Home Solar + Battery Payback Under 10 Years?Easily Doable!

Investing in a residential home battery and solar system is generally considered feasible if the payback falls within the battery warranty period – typically 10 years. According to Josh, that’s easily doable.

Josh Mylne is, among other things, a dad, a scientist, and an energy nerd. He’s extracted an obscene amount of data from his various apps and home energy bills and reckons his solar power, battery, heat-pump and induction cooking set-up is on its way to paying for itself in less than 10 years. Way less.

Josh lives in WA. Around 2005 Josh and his family moved from the UK back to Australia. They were skint, so due to budgetary constraints, out came the Excel spreadsheet to guide their finances.

They continued being a one-car family and used bicycles as much as practical. Fast forward to the COVID-19 pandemic and the decarbonisation journey began in earnest.

On Go The Solar Panels

In November 2020, they had rooftop solar power installed. The system consisted of 19 x 350W solar panels (6.65kW total), and a 5kw hybrid inverter.

After the solar panels were installed, the electricity bills dropped by about $950 p.a. But that wasn’t enough for Josh. He’s not a big fan of supply charges or the miserly feed-in tariff offered by Synergy in WA. He became a net exporter of electricity, but that didn’t reflect in power bills due to the difference in import tariff (30c per kWh) and feed-in tariff (2.5c to 10c  per kWh), and the aforementioned supply charge ($1.07/day). More work to do.

See Josh’s electricity usage graph below compared to the bills above. He became a net exporter in November 2020 but it didn’t reflect in his electricity bills. Not happy Jan.

Getting Off The Gas

The goal was to decarbonise their lives as much as possible, so getting off the gas was a priority. At the same time as the solar panels went on, the gas hot water system was replaced with an heat pump.

Gas usage immediately plummeted by 95%. The bill, however, dropped by only about 80% – still an impressive saving of $750 p.a. This was once again because of the despised supply charge. A kitchen renovation in July 2022 provided an excuse to ditch gas altogether when they installed an induction cooktop, providing further savings.

Having no gas bill now saves them $900 p.a. The only gas at their place now comes out of a bottle when they fire up the BBQ.

Home Battery + More Solar Power

Not content with the savings already achieved, the family decided to purchase a home battery in November 2021. At the same time a 10 kWh battery was installed, they threw another 7 x 350W solar panels on the roof, bringing it to a total of 9.1kW of solar capacity.

Josh must have been bouncing off the walls in excitement at the thought of expanding his Excel spreadsheet to include the battery. He’s now worked out they’ve become 93% self-sufficient for energy. To put it another way, only 7% of the electricity they use comes from the grid, averaged over the whole year.

He says that the battery gets them all the way through the night 310 days per year (84.9%), although it’s almost impossible to have a day that’s 100% self-sufficient as any spike of use that goes above 5kW has to be covered by the grid.

“After one full year with the battery, I analyzed how often the battery gets you all through the night to the next day (see the graph below). Out of the whole year, 341 days (93.4%) made it past 9 pm, 334 days (91.5%) made it past midnight, 320 days (87.7%) it got past 4 am, and 310 days (84.9%) it made at all the way through all night to the next day with no grid draw at all.”

I must admit at first glance I struggled to ‘get’ the graph below, which shows the ability of Josh’s battery to see them through the night without grid power, over a period of 12 months. The green is not actually a Christmas tree, but the data. The white bars (which look like the data) are, in fact, showing where the battery didn’t make the distance, and the grid kicks in. If you stare at it long enough it’ll make sense.

You can see the heatwaves in late summer where a few days in a row the battery was drained by 9 pm thanks to the air conditioner. Winter had the occasional super overcast/ rainy day when the battery didn’t fully charge, as you almost always see single white bars.

Hot tip for WA solar buyers:

Josh’s solar, battery and inverter are DC-coupled, giving him potentially more power to charge the battery than an AC coupled system would. As the battery is connected to the DC side of the hybrid inverter, its input is not limited by the 5 kW nominal inverter rating, and so can take advantage of all the unclipped power available from the solar array to charge the battery.

This works particularly well in systems where the DNSP rules limit single-phase inverter sizes to 5 kW (WA), and a bonus in battery systems where panel capacity is more than 133% of the inverter rating. Josh’s system ticks all the above boxes – Western Power inverter limit: 5 kW (single phase), Josh’s solar: 9.1 kW, Josh’s inverter: 5 kW. This could arguably be a significant factor in trimming down the payback time for a home battery.

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