Customers buying home batteries are often going for “whole home” backup on the assurance that going forward, everything will be solar powered.
People are however alarmed when bills lob in the letterbox with hundreds of dollars consumption charged out. How is that even possible?
Is Your Controlled Load Hot Water Wired In?
The answer is often controlled-load water heating, which, not everyone appreciates, is a separate supply to your house, often with its own meter (or a separate register).
Hot water can be 30% of your home’s energy consumption, and if it’s not wired in, your solar can’t power it and the monitoring app in your pocket can’t see it either.
Customers Feel Betrayed
People contact SolarQuotes for support because they’re bewildered about getting a big bill when the shiny shoe salespeople promised otherwise. It’s not unreasonable to expect when you’ve spent a lot of money on a heap of new hardware that your next power bill will be significantly cheaper.
However the confusion is perfectly understandable.
It’s not that dodgy solar companies are setting out to fleece people, but when you break the word down into three basic parts, assumed knowledge and assumed language makes an ass of u & me.
Assumption One
For an electrician it’s really basic. Hot water uses off-peak power, and of course it’s common knowledge that it’s metered separately to your daytime supply. Off-peak is cheap because the network only switches it on for a few hours, usually overnight, that’s why it’s called “controlled load” after all. Everybody knows that, right?
Assumption Two
For a customer who’s never really understood the intricacies of electricity billing and never needed to open their meter box to find the boost button, it’s also really basic. When you buy a solar power system with “whole home” backup, then it must cover the whole premises. Everything is included, right?
If you’re buying a solar power system with battery backup for blackout protection, you must document your wishes with the salespeople. Write down what you expect will work during a power outage.
I have to point out assumption one is an understandable failure. As someone immersed in the industry, it’s easy to forget the average punter just doesn’t have the base knowledge you do. However, it’s not rocket surgery to let the customer know that a resistive hot water heater is a big, heavy load you probably don’t want eating up your battery during a power outage.
Solving The Problem
The solution is simple. Have a contactor fitted to your switchboard to effectively create your own off-peak supply.
It should be outlined as a feature during the sales process and offered as an option when your solar is installed. If it wasn’t, well, I’d say it’s a little lazy on the solar retailer’s part. It’s a bit like installing a major new electrical load and not offering a switchboard upgrade to accompany it.
If they didn’t explain it to begin with, I’d be asking your installer to come back and do it for the cost of the parts. Be nice though – they’re the people you’ll need for warranty support, so you want a good relationship for the next ten years.
A tank on your house is a cheap way to soak up solar, it’s thermal energy storage, a battery you already own (This isn’t a real brand, it’s just food for thought).
Even if you didn’t opt in for “whole home” battery backup, a diversion or sunshine circuit makes great sense for solar-only households because it’s just the cheapest battery – one you already own.
There are a few different ways to get there, depending on your budget, type of hot water service and model of solar inverter.
If You Have A Heat Pump:
Relax, it likely has an onboard timer and has already been wired to run during the day. You probably won’t notice any difference because the incredible efficiency and thus low electrical demand of a heat pump means it’s just not expensive to run.
I have a heat pump running on controlled load because it’s cheaper than eating up a legacy premium feed-in tariff – interestingly, the monitoring run by Solar Analytics is smart enough to recognise the electrical signature of a heat pump, and they’ve emailed to alert me to change my behavior.
If You Have A Resistive Electric Hot Water Tank:
It probably needs 3.6kW, supplied for a few hours straight to keep it happy, and there are a few ways to get that.
Fronius have a dynamic resistive heating controller that works seamlessly with their solar, battery & EV charger ecosystem. Here the yellow is house consumption, red is water heating & grey is export to the grid after the water is hot.
You could install a timer (with a contactor) to bang the hot water on for 3 (possibly free) hours in the middle of the day.
This solar power system is a bit of a monster, but it shows a nice, square slab of yellow hot-water consumption under the grey solar curve.
Probably a better option is to use the same contactor, but control it with your solar inverter (or CatchControl) so it only comes on when the weather is sunny and you’ll otherwise be getting a pittance for exported energy.
Fronius have a dynamic resistive heating controller that works seamlessly with their solar, battery & EV charger ecosystem. Here the yellow is house consumption, red is water heating & grey is export to the grid after the water is hot.
Get a dynamic diverter to intelligently harvest every watt of surplus solar power. There are a few available but the GreenCatch device is a good example. Dynamic diverters aren’t compatible with heat pumps or some battery hybrid systems, however the most refined solution for resistive heating is the Fronius Ohmpilot, but this only works for a 3kW element and Fronius solar ecosystems.
The Best Of Both Worlds
This winter my own resistive hot water ran conventional, overnight, off-peak power for three reasons:
- Despite 15kW of solar on the roof, the yield wasn’t enough to reliably recharge my battery when the air conditioner was running it flat every night.
- It’s still cheap enough and doesn’t shorten the overall life of my battery.
- South Australia often has abundant wind energy in winter, so grid power is pretty green when we’re running a rolling average of 75% renewable per annum.
There are of course some sunny days in the low season though, so the contactor I’m using to divert solar power into the hot water tanks is actually a changeover relay.
At rest, it connects to the controlled load circuit, so the default position is to draw through and pay via the off-peak meter.
When it’s sunny, the relay changes over to daytime supply and the hot water “charges up” on solar power until the tank is hot. With some luck, the tank temperature doesn’t fall much overnight, so we don’t pay for any off-peak power because the thermostat is still open.
Conversely, if there aren’t enough sunny hours to fully heat the tank, off-peak will top it up after 11pm. This sort of control system often works best with a smaller element in the tank because there will be more hours in the day when you have enough surplus to power it.
This is an example of a solar-controlled contactor, with some labelling to keep the plumbers informed.
Sorry Queensland
There’s nothing in the AS3000 rule book against this kind of changeover device – indeed the GreenCatch diverter has dual inputs to make it work seamlessly, however the poles and wires people from Energy Queensland aren’t about to have you helping them flatten the duck curve.
Even though every other DNSP in the country ignores the technical retail regulations because soaking up daytime power helps everyone, Queensland insists that controlled loads must be hardwired to off-peak meters. If you want to switch them, you can’t have the cheap rate.
Though they were doing the same thing for EV charging, the rules have at least been updated recently, so Queenslanders don’t have to pay coal-fired prices for their own solar power.
To reiterate, Queensland is a great place for solar and you’ll do better than the deep south by simply running your hot water during the day, but you can’t top it up overnight with off-peak power.
GreenCatch is an excellent dynamic diverter, but it doesn’t always play well with battery systems. Check with your installer before buying one.
BE AWARE Of Solar Companies Promising ZERO BILLS
With high daily charges and low feed-in tariffs becoming the new normal, it’s simply not possible to guarantee a solar and battery system will deliver enough kilowatt hours to meet your needs and a heap more energy to earn export credits. It’s not impossible, but not probable either.
Even electricity enthusiasts playing with wholesale prices using Amber have noted that with the expansion of grid-scale batteries, there’s less market volatility, meaning fewer opportunities, lower price spikes and shorter windows to make profitable exports of electricity.
Don’t Use Your Battery For Hot Water
Plugging surplus solar into a thermal battery like a hot water tank makes great sense, but collecting solar power, shunting it into an expensive electrical battery, then shuffling it across into a bucket of water using a big dumb resistive jug element isn’t really smart.
When you include all the environmental, efficiency & capital costs, the energy coming from a battery is still pretty expensive. Some argue you are paying 50c each for warranted kilowatt hour delivered by a battery.
For more, read our dedicated guides to hot water heat pumps and to solar batteries.
About Anthony Bennett
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