Guide · Solar
Off-Grid Solar System Design for Queensland
Direct answer
Sizing and component selection for stand-alone off-grid solar systems in Queensland. Insolation, autonomy days, panel and battery selection, and inverter choice.
An off-grid solar system has to do everything a grid connection does, by itself, indefinitely. There’s no fall-back. No utility to call when production is low. Designing for that means accounting for the worst week of the year, not the average. This guide covers the sizing methodology used for a typical off-grid Queensland install, whether rural acreage, granny flat, or remote property.
Step 1: Calculate daily energy use
The first number you need is honest daily kWh consumption, not aspirational.
For a typical 3-bedroom house off-grid in Queensland:
| Load | Avg power | Hours/day | Wh/day |
|---|---|---|---|
| Refrigeration (single fridge/freezer) | 100W cycling avg | 24 | 2,400 |
| Lighting (LED throughout) | 30W avg | 5 | 150 |
| TV and entertainment | 80W | 4 | 320 |
| Electronics (computers, chargers) | 40W avg | 16 | 640 |
| Hot water (electric, heat pump) | 800W | 4 | 3,200 |
| Cooking (induction or electric) | 1,500W avg | 1.5 | 2,250 |
| Washing (machine + occasional dryer) | 600W avg | 1 | 600 |
| Pumps (water, septic) | 400W | 0.5 | 200 |
| Miscellaneous | 100W | 8 | 800 |
| Daily total | 10,560 Wh |
This is roughly 10–12 kWh/day for a typical household. Larger families or properties with workshops, sheds, or pool pumps push higher. Smaller households or those that gas-fuel cooking and hot water use less.
Hot water and cooking are the two biggest discretionary items. An off-grid design that uses gas for both reduces electrical load by 40–50%. Many off-grid Queensland installs use gas cooking and gas (LPG) hot water to keep the solar system size manageable.
Step 2: Multiply by autonomy days
Autonomy days = how many days the system runs from battery alone with no solar input.
Queensland is sunny on average but has runs of overcast weather, especially in the wet season. A reasonable design assumption is 2–3 autonomy days for SE Queensland, 3–5 for the tropical north.
For 10.5 kWh/day daily use × 2.5 autonomy days = 26.25 kWh of battery.
That’s the usable battery capacity. Apply the depth-of-discharge derating:
- LiFePO4 (80% DoD): 33 kWh nameplate (so 26.25 / 0.8)
- AGM (50% DoD): 52 kWh nameplate (rare for off-grid these days)
The choice between AGM and LiFePO4 for off-grid is increasingly clear-cut: LiFePO4 is significantly cheaper per usable kWh over a 10-year period, and has dramatically better cycle life. For new off-grid installs, LiFePO4 is essentially the only sensible choice unless budget is severely constrained.
Step 3: Size the solar array
Solar array sizing is based on:
- Daily energy use
- Average daily insolation (hours of equivalent full sun)
- Panel and system inefficiency
- Generator backup availability
Average daily insolation for Queensland (annual average):
| Region | Average daily insolation |
|---|---|
| Brisbane / Gold Coast | 4.7–5.0 kWh/m²/day |
| Toowoomba / Darling Downs | 5.0–5.3 kWh/m²/day |
| Townsville | 5.5–5.8 kWh/m²/day |
| Cairns | 5.0–5.3 kWh/m²/day |
| Far north (Weipa, etc.) | 5.5–6.0 kWh/m²/day |
But annual averages hide the worst weeks. Winter and wet-season weeks can produce 30–50% of average. For off-grid sizing, design for the worst month, not the average:
| Region | Worst-month average daily insolation |
|---|---|
| Brisbane / Gold Coast | 3.2–3.5 kWh/m²/day |
| Townsville (wet season) | 3.5–4.0 |
| Cairns (wet season) | 3.0–3.5 |
For a 10.5 kWh/day load with 3.3 kWh/m²/day worst-month insolation:
- Required array (idealised): 10.5 / 3.3 = 3.18 kW
- With system inefficiencies (15–25% losses): 3.18 / 0.8 = 3.97 kW
- For meaningful margin without generator: 5–6 kW
- For full autonomy with no generator: 8–10 kW (and substantial battery)
A 6 kW solar array is a reasonable starting point for the example load. Sized down if a generator is available, up if generator-free operation is the goal.
Step 4: Specify the inverter and charge controllers
Off-grid inverters are different from grid-tie inverters. They produce AC output without needing a grid reference signal. The dominant brands in Australian off-grid:
Victron MultiPlus / Quattro
The default choice for serious off-grid in Australia. The MultiPlus II 48/5000 (5kVA at 48V) is the standard for the example load size. The Quattro 48/8000 or 48/10000 for larger systems.
