aware of not over capitalizing. Your latest comments indicate that the Green Catch Relay is the way to go that directs solar to my HWS but also is able to ensure we are covered during intermittent weather. Can you advise what is the current best reasonable priced option (quote) please.

  4. Big problem is that most of water consumption is in the evening. If the system has the ability to take power at that point it will! During the day the consumption of hot water is very limited as most showers are in the evening therefore energy will still be taken from off peak rather than solar!

    1. I think the idea is that the relay takes excess solar power, and can be programmed to take excess solar power only – and/or other times. So it doesn’t matter when you shower, you’re showering with a large volume of stored hot water in the evening not electricity. The partially cooled storage tank is then brought back to temp the following day instead of instantly after you shower. That seems to be the whole idea of the product.

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Mark Cavanagh

Mark is the Owner and Manager of MC Solar & Electrical. He’s an Electrician, accredited solar installer/designer and an electrical contractor.

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