The short version
Key points
- The planned system has 25 kWh of nominal battery capacity, with approximately 22.5 kWh described as usable.
- The quoted installation cost is $23,290, with an estimated federal rebate of $8,370 and $10,000 available through a WA interest-free loan.
- Pat estimates the battery could reduce his household energy costs by about $80 per month, although the figures are based on averages and will be checked against real bills.
- The forecast assumes existing solar panels generate enough energy to charge the battery and excludes electric vehicle charging from the battery calculation.
- The model includes a 94% round-trip efficiency assumption and an estimated 2.5% annual degradation rate.
The system and household assumptions
The forecast is based on a four-person Western Australian household with existing 6.6 kW solar panels and a 5 kW inverter. Average solar generation is estimated at 26 kWh per day, although this varies by season and can fall on overcast days.
Household electricity use, excluding electric vehicle charging, averages about 18 kWh per day. Pat separates this into approximately 11 kWh during the day and 7 kWh overnight. The EV is expected to continue using grid power rather than the home battery.
The calculation also assumes a three-phase home, a DC-connected battery with a hybrid inverter, and a worst-case round-trip efficiency of 94%. Pat notes that electricity prices may rise, but does not include future price increases or possible falls in the forecast.
Battery, installer and quoted cost
The selected battery is a Growatt APX system made up of five 5 kWh units, giving 25 kWh of nominal capacity. The transcript states that approximately 22.5 kWh is usable. The system also includes a three-phase 5 kW Growatt hybrid inverter.
Pat chose Growatt partly because an older Growatt solar inverter at the property has been reliable, and because the installer reported a low return rate for Growatt inverters. He describes the equipment as a balance between price and performance, while noting that the companion app is functional but not especially user-friendly.
The installer selected is Sunterra. The quoted system total is $23,290. The quote includes items such as battery installation, a bollard for garage protection, an emergency power supply control box, a smart meter and inverter replacement. The three-phase configuration and DC-coupled design may make this installation more expensive than some alternatives.
Rebate and payment structure
The estimated federal rebate is $8,370, leaving a quoted amount of $13,620 before the WA financing arrangement. Pat says the WA government subsidy had previously been more generous, and that the change reduced the benefit compared with an earlier potential $5,000 difference.
Rather than paying the full amount upfront, Pat plans to use a WA government interest-free loan of up to $10,000 over 10 years. That equates to repayments of $83.33 per month. The remaining upfront payment is approximately $3,620.
He says account-keeping and setup fees that he initially expected were waived for this battery arrangement. He also estimates that retaining $10,000 in a mortgage offset account, assuming a 6% mortgage rate, could save $2,742.95 over 10 years. That is a personal financial comparison rather than a guaranteed return.
Estimated savings and payback
Pat’s average monthly household electricity usage cost is estimated at $161.94. His wider bill is about $242.15 per month, including approximately $41 for EV charging and $38 for the supply charge, neither of which is included in the battery’s offsettable household usage in the same way.
The battery is intended to use excess solar during the day and supply the home during the evening peak. The home has a time-of-use tariff, with a daytime rate of 8.6 cents per kWh between 9 am and 3 pm and a peak rate of 53.8 cents per kWh between 3 pm and 9 pm.
After including the estimated $83.33 monthly loan repayment and the supply charge, Pat calculates that the household could be about $80 better off per month. He describes this as nearly $1,000 per year and estimates that the $3,620 upfront payment could be recovered in about 3.7 years. These are forecasts based on averages, not measured installation results.
Capacity, degradation and limitations
The forecast assumes the battery can cover typical household usage, but a 5 kW inverter may not suit homes with substantially higher demand, such as properties with electric cooking or older, less efficient air conditioning. A larger inverter could increase the installation cost and might require changes to the existing solar array.
Pat includes battery degradation in the model. Using the stated Growatt estimate of about 2.5% degradation per year, he expects usable capacity to be lower after 10 years, estimating approximately 19.4 kWh at that point. The battery warranty is stated as 10 years, while the installer offers a 10-year total warranty including a five-year extended warranty and workmanship coverage.
The installation was scheduled for 13 August 2025. Pat planned to compare the forecast with several months of real electricity bills after installation.
TechManPat’s conclusion
I think this installation can make sense for my household because the interest-free loan limits the upfront cost and my tariff creates a large evening price difference. I do not think a 25 kWh battery is suitable for everyone, and my forecast depends on existing solar generation, household usage, tariffs and the rebate. I’m expecting about $80 per month in savings, but the installation results will show whether that estimate reflects reality.
This knowledge-centre summary is based on the linked TechManPat video and reflects the information available when it was published. Check current pricing, availability and policies before acting.
