Solar Hot Water vs Solar PV with Diverter NZ: 2026 Comparison

Key Takeaways

• Water heating represents approximately one-third (30%) of a typical New Zealand household electricity bill.
• Dedicated solar thermal systems cost $7,000 to $9,000 installed, require building consents, and carry no government grants.
• Solar PV paired with an electronic diverter turns an existing electric cylinder into a thermal battery.
• With retail power costing 30c+ per kWh versus export rates of 8 to 12c per kWh, a diverter pays for itself in roughly 18 months.
• Hot water heat pumps cost around $7,000 installed and cut annual cylinder running costs from $480 down to $180 according to EECA data.

The water heating dilemma in New Zealand homes

Water heating is about a third of a typical NZ household's energy bill, which makes it the single biggest thing the sun could be paying for. For decades, solar water heating meant installing solar thermal roof collectors: copper pipes circulating fluid through flat plates or evacuated glass tubes to heat water directly.

However, the dramatic price drop of photovoltaic (PV) solar panels has transformed home energy economics. Today, Kiwi homeowners face a strategic choice between traditional solar thermal, rooftop PV paired with an electronic diverter, or a standalone hot water heat pump.

Compare retail buyback pricing in our comprehensive analysis of solar-buyback-rates-by-retailer-new-zealand.

Comparing the three technologies

Traditional solar thermal collectors: pros and cons

A well-installed thermal system covers 50 to 75% of a household's hot water and lasts decades. Because hot water is around 30% of a typical power bill, a thermal system that performs well trims something like 15 to 20% off the total bill.

However, solar thermal faces three major hurdles in New Zealand:

1. High upfront capital costs: Current NZ suppliers list installed solar water heating systems between $7,000 and $9,000, depending on the site and whether the cylinder is replaced at the same time. These systems commonly pair roof collectors with a 300 litres cylinder for a family, sized at roughly 75 litres per square metre of collector.
2. No government subsidies: There are no government grants for solar water heating; the old subsidy scheme ended years ago.
3. Council consent and maintenance overhead: Because the system involves plumbing and roof penetrations, installation typically needs a building consent, which is a cost and delay PV panels usually avoid. Furthermore, glycol loops, relief valves, and sensors require ongoing plumbing maintenance.

The PV diverter: your cylinder as a thermal battery

An electronic solar diverter (such as a catch-power, eddi, or Solar-Log unit) is a small controller installed adjacent to your switchboard. It monitors generation in real time.

When the panels are making more than the house is using, the diverter sends the surplus into the cylinder's element instead of exporting it. Your ordinary cylinder becomes a thermal battery, storing the afternoon's sunshine as the evening's showers.

The economic arbitrage

Exported electricity earns a buy-back rate of typically 8 to 12c per kWh, while the grid power you avoid buying costs 30c or more. Every diverted kWh is worth the difference, which is why a diverter on a typical household pays for itself in roughly 18 months.

Crucially, for PV with a diverter, no new cylinder is required: the diverter feeds your existing electric cylinder's element. See overall system costs in our guide to solar-panel-costs-new-zealand.

When does a hot water heat pump make sense?

If your existing hot water cylinder is near the end of its operational life, a heat pump cylinder is a compelling alternative. Hot water cylinders last about 15 years, and EECA reports that nearly half of NZ homes with electric cylinders are running one more than 10 years old.

EECA's figures put a traditional electric cylinder at about $480 a year to run and a hot water heat pump at about $180, from an installed cost of around $7,000. Running a heat pump during peak daylight solar hours delivers ultra-efficient heating with minimal power draw.

Calculate household self-consumption sizing strategies in solar-system-sizing-self-consumption-new-zealand.