Learn about solar technology

If they’re well maintained, you can expect solar panels to last 25–30 years, with only a small decrease in output of around 0.5% per year.

The ‘size’ of your solar system refers to the total power-generating capacity of your panels, measured in kilowatts (kW). 

The system size depends on both:

• the number of solar panels
• rated capacity of the panels. 

For example, a typical home solar system might include 13 x 400-watt (capacity) panels, so the system size would be 5,200 watts or 5.2 kW.

The size of your inverter is either equal or slightly less than the total capacity of the panels. If you’re also looking at a battery, the storage capacity should be considered in relation to the size of your system.

• A string inverter is the most common and cheapest type. It's best when your panels will get full sun, and goes on the wall of your home.  
• Microinverters are more expensive, but a good option if shade will hit some of your panels. Microinverters go on your roof, one for each solar panel.

You’ll likely need to replace the inverter after about 10–15 years, depending on its quality.

Having around 20% more solar panel capacity (kW) than inverter capacity tends to produce the best value for most New Zealand homes.¹ This is known as 'overpaneling’ and works because panels are rarely generating power at their full capacity. 

With overpaneling, you’ll lose a small amount of potential power output when the inverter reaches its maximum (on peak sunny days in summer), but your solar system will generate much more output over the course of the year with only a minor additional install cost.    

As there are different kinds of inverters, a range of sizes and varying prices, it’s wise to ask a solar expert for guidance.

A solar battery should last 10 years or more, depending on the battery type and quality, plus how intensively it’s used.  

A battery should still be operating near its full capacity after 10 years, and may not need to be completely replaced at this point. In most cases, additional capacity can be added to the system by either adding more solar panels, or a supplementary battery.  

Batteries only need replacing if they are no longer in working order.  

Most home batteries are lithium-ion – a high performing, low maintenance option that has a good lifespan. There are two common sub-types of lithium-ion battery - nickel manganese cobalt (NMC) and lithium iron phosphate (LFP). Compared to the NMC type, LFP batteries are sightly bulkier, but offer better thermal stability, and so are often favoured for home installations.  

Lead-acid batteries have been used for many years, but are much bulkier, have lower efficiency, and a shorter lifespan, and so are unlikely to be recommended  today.  

Some batteries are separate from the inverter unit, and some have the inverter built into the battery. It’s good to discuss with your installer what suits you best, as it may affect your costs if you decide to add a battery later. 

Work is underway to introduce vehicle-to-grid and vehicle-to home technology, which will enable you to use your electric vehicle as flexible storage - drawing power for your house when needed, or even selling back to the grid. A small number of vehicles and chargers are capable of this now, however it is not widespread.  

Key things that we recommend you look for in a battery are: 

• Efficiency – At least 85% (meaning you get around 85% of the energy out, compared to what goes in when it is charged).  
• Degradation – A battery should still have at least 60% of its usable capacity after 3,000 charge-discharge cycles. This should be explained in the manufacturer’s warranty.  
• Measurement – Some batteries are able to ‘report’ on their state of charge, useable capacity, and operating mode. 
• Product response – A useful ability for a battery is for the owner to be able to set minimum and maximum state of charge, and operating mode. 

In New Zealand, there are rules around where you can install a home battery, including: 

• The type of space they are installed in – either outdoors or in non-habitable spaces (like a garage or utility room).  
• The distance from exits, windows, ventilation openings, gas equipment and potential vehicle impact.  
• Sufficient fire-resistant protection. 

To make sure you have yours installed safely, make sure you work with an experienced installer. Solar can be installed by solar providers and registered electricians. 

More on finding installers

If you do get a battery, you should make sure that you are on a ‘time of use’ plan with your power company. This means you can use your own energy when power is expensive (in the mornings and evenings) and use grid power to top up when it’s cheap.

Usually, your battery management system will be configured with information on your pricing plan. If you do change plans, you may need to reconfigure your battery to get the most benefit from this. Your installer can advise you on how to get this done.  

Find out more about optimising your system

1. Understanding the value of residential solar in NZ | EECA(external link)