Solar Battery Storage NZ: 10.35kW Solar + Tesla Powerwall 3, Arrowtown

Project Overview

Kate Giles was relocating from the North Island to a new, fully electric home in Arrowtown, and she knew the power bills were going to be high the moment she moved in. Everything in the house runs on electricity aside from the log burner: underfloor heating, electric hot water, and an EV she drives around the Basin. Future Energy paired a 10.35kW Suntech solar array with a Tesla Powerwall 3, sized specifically to offset the underfloor heating running overnight through Arrowtown’s cold winters, and added a hot water timer to shift heating into daylight hours. The result is an estimated $4,272 in first year savings and full home backup if the grid ever goes down.

The Challenge

Arrowtown winters are cold, and Kate wanted her underfloor heating running around the clock through them, on top of electric hot water and charging an EV at home, in a fully electric property with no gas connection. Moving from the North Island, she knew the power bill without solar would be substantial. The catch was timing: underfloor heating draws heavily overnight and into the evening, long after the sun has gone down, so panels on their own would never cover it. The battery had to be sized around that overnight draw specifically, not just around daytime consumption.

There was a strong north facing roof to work with, which made the panel side of the decision straightforward. As Future Energy’s advice put it at the time: let’s fill it with panels. The harder part was making sure the battery and the load timing worked together so the underfloor heating stayed on through the night without leaning on the grid.

The System

23 x Suntech 450W Panels – 10.35kW Total

Twenty three Suntech STP450S-H48-Nth+ panels were installed across the north facing roof, split into two strings: twenty panels at 340 degrees and three at 339 degrees, both at a 30 degree tilt, for a combined 10.35kW array. Modelled to generate 37.4kWh a day against a household consumption of 30kWh a day, the system produces a genuine daily surplus even through an all electric, underfloor heated home. Every panel carries a 25 year warranty.

Tesla Powerwall 3 – 13.5kWh Usable with Full Home Backup

The Tesla Powerwall 3 pairs a 10kW inverter with 13.5kWh of usable battery storage, sized to carry the home’s overnight underfloor heating draw without falling back on the grid. With the Tesla Gateway installed, the system also delivers full home backup in a power outage, an important factor for a household this dependent on electricity for heating, hot water and EV charging. In year one the battery is modelled at 248 discharge cycles, backed by a 10 year battery warranty and a 10 year inverter warranty.

Hot Water Timer – Shifting Load Into Daylight Hours

Alongside the solar and battery system, Future Energy installed a hot water timer so the cylinder heats during daylight hours when the panels are producing rather than drawing from the battery or grid overnight. It is a small addition that works alongside the underfloor heating strategy, keeping more of the home’s most predictable loads running on free daytime solar.

The Results

• 10.35kW solar array across 23 Suntech 450W panels on a north facing roof
• 13.5kWh usable battery storage via Tesla Powerwall 3, with full home backup during outages
• 37.4kWh of clean electricity generated every day against 30kWh of daily consumption
• 248 battery discharge cycles modelled in year one
• $4,272 estimated first year savings on a current annual bill of around $4,910
• Annual electricity bill reduced to an estimated $637
• An estimated 1.7 tonnes of CO2 offset per year
• 25 year panel warranty, 10 year battery warranty, 10 year inverter warranty, 10 year installation warranty

All electric home. Underfloor heating covered. Power bill cut by thousands.

Frequently Asked Questions

How do you size solar and battery storage for a home with underfloor heating?

Underfloor heating draws power overnight and into the evening, well outside the hours the panels are producing, so the battery has to be sized around that load specifically rather than just around general household consumption. For Kate’s Arrowtown home, that meant pairing a 10.35kW solar array with a 13.5kWh Tesla Powerwall 3, big enough to carry the underfloor heating through the night without drawing from the grid, even through a Central Otago winter.

How does the Tesla Powerwall 3 keep the lights on during a power outage?

The Powerwall 3 installed at this Arrowtown property includes the Tesla Gateway, which enables full home backup. If the grid goes down, the system switches to battery power automatically, keeping the home running rather than just backing up a handful of circuits. For a fully electric household with underfloor heating, electric hot water and an EV, that resilience was one of the deciding factors in choosing Powerwall 3 over a smaller or partial backup system.

Is Tesla Powerwall 3 a good fit for an all electric, EV charging household?

Yes, particularly where a home has no gas connection and relies on electricity for heating, hot water and vehicle charging. The 13.5kWh of usable storage in a Powerwall 3 gives a genuinely electric dependent household like this one enough capacity to cover overnight loads such as underfloor heating, while the 10kW inverter is sized to handle whole home demand rather than just topping up a few appliances.

What does a hot water timer do, and why pair it with solar?

A hot water timer schedules the cylinder to heat during the hours solar panels are producing, rather than whenever hot water happens to be used. Paired with solar, it means more of the home’s hot water is heated on free daytime generation instead of drawing from the battery or grid overnight, complementing a battery sized around loads like underfloor heating that genuinely need to run after dark.

Considering solar battery storage NZ for an all electric home? Future Energy designs and installs residential solar and battery systems across New Zealand, sized around how you actually use power, from underfloor heating to EV charging. Talk to us about a system sized for your home, your roof, and your energy goals.