The Death of Hot Water Timers and the Fronius Hot Water Relay
Hot Water Timers and Hot Water Diversion has been a topic surrounding solar systems since the option became available. The idea is simple; when installing solar you remove your electric hot water system from your off-peak meter, and instead run it on your main meter – T11 (which is what the solar is connected to) – and time it’s heating to coincide with the solar production hours. This used to be done simply with a hot water timer – the dumb method, but now can be done using an inverters “Load Management” function – the smart method. There are other options, but we’ll touch on them in future posts. This post simply aims to cover both methods, and why, as a result of increased feed-in tariffs, Solar hot water timers, and the Fronius Hot water relay, are dead.
The Solar Hot Water Timer
With a hot water timer, installers set the timer up to come on for a set amount of hours during the period when they expect solar production to be at it’s highest, and when they expect the homeowner will be sending the most power back to the grid. The problem here is, it’s not guaranteed that period will always be the time of highest production due to rain or clouds, and the homeowner may have different usage habits on different days. If the period where the timer flicks on, is not when enough surplus power is being generated, the hot water will be heated from the grid, at the increased T11 rate. In this case, you can often lose money installing timers, hence, the dumb method. Surprisingly, many sales companies in Brisbane are still selling hot water timers as a load control. I’ll give an example later in this post of why it’s not an ideal method.
The Fronius Hot water Relay
Now for the smart method; using the Load Management Function of an inverter. The best one on the market is the Fronius Inverter’s Data manager card connected with a third party relay. For simplicity, we refer to this as the Hot water Relay, and it’s the method we use (well, used, but I’ll get to that). The way it works is when a Fronius hot water Relay and Fronius Smart Meter are integrated into a solar system, they have the opportunity to smartly decide when the hot water unit turns on. The hot water relay is able to use the information from the smart meter to identify when a system is sending a large excess back to the grid. When that excess exceeds the kW demand of a hot water unit, it tells the hot water unit to switch on. When the excess is under the kW demand of the hot water unit, it turns off and tries again later. Below is a nice example of a hot water relay in action:
The green is our solar production, and the filled in grey is our household load. The large spikes are the hot water unit turning on, and you can clearly see the hot water relay controlling the unit to only turn on when the solar production is at its highest, and when it knows it won’t need to draw from the grid. This is very beneficial on cloudy days, on days that may have morning rain, or basically for any household that has varying day by day usage habits. However, the hot water relay, like a hot water timer, still requires excess solar production equal to the full capacity of the hot water unit (meaning a 3.6kW hot water element will require the full 3.6kW of solar production all at once), and as such on rainy days, when the excess is not there, it will need to draw a portion from the grid to ensure the hot water unit receives heating just like a hot water timer.
Below is a good example of the hot water unit working well early, but then it hits a time when it can no longer wait for excess solar production, and it needs to draw from the grid. The unfilled section is bought from the grid at the highest rate ( T11, 27c/kWh) :
Who killed the Fronius hot water relay?
Using a portion of T11 power from the grid used to be fine, as it is far superior to a hot water timer, and even with this occasional draw from the grid at the increased T11 rate, they still made financial sense. A hot water timer may help a hot water unit achieve 50% coverage from solar, while a hotwater relay may achieve 70%. They both cost the same, and the hot water relay is actually more reliable. However, as good as relays are, since July 1st 2017, they rarely make financial sense.
Feed in tariffs
On July 1st, 2017, feed-in in tariffs greatly increased, in some cases from 6c per kWh to 14c per kWh, so the math behind hot water diversion becomes more complicated. This below is an example of an Excel spreadsheet calculation of a hot water relay’s payback period on a 6c Feed In Tariff – what it used to be. Most of the values we can read directly from a power bill, but the section we estimate is the percentage of the excess hot water kWh draw that will be covered by the solar + relay combo. In the below case we estimated the hot water relay will account for 70% of the heating, and the remaining 30% resulting from rainy days, higher base load etc, will need to be drawn from the grid. The calculation importantly takes the feed in tariff into account, as we have to remember the excess used to heat the hot water unit could have been sent back to the grid for 6c per kWh:
(EXCEL SPREADSHEET CALCULATIONS – 6c FEED IN TARIFF)
$34.20 better off a quarter than the off peak rate, $136.80 better off a year.
However, the calculation becomes worse the higher the feed in value becomes. Below is the exact same estimate, but calculating off a 14c Feed In Tariff instead (Origins Solar Boost rate as of July 1st 2017):
(EXCEL SPREADSHEET CALCULATIONS – 14c FEED IN TARIFF)
A whopping $2.40 a year in savings, you’d have to be a pretty good (corrupt) salesman to convince someone to proceed with this kind of payback period. And just for an extra laugh, let’s look at a timers current payback period:
(EXCEL SPREADSHEET CALCULATIONS – USING A TIMER & 14c)
I’m probably being generous anticipating a hot water timer to cover 50% of the hot water heating, but even if it does exceed expectations to say 60%, you’re nearly always guaranteed to lose money if you install a timer.
Is the Fronius hot water relay dead?
No, I’d say it’s more like semi-retirement.
(Incidentally, its role has been replaced by the Fronius Ohmpilot, but that’s for another post).
But there’s still plenty of situations where the Fronius hot water relay may be of use. For example:
- Feed in rates are not guaranteed
- Off peak rates may increase
- If you have a 10kW system, your solar may cover more than 70% of the hot water heating.
- It may not be viable to connect your hot water system to off peak.
However, it’s still important to have a chat with a solar consultant, work out your household loads and times they’ll come on, run through your bill, and work out if removing an off peak meter in favour of a relay is worth it. This is what we’re here for, and we’re constantly re-evaluating market changes to ensure when we come out to quote, we are offering the best, up to date advice.
Update 14/10/2021 – Solar Feed in Tariffs have been dropping lately and so the “Fronius Hot Water Relay” or “Ohm Pilot” is getting closer to being a smart option again. Since Feed in Tariffs go up and down all the time, it’s probably still a better choice to keep your hot water on off peak!
And remember, if anyone tries to sell you a hot water timer, run for the hills.
Ben Neville
Technical Sales Support
MC Electrical
