Sizing a Battery and Solar System with Phisaver Data

This follows how I sized a solar and battery system for my house, using energy monitoring data from phisaver. An example of the data is below:

Sizing the System

Firstly I checked the daily consumption, and night-time consumption:

  • Daily average of 13.5kWh/day.
  • Nightly consumption of 5.5kWh/night

Then we check the solar production in Brisbane:

  • 4.0 kWh/day/kWpeak including losses (4.5kWh/day without losses)
  • Ranges from 2.9 to 5.8 in Summer
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
5.5 5.4 4.5 4.0 3.2 2.9 3.1 3.8 4.8 5.1 5.8 5.8

So this means we want, in order to approximately be “net zero” each day:

  • Need 13.kWh/day, with minimum production of 2.9kWh/day, so 4.7kW PV system
  • Need 5.5kWh battery to run through the night. This could be reduced through better night-time usage. So say 4.0kWh battery.

Checking the System

A supplier provided system is available at 6.5kW PV and 3.3kWh battery at a price of about $13,000. This will be installed shortly, and I’ll post the results.

Checking the Payback

The Queensland government is providing $3000 funding, plus an interest-free $10,000 loan. Looking at 10 years:

  • My current electricity cost:
    • $1,500/year including $365 service charges
  • With a new system:
    • Consume 0.7kWh/day (4.0kWh – 3.3kWh battery) : $63
    • Service charge: $365
    • Export excess @ 9c. Need 4.7kW and will have 6.5kW panels: receive $237
    • Loan repayment: $1000
  • Hence:
    • Save $382 per year for 10 years
    • Then $1,382 per year after 10 years when loan is repaid

Update after Installation

The solar is installed. In the end I went with a 6.5kWh battery and 6.5kW panels and 5.0kW inverter. After a few days running, I’m choosing a retailer and checking costs:

Taking the following assumptions from my energy monitoring system at

Consumption12kWhFrom phisaver 12 month average
Import4kWhEstimate, with battery based on phisaver
Export6kWh20kWh solar average - 6kWh battery charge - 8kWh self consumption
Current375$/qCurrent bill without solar, about 10kWh/day

And using AGL who have a good solar feed-in rate of 20c/kWh:

Total Saving
2AGLsolar0.250.210.8$73$292 $1,208
3AGLbudget0.190.111.16$106$423 $1,077

Selecting plan 2, the “Solar Saver”, will cost about $290 a year. This saves $1,200 a year, which covers the $1000/year loan repayment.

So we’re $200/year ahead, and have minimal grid import. After 10 years, we’re $1,200/year ahead.

The assumptions are a little conservative, and with the help of Phisaver (and maybe a new fridge!) bills can be close to $0.

Further Details

If you’re really keen, see a detailed spreadsheet of the original estimates.





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