For households looking to get more out of rooftop solar, a hot water tank can act a bit like a low-tech battery.
That was the idea behind a question from an Australian family trying to decide whether a single-element electric hot water system would be enough.
Here's what to know
In a Reddit thread, the original poster described a home with two adults and one baby that is considering swapping gas hot water for an electric system sized at 83 gallons (315 liters) or more.
The plan was to run it during sunny midday hours to use rooftop solar, and the poster asked: "If the water is heated during the middle of the day, could I expect it to last until the next morning for hot showers?"
Responses on the r/AusRenovation subreddit suggested that for a household of that size, a single-element tank is often sufficient. One commenter who had switched from gas to a 66-gallon (250-liter) single-element system wrote, "I have kept the water stored at 65 degrees and the water was fine for the next day showers."
Commenters also said the second element is mainly useful for speeding up recovery after hot water is used, rather than improving efficiency.
"Single will be fine, dual is to increase the recovery rate," as one commenter put it. "Typically used in commercial places."
A heat pump water heater could be another upgrade, as it could improve efficiency and save money on the home's energy bills.
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More background
A common way to manage home energy use is to choose a larger tank, heat it when solar output is highest, and rely on that stored hot water later.
That does not mean a dual-element system has no upside. It can still be useful for larger families or for homes that sometimes want a faster hot-water top-up.
One commenter with a dual-element 79-gallon (300-liter) system and a my Eddie solar diverter said it "works really well." They added that after several dark days, you can switch on both elements for a faster boost.
Several commenters said a heat pump could be the better buy. One wrote, "I swapped a resistive hot water system for a heat pump and it's been amazing," while another said a heat pump is "twice as efficient as a storage tank system."
Another commenter said reducing the electricity needed for water heating can free up room for other big household loads, including EV charging and a house battery.
What can be done?
For a home with two adults and one child, the discussion suggested that a commercial-style twin-element tank may not be necessary.
If the budget allows, a smart heat pump may unlock even greater savings. Cala makes customizable smart heat pump water heaters that help homeowners decrease their energy bills by heating water exactly when it's needed.
For people who want a more tailored approach, Cala is built around the idea that smarter timing can reduce waste while keeping enough hot water on hand for when it is actually needed.
Where can I learn more?
• In Australia, hot water could rival 2 million home batteries for shifting solar power.
• You can save over $800 by switching to a heat pump.
• Many homeowners find heat pump water heaters pay off through lower operating costs.
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