Features:
- Inverter, battery charger, and AC transfer switch in one unit
- Works seamlessly with grid power if available (grid-interactive when needed)
- Excellent integration with Victron monitoring (Cerbo GX, VRM)
- Solid track record over decades of design refinement
SMA Sunny Island
Higher quality but more expensive. Used in some larger off-grid systems and increasingly in commercial/industrial off-grid.
Selectronic SP Pro
Australian-made, popular in serious off-grid installs. Particularly common in rural NSW and Queensland. Excellent product but requires specific dealer support.
Schneider XW Pro
Used in some installations. Capable but less common in Australia than the alternatives.
For most Queensland off-grid residential installs, Victron MultiPlus II is the right answer.
Solar charge controllers
Sized to the array. Victron MPPT 250/100 (handles up to 5.7 kW at 48V) for the 6 kW example. Multiple controllers for larger arrays, allowing different array orientations to feed different controllers.
Step 5: Generator backup
A generator is the off-grid system’s emergency backup. For most installs:
- Sizing: Approximately the same as the inverter. 5 kVA generator for a 5 kVA inverter.
- Type: Diesel preferred for reliability and fuel availability; petrol acceptable for smaller installations
- Auto-start: Connected to the Victron via a 2-wire signal so the inverter starts the generator when battery state of charge drops to a configured level
- Run time: Designed for 0–3 hours per week in normal operation, more during extended overcast periods
A reasonable generator setup runs 100–300 hours per year on a properly sized off-grid system in Queensland. Properties without backup are exposed to extended outages during the wet season.
Step 6: Integration with the rest of the property
A modern off-grid system integrates with:
- Home automation: Home Assistant for monitoring, alerts, and load shedding
- Smart loads: pool pumps, water heaters, and EV chargers that run only when solar excess is available
- Tank monitoring: water and fuel level integrated into the same dashboard
- Communications: multi-WAN router with NBN, 5G, Starlink as available
- Security: cameras and alarm systems integrated with the same infrastructure
This integration is what separates a well-engineered off-grid system from a collection of components. It’s what lets you see at a glance whether everything is healthy, and intervene before small issues become outages.
Common Queensland off-grid scenarios
Granny flat or studio behind a main house
Typical daily load: 4–7 kWh Typical system: 3–4 kW solar, 10 kWh LiFePO4 battery, 3 kVA inverter Typical install cost: $25,000–$40,000
Rural acreage main residence
Typical daily load: 8–14 kWh Typical system: 6–8 kW solar, 20–30 kWh LiFePO4 battery, 5 kVA inverter, generator backup Typical install cost: $50,000–$90,000
Hobby farm or remote property with workshop
Typical daily load: 12–25 kWh Typical system: 10–15 kW solar, 40–60 kWh LiFePO4 battery, 8–10 kVA inverter, generator backup Typical install cost: $80,000–$180,000
These figures are indicative. Actual sizing depends on specific loads, regional conditions, and design choices.
Common design mistakes
The patterns we see most often on retrofit assessments of existing off-grid systems:
- Battery undersized for autonomy. Owner runs out of battery in week 2 of overcast weather and has to run the generator daily.
- Solar oversized but battery undersized. Excess solar production during the day is dumped to the load (or wasted) because the battery can’t absorb it.
- No load shedding. When state of charge is low, all loads remain available, including discretionary ones, until the inverter cuts out completely.
- Generator not auto-starting. Owner has to manually start the generator at 11 PM when the system shuts down.
- No monitoring. No idea what’s happening, no early warning of degrading components, no data to support warranty claims.
- No comms infrastructure. Internet is limited to whatever 4G signal reaches the property, with no failover.
Common questions
Can I add off-grid capability to a grid-connected house later? Yes. The system is essentially the same: solar, battery, inverter, with an automatic transfer switch added to disconnect from the grid during outages. Going fully off-grid (disconnecting the grid permanently) is more involved but the technology is identical. Most “hybrid” installs (Victron MultiPlus with grid input) can run as off-grid by simply disconnecting the grid.
What if I have an EV or plan to get one? Add the EV’s expected daily charging to the daily load calculation. A typical EV charging once a week from empty might add 60–80 kWh once weekly, or 8–11 kWh/day averaged. This significantly increases system size requirements. Many off-grid systems with EVs use scheduled charging (only when solar is in excess) and a generator backup.
What’s the lifespan of a properly designed off-grid system? Solar panels: 25 years (typical warranty), 30+ in practice with declining output. LiFePO4 batteries: 12–20 years to 80% capacity. Inverter: 10–15 years. Solar charge controllers: 10–15 years. Generator: 15+ years with proper maintenance.
The system as a whole has a 20–25 year design life with periodic component replacement.
Can I install off-grid solar myself? Some of the work, with appropriate expertise. Battery installation, PV panel mounting, and basic DC wiring is owner-accessible if you understand the safety requirements. The grid-connection inverter installation, the AC distribution, and the compliance certification require licensed work. For most owners, the practical answer is professional install with optional owner-completed prep work (panel mounting hardware, conduit runs, cable trays).