How Can You Add Batteries To An Existing Solar System?

Battery storage capable solar PV systems require special wiring, and some require a special inverter to interface with the batteries properly.

Inverters that can work with batteries normally cost more and have only been on the market for the last few years – so chances are very high that many people considering adding batteries to their solar power system do not have one. However, some batteries can be added to a solar power system without requiring an additional inverter.

Most people (who use average/above-average levels of electricity in their homes) will also need to strongly consider upgrading their current solar power system size if it’s less than 5kW.  We recommend having much more than this, but 5kW is around the minimum required to produce enough power on overcast days or in the winter to at least partially charge the battery.

Usually, the best option is to install a completely new solar system rather than expand an old one.  It can replace the one that’s already there or — provided there’s room — can be installed as a second, separate system.  But here we’ll just look at bringing an existing rooftop solar system up to 5kW.

Let’s use the example of Gary, who bought a 3kW solar system some years ago, uses the typical amount of electricity for an average Aussie home (about 16kWh/day, for 35c/kWh) and wants to add a Tesla Powerwall to his home. Gary will require the following:

  • 6 x 335W panels (to bring his system size up to 5kW): ~$1,000 + installation
  • Upgraded inverter: ~$1,500 + installation
  • Tesla Powerwall battery + gateway: $12,100 (as of early November 2023) + installation 

Total cost: $14,600 + installation

Gary balks at this cost – he only pays $250/quarter for his electricity at the moment, with his 3kW system offsetting half of his electricity usage. Even if adding a Powerwall completely wiped out electricity charges on his bill, $1,000 worth of savings per year means that even after ten years, he’s only saved $10,000 – the battery upgrade won’t pay for itself for many more years!

The worst part is – the Powerwall only has a warranty of ten years (as do most other solar battery manufacturers). So it might reach the end of its lifespan before he’s even recouped his investment.

In conclusion? It’s expensive, but you can add batteries to an existing solar system. Don’t expect it to make financial sense unless you are on a time-of-use tariff or until solar battery prices drop significantly. Until then, the best bang for buck for most people is a roof full of solar.

>> Next: “Battery ready” solar systems, and how to buy one >>

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