Tool · Bill kWh to kW system
Solar system size calculator
Work out the right kW system for your home from your quarterly bill kWh, daily use pattern, and planned EV or heat pump. Output uses real Bureau of Meteorology + CEC city-yield data, not vendor PR.
★Key takeaways
- ✓Method: (daily kWh use / city kWh-per-kW-per-day) + 25% buffer for cloudy days and shoulder seasons + extra capacity for EV / heat pump plans.
- ✓6.6kW system is the most common residential install. Yields ~7,000-9,500 kWh/year depending on city, enough for a typical family using ~18-22 kWh/day.
- ✓Match panel oversize to inverter capacity. CEC allows 1.33:1 panel-to-inverter ratio. Clipping is real but small (2-5% of annual production).
- ✓Future-proof for EV + heat pump: add 3 kW for an EV, 1-2 kW for heat pump hot water, 2-3 kW for full electric heating.
- ✓State export limits matter. Residential single-phase: typically 5 kW inverter max. Three-phase: up to 15 kW. Battery-ready means hybrid inverter + DC coupling.
Calculator
Estimate the right kW for your home
Enter your quarterly bill kWh, your usage pattern, and any planned big-load additions. Output is the system kW that covers your annual demand with a small buffer.
Look for "Total usage" or sum the kWh on your bill.
Daily usage
20 kWh
from bill + load additions
Recommended system
6.6 kW
north-facing equivalent
Annual production
9,400 kWh
indicative, north-facing
Method: (daily kWh / city yield) + 20% buffer. Rounded to standard system sizes (5, 6.6, 8, 10, 13, 15 kW). Add a battery if your usage pattern is "mostly evening".
Profiles
Household profile to system size
| Profile | Daily kWh | Recommended kW | Notes |
|---|---|---|---|
| Small home, mostly day-use | 8-12 | 5-6.6 kW | Pensioners, work-from-home, AC + dishwasher run midday. Battery optional. |
| Average family, mixed | 18-25 | 6.6-10 kW | Most common. School-aged kids, evening peak. Battery worth modelling. |
| Large home, high evening | 25-40 | 10-13 kW | Big home, multiple AC zones, electric cooking, evening peak. Battery recommended. |
| Family + EV charging | 30-50 | 10-15 kW | EV charges overnight. Oversize panels so a future battery can fill from excess midday solar. |
| All-electric home (no gas) | 35-55 | 13-15 kW | Heat pump hot water + heat pump heating + induction cooking + EV. Battery near-essential. |
| Off-grid / hybrid | 20-40 | 13-20 kW + 20-40 kWh battery | Specialist install. Different design rules; talk to off-grid specialist not general retailer. |
City yield
Production per kW installed by city
A north-facing array at typical Australian residential pitch produces this many kWh of electricity per kW of installed panels per day on average. Real production swings +/- 15% for east-west splits, shading, dust + tilt.
| City | kWh per kW per day | Peak sun hours | STC zone |
|---|---|---|---|
| Darwin | 4.6 | 5.8 | Zone 1 |
| Cairns | 4.4 | 5.5 | Zone 1 |
| Brisbane | 4.2 | 5.2 | Zone 2 |
| Perth | 4.4 | 5.4 | Zone 2 |
| Sydney | 3.9 | 4.8 | Zone 3 |
| Adelaide | 4.0 | 5.0 | Zone 3 |
| Canberra | 4.1 | 5.0 | Zone 3 |
| Melbourne | 3.6 | 4.3 | Zone 4 |
| Hobart | 3.3 | 4.0 | Zone 4 |
Source: Bureau of Meteorology solar exposure data + Clean Energy Council typical-performance ratings for north-facing arrays at 25-30 degree pitch. bom.gov.au.
