How to time your hot water system around solar in Perth

If your Perth home has solar and an electric hot water system, move the timer from its overnight setting to run between 9am and 3pm instead. That's when your panels are producing the most, and using that power to heat water is worth more than exporting it. For most households, the shift is worth roughly $340 to $570 a year, depending on how much hot water you use, and it takes about ten minutes with a screwdriver.

Most Perth homes with electric hot water are still set up the old way: heat overnight, cheap and quiet while everyone sleeps. That made sense once. It stopped making sense the day solar panels went on the roof.

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Why was hot water set to heat overnight in the first place?

Before solar, Synergy ran a legacy "off-peak" hot water program: a lower rate for power drawn overnight, so households heated water cheaply while demand on the grid was low. Plenty of homes signed up, and the timer has stayed on that setting ever since.

That program is largely gone. Synergy's current residential tariffs, A1 and Midday Saver, don't carry a specific hot-water off-peak rate for most customers. If your hot water is still billed on an old dedicated hot-water tariff, it's worth checking directly with Synergy, but many households are simply on standard A1 with a timer left running on overnight habit rather than any pricing advantage.

On the A1 tariff (33.2621c/kWh flat), overnight power costs exactly the same per unit as daytime power from the grid. There's no rate-based reason left to heat water at 2am instead of 2pm.

How much can shifting your hot water timer actually save you?

Here's the mechanism. Once your solar system generates more than your house is using at that moment, the surplus goes one of two places:

• Exported to the grid at 2c/kWh (most solar-hours surplus, including the whole 9am to 3pm window, falls under DEBS' off-peak export rate)
• Used to heat your hot water, avoiding 33.2621c/kWh of grid import you'd otherwise pay

The gap between those two, 31.26c/kWh, is what you're leaving on the table every time solar-hours surplus gets exported instead of used.

Worked example, for a 250L electric storage system:

1. Hot water heating typically runs 3 to 5kWh/day (it varies with household size, thermostat setting, and incoming cold-water temperature)
2. At 4kWh/day, grid-heated hot water costs 4kWh x 33.2621c = $1.33/day, or about $486/year
3. Shift that same 4kWh to solar hours: you lose 4kWh x 2c = $0.08/day of export income, but avoid the $1.33/day of import
4. Net saving: $1.25/day, or about $456/year, for a household shifting 4kWh/day from grid to solar

(This assumes the shifted kWh would otherwise have been exported. Use less hot water than that, and the saving scales down with it: a 3kWh/day household saves closer to $340/year, a 5kWh/day household closer to $570/year.)

What's the best time of day to run your hot water?

Perth's solar generation peaks at roughly 10am to 2pm. In practice, the timer window to aim for is:

9am to 3pm. That's DEBS' off-peak export window too, so any solar surplus you don't use for hot water is only earning 2c/kWh anyway. Self-consuming it via the hot water element is worth more than letting it flow to the grid.

A well-insulated hot water tank holds its heat far better than most people expect. Water heated mid-morning is generally still hot enough for a shower that evening, and the small amount of heat a tank loses overnight makes no real difference at the tap.

If your system has a boost element: some heat pump hot water systems top up with resistive heating when demand outpaces what the heat pump alone has provided. If that's your setup, see our heat pump hot water guide for how scheduling differs from a standard electric storage system.

How do you change your hot water timer?

The mechanism depends on what's controlling your system:

1. Mechanical timer on the unit. Most older electric storage systems (Rheem, Vulcan, Dux, Rinnai electric storage) have a wheel or slider timer, either on the unit itself or in a nearby timer box. Find the "on" pegs, move them to cover roughly 9am to 3pm, and turn off the overnight segment.
2. Digital timer switch (Legrand, HPM). Some homes have a separate digital timer between the meter box and the hot water unit, showing the current time and programmed windows. Reset it to the same 9am to 3pm window using the timer's own manual.
3. Smart hot water controller (Swell Energy, Reposit, and similar). These are usually added after solar goes in, specifically to automate this kind of scheduling. Check the app: it may already be running solar-first, or it may need reconfiguring.
4. Heat pump hot water. Reclaim Energy, Sanden, Rheem AMBIHEAT and Stiebel Eltron systems have their own scheduling logic, and the ideal window depends on the specific unit. Check the manual for a day/night mode switch or digital timer, and see our heat pump hot water brand comparison if you're weighing up options.
5. Solar diverter (Catch Power, SolarEdge Home Hot Water Controller, myenergi EDDI). If you already have one of these installed, it's handling this automatically: it watches for solar surplus in real time and redirects it to the hot water element. No timer change needed. If you don't have one yet, our solar diverter guide covers how they compare to simply adjusting a timer.

Should you boost the temperature for legionella protection?

Legionella bacteria stop multiplying above 60°C, which is why electric storage hot water systems are set to at least 60°C as standard (some run at 65°C for extra margin).

If your system heats once a day, during the solar window, and the thermostat stays at 60°C or above, legionella risk stays well-controlled. The tank holds that temperature through the rest of the day and overnight.

Don't drop the setpoint below 60°C to save power. That's a genuine legionella risk, not a modest one. Keep the setpoint at 60 to 65°C; the only thing that should change is when the system heats.

What if your hot water demand is bigger than solar can cover?

A big household, several showers back to back in the evening, or guests staying over can all push demand past what the solar window heated. When that happens, the system tops up from the grid. That's normal, and it's exactly what it would have done anyway on the old overnight schedule.

A few ways to reduce how often that happens:

• A bigger tank. A 315L tank holds more pre-heated water than a 250L, so there's more buffer before you run cold in the evening.
• A wider timer window. Stretching to 9am to 3pm captures more solar generation than a tighter 10am to 2pm window.
• A small evening boost. A short second heating period, say 6pm to 7pm, as a fallback. It draws from the grid, but only fires if the tank has already cooled down significantly, not every night.

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Shifting your hot water timer from overnight to 9am to 3pm is one of the highest-value free changes a Perth solar owner can make, worth roughly $340 to $570 a year depending on how much hot water your household uses, for about ten minutes with a screwdriver. If your installer never mentioned it, do it yourself: find the timer, whichever type it is, move the "on" window into solar hours, and leave the overnight segment off.