How should a hot water cylinder run on solar?

For a lot of New Zealand houses the electric hot water cylinder is the largest load you can move into the day without buying a battery. Solar energy for your home is worth the most when it runs something you would otherwise buy from the retailer. A storage cylinder is a tank of heat. If you heat it while the panels are producing, you use solar power for your home instead of exporting that energy at a lower buy-back rate.

This is a controls and tariff question, not a reason to rewire the cylinder yourself. Element cylinders, heat-pump water heaters and gas or wetback systems behave differently. A licensed electrician should change any supply, timer or relay. The solar installer should say whether their design assumes the cylinder can run at midday.

Why the cylinder beats a slightly better buy-back rate

You pay more to import a kilowatt-hour than a retailer pays you to export one. Households often see a gap of roughly three to four times, depending on the plan. A unit of electricity that heats water at noon avoids the import price. The same unit exported earns the buy-back. Moving the cylinder is usually worth more than shopping for a buy-back rate a cent or two higher.

EECA's public savings figures, about $1,000 or more a year for a typical home and about $1,246 in their Auckland example for a 5 kW system, assume you use a decent share of the generation in the house. A cylinder left on a night schedule will not produce those figures by itself. The buy-back guide is the rate side of the same point: https://homesolarinstallation.co.nz/blog/solar-buyback-rates-nz/

Check current plans before you lock a story to one rate. Powerswitch lists them: https://www.powerswitch.org.nz/solar-rates

Ripple control and timers

Many New Zealand cylinders are already on a controlled or night rate. The lines company can switch that circuit with ripple control. A timer you add has to work with that relay, not fight it. If the network turns the cylinder off during the day, a hopeful timer will do nothing.

Ask the electrician what your cylinder circuit actually is. Some houses have a continuous supply and can take a simple daytime timer. Some are on a controlled supply that is only available in set windows. Some have both an element and a booster. The right change depends on which of those you have. Guessing from the switch colour is not a design.

A solar diverter or a relay that heats water only when the inverter sees surplus is another option. It can mop up export that would otherwise leave the house. It is extra hardware and extra setup. Ask what it costs, which cylinders it suits, and what happens on a dull day when you still need hot water by evening. You may still want a backup boost.

Element cylinder, heat-pump cylinder, gas

A standard electric element cylinder is the straightforward case: a few kilowatts for a few hours, stored as hot water. Heating it late morning or early afternoon matches a north-facing array better than heating it at 1am, if the supply allows it.

A heat-pump water heater uses fewer kilowatt-hours for the same shower because it moves heat rather than making it with an element. That is good for the bill. It also means solar has less cylinder load to absorb. Do not size an array on the assumption of an old element if you are about to replace it with a heat pump. Tell the designer which cylinder you will have.

Gas, instant gas, and a wetback on a wood burner do not move onto the solar electric supply. Solar will not cut a gas water bill. If you are comparing solar with a shift to electric hot water, price both the cylinder change and the array. The electrical work for a new cylinder circuit belongs with a licensed electrician. Tromar Electrical is one option for that scope: https://tromarelectrical.co.nz/

How much load is realistic

A typical household cylinder element is often around 3 kW. Runtime depends on how much hot water you use and how cold the inlet is. In winter the inlet is colder and showers are longer, so the cylinder wants more energy in the season when the array produces less. A summer timer setting can be too optimistic in July.

You do not need a perfect model to have a useful quote conversation. Say how many people live in the house, whether you are home at midday, and whether the cylinder is electric. Ask the installer to show the cylinder as a daytime load in the self-consumption estimate, or to say they left it out.

If the export limit on your street is tight, daytime water heating is one of the cleanest ways to use surplus before the inverter clips. The export guide explains clipping: https://homesolarinstallation.co.nz/blog/solar-export-limits-nz/

Safety and who does the work

Do not move wires, bridges or thermostats because a video showed it. Cylinder thermostats are a scald and a compliance issue. The element circuit is prescribed electrical work. The person who changes it should certificate what they changed.

Legionella control still applies. Water stored too cool for too long is a health issue. A timer that never lets the cylinder reach a safe temperature is a bad timer. Ask the electrician and, if you have a plumber on the cylinder, the plumber, what minimum temperature they want and how often a boost should run.

If the solar install and the cylinder change are different visits, write down who is responsible for the final switching arrangement. A solar array commissioned while the cylinder is still locked to midnight will not show the saving the quote described.

What to ask for in the design

Keep the cylinder in the same conversation as the kilowatts.

• Is the cylinder electric, heat pump, gas, or wetback, and can its supply run in the day?
• Is ripple control going to block a midday timer?
• Will a diverter or relay be included, and what does it cost?
• What self-consumption figure did you use with the cylinder on a day schedule, and without it?
• Who certificates the cylinder circuit if it is altered?

Houses across the north of Auckland

Electric cylinders on solar are a house-by-house setup in Algies Bay, Arkles Bay, Army Bay, Big Omaha, Coatesville, Dairy Flat, Dome Forest, Dome Valley, Glorit, Gulf Harbour, Hatfields Beach, Helensville, Huapai, Kaukapakapa, Kawau Island, Kumeu, Leigh, Matakana, Muriwai, Omaha, Orewa, Puhoi, Riverhead, Snells Beach, South Head, Waimauku, Warkworth, Wellsford, Bayswater, Belmont, Campbells Bay, Castor Bay, Devonport, Forrest Hill, Hauraki, Milford, Narrow Neck, Stanley Point, Sunnynook and Takapuna. A full-time home in Takapuna, Milford, Forrest Hill or Sunnynook with people in the kitchen at noon is a different hot-water pattern from a bach at Leigh, Matakana, Omaha or on Kawau Island that is empty midweek. Rural supplies around Wellsford, Glorit and Dome Valley may have different controlled-load hours from a North Shore street. Ask about your circuit.

Home Solar Installation NZ can introduce a local partner for a solar quote that includes how you heat water. The form is free and there is no obligation. EECA's savings context, which depends on using solar in the house, is here: https://www.eeca.govt.nz/for-homes/solar-for-homes/solar-costs-and-savings/