How to time your hot water system around solar in Perth

Many Perth homes have electric hot water systems set to heat on a timer — often overnight, a legacy of pre-solar "off-peak" electricity programs. Once you have solar installed, heating water overnight from the grid at 33.26c/kWh when you could be heating it from midday solar at effectively 33.26c avoided (vs 10c export) is leaving money on the table.

Shifting your hot water timer to solar hours is typically the single most impactful free change you can make after solar installation.

Why overnight hot water heating made sense before solar

Before solar, Synergy's legacy "off-peak" hot water program charged a lower rate for power drawn during off-peak hours (usually overnight). Homeowners signed up for this program to heat hot water cheaply at night.

This program has changed. Synergy's current residential tariffs (A1 and Midday Saver) do not have a specific "hot water off-peak rate" for most residential customers. If you are still on a legacy hot water tariff arrangement, check with Synergy — many households are now on standard A1 and their hot water timer is simply set to overnight from historical inertia.

On the current A1 tariff (33.2621c/kWh flat rate), overnight hot water costs the same per unit as daytime hot water from the grid. There is no tariff-rate reason to heat at night rather than at midday.

The solar hot water calculation

When your solar system is generating surplus power (generation > household consumption), that surplus either:

• Exports at 10c/kWh (via DEBS to the grid), OR
• Runs your hot water (avoiding 33.26c/kWh grid import you would otherwise pay)

The difference: 23.26c/kWh for every kWh of hot water heating shifted from grid-import to solar-surplus.

For a typical Perth home with a 250L electric storage hot water system:

• Hot water heating: approximately 3–5kWh/day (varies by household size, temperature setting, incoming cold water temperature)
• At Perth's 2026 tariff, 4kWh/day of hot water heating costs: 4kWh × $0.3326 = $1.33/day = $485/year from the grid
• Shifted to solar surplus: the 4kWh no longer exports at 10c (saves $0.40/day in lost export), saves $1.33/day in avoided import. Net saving vs grid: $1.33 - $0.40 = $0.93/day = $340/year

(Assumes the 4kWh would otherwise have been surplus and exported. If your solar surplus is less than 4kWh, the saving is proportionally less.)

When to heat: the optimal solar window in Perth

Perth's solar generation peaks between approximately 10am and 2pm. The practical timer window for hot water is:

Recommended timer: 10am–2pm (or 9am–3pm to capture more of the DEBS Super Off-Peak window)

Hot water storage systems maintain temperature for 18–24 hours in an insulated tank. Water heated at 11am will remain hot for use at 6pm. A 250L tank with R3.5+ insulation loses approximately 2–4°C overnight — insignificant for most household shower temperatures.

Important for households with a thermostat boost: Some modern heat pump hot water systems have a boost element that runs when demand exceeds what the heat pump has pre-heated. If you have heat pump hot water, see the heat pump solar guide for different scheduling considerations.

How to change your hot water timer

Type 1: Mechanical timer on the hot water unit Most older electric storage hot water systems (Rheem, Vulcan, Dux, Rinnai electric storage) have a mechanical timer — a wheel or slider on the unit itself or on a dedicated timer box near the unit.

To change: locate the timer, usually labelled with 24 hours or 12-hour AM/PM segments. Find the current "on" pegs or segments and move them to approximately 10am–2pm (or 9am–3pm). Turn off the overnight segment. Confirm the "on" segment covers mid-morning to mid-afternoon.

Type 2: Digital timer (Legrand, HPM brand timers) Some Perth homes have a separate digital timer switch installed between the meter box and the hot water unit. These display the current time and programmed on/off times. Reset the on/off window to 10am–2pm. Refer to the specific timer's manual for programming steps.

Type 3: Smart hot water controller (Swell Energy, Reposit, etc.) If you have a smart hot water controller installed (typically added post-solar to automate scheduling), consult the controller's app. These systems may already optimise timing but may need to be reconfigured for solar-first mode.

Type 4: Heat pump hot water (different approach) Heat pump hot water systems (Reclaim Energy, Sanden, Rheem AMBIHEAT, Stiebel Eltron, etc.) have different scheduling considerations. Heat pump mode runs during the day optimally, but the scheduling window depends on the specific unit. See your heat pump's manual for timer setup — most have a day/night mode switch or digital timer.

Type 5: Solar diverter (Catch Power, SolarEdge Immersion, myenergi EDDI) If you have a solar diverter installed, it already handles this automatically — it monitors solar surplus and diverts it to the hot water element in real time. You do not need to change the timer; the diverter does the scheduling.

What about temperature: should you boost temperature for legionella protection?

Legionella bacteria cannot survive above 60°C. Australian standards (AS/NZS 3500) recommend electric storage hot water be set to 60°C (some systems set to 65°C as margin). At 60°C, the tank is safe.

If your hot water system only heats once per day (during the solar window), and the thermostat is set to 60°C, legionella risk is well-controlled. The tank will hold temperature for the remainder of the day and through the night.

Do not reduce setpoint below 60°C to "save energy" — this creates genuine legionella risk. The setpoint should stay at 60–65°C; the only change is the time of heating.

What if my hot water demand exceeds what solar can cover?

If your household uses more hot water than the solar window can heat (e.g., large family with multiple showers in the evening, or guests), the hot water system will draw from grid to maintain temperature. This is normal and expected.

Options to ensure adequate hot water with solar timing:

• Larger tank: A 315L tank stores more pre-heated water than a 250L, reducing the risk of running cold in evening
• Extend the timer window: Widen to 9am–3pm to capture more solar generation
• Add a boost period: Set a secondary evening boost (6pm–7pm) as a fallback; the evening boost draws from grid but only activates if the tank has cooled significantly

Shifting your electric hot water timer from overnight to 10am–2pm is one of the highest-value free actions a Perth solar owner can take after installation. The saving is approximately $300–$400/year for most Perth households — achieved in 10 minutes of timer adjustment. If your installer hasn't mentioned it, do it yourself: locate the timer (mechanical, digital, or app-based), move the on-segment to solar hours, and leave the off-period at night.