Inverter sizing
Panel-to-inverter oversize + clipping
The CEC allows panel arrays up to 133% of inverter capacity. Excess production above the inverter rating is "clipped" and not converted to AC, but only during peak hours on perfect summer days. Net annual loss is small.
| Panel size | Inverter | Clipping | Notes |
|---|---|---|---|
| 5 kW | 5 kW | None | Matched 1:1. Conservative. |
| 6.6 kW | 5 kW | ~2-4% peak hours | Most common config in NSW/VIC/QLD where retailer limits export to 5kW. |
| 10 kW | 8 kW | ~3-5% peak hours | Larger system inside SAPN/Ausgrid/Energex export caps. |
| 13 kW | 10 kW | ~4-7% peak hours | Common for battery-ready installs. Single-phase 10kW inverter is typical max. |
| 15 kW | 10-15 kW | Varies | Likely requires three-phase. Network application required in most states. |
Roof + export limits
Two hard constraints on system size
Roof area
Modern 440W+ panels are ~1.13m x 1.75m (~2sqm each). A 6.6kW array of 15 panels needs ~30sqm. A 13kW array of 30 panels needs ~60sqm. Subtract dormers, vents, AC condensers + shaded sections. Most homes can fit 6.6-10kW without compromise; 13kW+ usually requires a second roof face.
State export limits
Residential single-phase export caps are 5kW (NSW, VIC, QLD, SA, WA) or 10kW (TAS, ACT in some areas). Three-phase residential typically allows 15kW inverter. Larger systems require a network connection application + approval, which can take 4-8 weeks. Some networks (especially regional WA, parts of SA) restrict new export connections entirely.
Common questions
System size questions
Why is 6.6kW the most common residential system size?
Two reasons. First, in most states the residential single-phase export cap is 5kW inverter capacity, and a 6.6kW panel array on a 5kW inverter is the largest CEC-approved oversize ratio (1.33:1). Second, the price difference between 5kW and 6.6kW is small (a few hundred dollars of panels) but the extra production over 25 years is meaningful. The math points everyone to 6.6kW unless the home cannot fit the panels or has three-phase already.
What does panel-to-inverter oversize ratio mean?
The CEC allows panel array capacity up to 133% of the inverter rated capacity (e.g. 6.6kW panels on 5kW inverter). Excess panel production above the inverter rating is "clipped" (not converted to AC). Clipping happens only at peak hours on perfect summer days and is typically 2-5% of annual production. The economics favour oversize because panels are cheap, inverters are expensive, and clipping is small.
Should I get a 3-phase install instead of single-phase?
Yes if (a) you already have three-phase power, (b) you want to install >10kW of panels, (c) you have a battery-ready ambition >10kWh or (d) you have an EV charging at home. Three-phase inverters cost ~$1,000-$2,000 more but unlock larger system sizes and avoid the single-phase export-limit constraints. Upgrading a home from single-phase to three-phase is expensive (usually $3,000-$8,000); only worth it if you have multiple reasons to do so.
How does an EV change my system size decision?
An EV adds roughly 8-15kWh per day if charged at home (varies with km driven). That is roughly equivalent to 2-3kW of extra panel capacity. If you have, or plan to have, an EV: size for current usage + 3kW extra for the car. Charge overnight on off-peak if you do not have a battery, or charge midday from solar if you work from home.
How do I read my bill to know my daily usage?
Find the kWh figure for the billing period and divide by the number of days. Example: bill shows 1,440 kWh for 90 days = 16 kWh/day. The bill usually shows the daily figure already; look for "Average daily usage". Compare across all four seasons (winter is typically higher with heating). The annual average is the right baseline for sizing.
Can I just install the biggest system possible?
Three constraints push back. (1) Roof space limits panel count: each modern panel needs ~2sqm and most homes fit 18-30 panels max. (2) State export limits cap inverter capacity (commonly 5kW single-phase residential, 15kW three-phase). (3) Larger systems hit diminishing returns because mid-day excess is exported at low FiT, not at retail rate. A right-sized system that matches household demand pattern is more economic than a maxed-out system.