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 phisaver.com:
| Item | Value | Notes | |
|---|---|---|---|
| Consumption | 12 | kWh | From phisaver 12 month average |
| Import | 4 | kWh | Estimate, with battery based on phisaver |
| Export | 6 | kWh | 20kWh solar average - 6kWh battery charge - 8kWh self consumption |
| Current | 375 | $/q | Current bill without solar, about 10kWh/day |
And using AGL who have a good solar feed-in rate of 20c/kWh:
| Plan | Who | Name | Import $/kWh | Export $/kWh | Service $/day | Total $/day | Total $/Qtr | Total $/Year | Total Saving $/year |
|---|---|---|---|---|---|---|---|---|---|
| 1 | AGL | std | 0.25 | 0.1 | 1 | 1.6 | $146 | $584 | $916 |
| 2 | AGL | solar | 0.25 | 0.2 | 1 | 0.8 | $73 | $292 | $1,208 |
| 3 | AGL | budget | 0.19 | 0.1 | 1 | 1.16 | $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.