Solar Hot Water: Timer, Diverter or Battery?

This is about solar electricity heating water

This guide covers solar photovoltaic panels supplying electricity to water heating. A solar thermal system, which collects heat directly, is a different installation. The timer and diverter discussion below mainly concerns a compatible electric resistance cylinder.

A gas continuous-flow heater has no electric cylinder element for a diverter to control. A hot water heat pump needs its own control assessment; do not assume a device intended for a resistance element can control the heat pump.

Before asking for prices, note the heater’s model, approximate age, storage capacity and existing controller. Give the installer a picture of the label if it is safely accessible.

What each option actually does

EECA describes timers and diverters as ways to shift water heating into solar hours. A cylinder stores heat for taps and showers; a battery stores electricity. That difference matters when deciding which need you are paying to meet.

A timer suits a predictable heating schedule

A timer gives the heater a window to operate. If clouds arrive while the element is heating, the schedule does not by itself prevent grid use. Ask the electrician how that window interacts with the thermostat and any existing controlled supply.

Take your household routine to the discussion: morning showers, evening baths, guests and any periods when you already run short. Request settings that support that routine, plus a clear explanation of how to override them when needed.

Ask for the installed price and the expected change in imported electricity, using your current tariff. A daytime setting should be assessed against the actual supply arrangement and household use, rather than treated as automatic free hot water.

A diverter responds to available surplus

A suitable diverter can vary power to a compatible element as spare generation changes. Once the cylinder reaches its thermostat setting, it cannot keep absorbing energy indefinitely. The diverter and timer model prepared for EECA explains the distinction between surplus-following heating and scheduled heating.

Ask the installer to show where surplus is measured and confirm compatibility with the cylinder, element and existing controls. Have the quote explain the boost or top-up function and what happens when the day’s solar is insufficient.

If a battery or solar-aware EV charger is also proposed, ask which gets first use of surplus power. Have the installer document the priority and demonstrate it at handover. Do not count the same spare kilowatt-hour as a saving for both the cylinder and the battery.

A battery should solve more than a timing problem

A battery can retain electricity for use after solar generation falls away. If your main aim is evening lighting, appliances or suitable outage backup, compare it against those needs. EECA’s system planning guide explains storage and the additional requirements for backup.

Ask for two forecasts using the same household demand: solar with the proposed hot-water control, and solar with that control plus a battery. The second forecast should show the additional benefit attributed to storage, along with losses and operating assumptions.

For a fair comparison, include installation, any extra control equipment and the warranty terms. Our battery or no battery guide takes you through the wider decision.

Replacing the cylinder? Compare a hot water heat pump too

EECA identifies hot water heat pumps as using less electricity than conventional resistance water heating, and they can work with solar. Read EECA’s hot water heat pump guidance. If replacement is already due, get that option priced before spending on controls for the old cylinder.

Ask which scheduling or solar controls the specific model supports, how its backup element operates and how the proposed settings affect hot-water availability. Keep the cost of replacing the heater separate from the cost of adding solar.

For a new house, bring this decision into the electrical and plumbing design. Our solar-ready home checklist covers what to agree before the walls are lined.

Keep the winter top-up and safe temperatures in the design

Ask for the forecast by month and an explanation of the cold-weather settings. A summer day with spare generation does not establish whether the same schedule will cover winter demand. For Canterbury households, include the actual property’s generation forecast and usual winter hot-water routine in the assessment.

Have the electrician or plumber retain the required heating cycle, storage temperature and delivery-temperature controls. Do not lower the cylinder thermostat to chase solar savings. MBIE’s hot-water guidance explains the need to control both legionella and scalding risks.

• What triggers a grid top-up, and when does it happen?
• How do I get extra hot water for visitors?
• What happens if the controller or its communications fail?
• How are the cylinder, battery and EV priorities set?
• What settings and operating instructions will I receive?

A useful handover includes a demonstration of the controls. Ask the installer to leave the normal settings and override procedure in writing.