A solar diverter is a controller that watches the electricity flowing between your home and the grid and, whenever it detects export, sends that surplus into the heating element of an electric storage hot-water tank. It varies the element's power continuously, so the tank absorbs roughly what the panels would otherwise export while drawing little or nothing from the grid. The result is surplus solar stored as hot water, which is worth considering when feed-in tariffs pay only a few cents per kWh.

Measuring the surplus

A diverter needs to know, moment by moment, whether power is flowing into or out of the home. It gets this from a current transformer (CT), a clamp fitted around the main grid cable in the switchboard, together with a voltage reference. From the size of the current and its timing relative to the voltage waveform, the controller calculates both the amount of power and its direction.

The measurement repeats many times a second. When the kettle switches on, the export figure drops immediately and the diverter reduces the element power to match. When a cloud passes, the same thing happens. CT placement is critical: the clamp must sit where it sees the whole home's net flow, the same point a smart meter measures, as described in smart meters and solar. A clamp on the wrong cable leads to heating water from grid power or to missed surplus.

Proportional control of a fixed element

A typical storage tank element is a simple resistor, commonly rated somewhere between about 2.4 and 3.6 kW. Left alone, it is either fully on or fully off. A diverter makes it behave like a variable load using solid-state switching:

• Burst-fire control switches whole mains cycles on and off at the point where the voltage crosses zero. Passing one cycle in three, for example, delivers about a third of the element's power on average.
• Phase-angle control switches part-way through each cycle. It gives smoother control but creates more electrical noise, so many diverters favour burst-fire methods designed to stay within interference and flicker limits.

Because energy meters add up energy over time, the rapid on-off switching averages out, and the meter sees a steady small load that cancels the export. The table shows the effect with a 3.6 kW element.

Once the water reaches its thermostat setting, the element stops drawing power regardless of the diverter, and export resumes.

How much water can surplus solar heat?

Heating water takes about 1.16 watt-hours per litre for each degree Celsius. Raising a 250-litre tank by 40°C therefore needs roughly 11.6 kWh. For context, a 6.6 kW system in Sydney typically produces roughly 24-27 kWh a day on average, so a full reheat can use a large share of a day's output before household use is counted. Tank losses and how much hot water the household draws change the real figure, and on cloudy or short winter days the surplus may fall well short.

A diverter therefore works alongside the tank's normal temperature controls rather than replacing them. Australian plumbing requirements call for stored hot water to be heated to at least 60°C, so systems keep a backup: a timed boost at a cheaper time, the existing controlled load circuit, or a controller that tops up only if the water has not reached temperature by late afternoon.

Diverters compared with timers and heat pumps

When a diverter makes sense

The value of a diverted kWh is the difference between what it would earn as export and what it would otherwise cost to heat the same water. That leads to a few useful checks:

1. You export a lot around midday. A diverter needs regular surplus to earn its keep.
2. Your feed-in tariff is low. With NSW feed-in rates commonly a few cents per kWh, each exported kWh earns little. The reasons are explained in understanding feed-in tariffs.
3. Your tank is in good condition. If it is near the end of its life, a heat pump may be the better replacement.
4. You know what the tank costs to run now. If it heats on general usage rates, commonly 30-45 c/kWh, diverted solar displaces expensive energy. If it already runs on a cheaper controlled load tariff, the saving per kWh is smaller.
5. Surplus priorities are clear. In homes with a battery, decide deliberately whether the battery or the tank fills first.

Moving an element off a controlled load circuit and fitting a CT inside a switchboard is electrical work, so it must be carried out by a licensed electrician.

Next steps

A Smart Solar Diverter is listed in the Blue Energy Solar market from $890 supplied and installed, and Heat-Pump Hot Water from $3,490 for homes where the tank is due for replacement. Both prices are indicative and confirmed after a site assessment. To compare the two against your own export pattern, request a free assessment from Blue Energy Solar.

Frequently asked questions

Can a diverter send surplus solar to loads other than hot water?

Proportional control only suits purely resistive loads, such as heating elements, some panel heaters and heated towel rails. Some diverters include extra outputs that switch to a second resistive load once the tank is hot. Motors, pumps and appliances with electronics should not be fed a chopped supply; instead they can be switched fully on by a relay or smart controller when enough surplus is available.

Does a diverter work with a gas or solar thermal water heater?

A diverter needs an electric element to heat, so it cannot work with a gas-only water heater. Solar thermal systems with an electric boost element can use one, but the rooftop collectors already heat the water on sunny days, leaving less for a diverter to do. In those homes the main benefit is replacing some grid-powered boosting on marginal days.

How well does a diverter perform in winter?

Output is lower in winter because days are shorter and the sun sits lower, while colder incoming water needs more energy to reach temperature. A diverter will usually still contribute on clear days, but a larger share of heating comes from the backup boost. Reviewing the boost timer each season helps avoid the element running from the grid just before surplus solar arrives.