Energy tip: Non-TOU metering and Solar Hot Water

Traditional metering (non time-of-use) often involves two meters.

Non-TOU meters bill at a stepped rate: 19.09c/kWh for the first chunk of 1750kWh per 91 days; 28.05c/kWh thereafter

The second meter bills the “Offpeak” circuit. This circuit is controlled by the energy company, charged at a lower rate, and used only for fixed installation of hot water and space heating appliances. There are two offpeak rates, and consumers can select which rate they are on.
- Offpeak 1 is currently 8.36 cents/kWh and available between 10pm and 6am
- Offpeak 2 is currently 12.1 cents/kWh and available for 16 hours per day, including 6 hours between 8pm and 7am and 4 hours between 7am and 5pm

The rate for solar hot water boost circuits are by default configured to Offpeak 2, which means they come on during the day. Mine used to come on an 8am, just when the sun starts to shine, thereby substantially negating the intent of the system, costing huge amounts of money, in return for year-round reliable hot water.

I tried changing rates, however Energy Australia will change tarrifs only once per year for free; more often costs $300. So even if Offpeak 1 was ok during the summer, I couldn’t switch to Offpeak 2 in the winter.

TOU metering, plus a timer, enables minimisation of energy used for solar hot water boost. “Offpeak” energy from a TOU meter is 8.8 c/kWh, about the same as non-TOU Offpeak (8.36). TOU “shoulder” rate is 14.96 cents, very slightly more than Offpeak 2 (12.1).

During summer, I set the timer to enable the solar boost circuit only during offpeak times. During Winter, I extend the timer to include some “shoulder” time.

Some understanding of when hot water is used, and the effect of the weather, can assist the planning process. Offpeak stops at 7am, too soon to heat water after morning showers. On a cold day, the timer could allow some heating into the shoulder period (as late as 2pm, or else after 8pm at night in time for evening showers).

While I can manage the timer manually, I can still run out of hot water if we have a few cold days in summer. This situation is recoverable quickly by bypassing the timer. But a better way is to fully automate using the WattWatchers network. This includes a weather forecast input, and some AI behavioural learning, and control over a relay instead of a timer.

Even without automation, a bit of intelligent experimenting and behavioural change can achieve significant reductions in energy. Showering at night is more likely to make use of solar energy than showering in the morning. Restricting the timer settings within the Offpeak period may also help, depending on when hot water is used.

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