Plan your solar system

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.

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

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.

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. 

You’ll lose a small amount of potential power output when the inverter reaches its maximum (on sunny days in summer), but your solar system will generate much more output over the course of the year with only a minor additional 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 only needs replacing if it no longer delivers what you need from it.

Adding a battery to your system allows you to store the solar energy your system produces, so you can use it later. This can reduce or even eliminate your power bills.   

Typical capacities are roughly 5 to 10 kWh, 10 to 15 kWh, or 20+ kWh. A small solar system won’t fully utilise a large battery, so avoid getting a bigger one than you need. 

Starting without a battery and adding one later will keep your initial installation costs down while giving you time to see the impact of solar on your power bills without a battery. 

You will need to check with a solar installer whether your system is compatible with a new battery.

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 (at night) and use grid power to top up when it’s cheap.

Find out more about optimising your system

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