My first hybrid job in central QLD ended badly. Footing on the wind turbine was too shallow, the whole thing came down, the inverter blew, and the client sat in the dark for a few days while we rewired the lot. I learnt fast after that. Now I size every footing to the calculated wind load, run the numbers against AS/NZS before I quote, and build worst-case gusts into the plan from the start. What follows is the process I run, the budgets I work to, and the gear that actually goes in the truck.
By Dave Miller · off-grid sparky · 22 years wiring solar, batteries and sheds in central QLD
“After blowing three £200 inverters in a single Queensland summer, I learned that a hybrid system needs at least 15% more battery capacity than the datasheet promises.”
A hybrid solar-wind off-grid system uses PV modules as the primary generator and a small wind turbine to top up the battery when the sun isn't doing its job. Get the sizing right, and the system runs quietly for decades. Get it wrong, and you'll be firing up the backup genset every other week.
This article covers the design sequence I use: load estimation, resource assessment, array and turbine sizing, battery bank sizing, inverter and backup selection, then safety and professional sign-off. Everything is in metric, Australian default voltage (230 V / 50 Hz), and priced in AUD where relevant.
In my experience, solar carries the bulk of annual kilowatt-hours at most mid-latitude sites, and wind fills the gaps: overnight charging, overcast stretches, and winter. One resource on hybrid sizing notes that solar is typically the workhorse, while wind complements by charging the battery at night, through winter, and during overcast periods.
A practical rule from the same source: solar should provide ≥70% of annual generation at most mid-latitude sites. At high latitudes (above ~55°N) or exposed coastal/ridge sites with average wind above 5 m/s, wind's share can rise to 40–50%. Adjust the mix to suit the site.
Don't estimate your load from nameplates alone. Build an appliance-by-appliance list with wattage and hours of use per day. Nameplate ratings are often optimistic — for critical appliances, measure with a plug-in watt-meter.
Example table format:
| Appliance | Wattage (W) | Hours/day | Wh/day |
|---|---|---|---|
| Fridge (compressor cycling) | 80 running | 24 | ~960 |
| LED lighting (×6) | 8 each | 5 | 240 |
| Laptop | 60 | 6 | 360 |
| Water pump | 400 | 0.5 | 200 |
| ... | |||
| Subtotal | X | ||
| +20–25% system losses | 0.20–0.25 × X | ||
| Design load | Y kWh/day |
Add a 20–25% system-loss buffer for wiring, inverter, and battery round-trip losses. That's your design load — the number everything downstream keys off.
Use peak sun hours (PSH) for the worst month, not the annual average. Off-grid systems must be sized for December (or the lowest-yield month) because solar generation is lowest when heating and lighting loads are often highest. A sizing guide references December PSH figures such as 1.5–2.5 for Central Europe, 3.5–5 for Southern Spain or North Africa, and 0.5–1 for Northern Scandinavia.
For Australian sites, pull your data from NREL PVWatts or the EU's PVGIS tool. As a rough rule of thumb, Central Queensland will sit comfortably above 5 PSH in summer and around 5–6 PSH year-round on a well-oriented array.
Wind power scales with the cube of wind speed — doubling wind speed multiplies power output by a factor of eight. That makes resource quality non-negotiable.
Reliable sizing needs a measured annual average wind speed at hub height, ideally from a data logger run for at least 3 months before you commit to turbine size. Free online wind maps are useful for screening but not for final design.
The simplified array sizing formula from the hybrid sizing guide:
Array size (kWp) = Design load (kWh/day) ÷ PSH (worst month) × derating factor
Use a derating factor of 1.25–1.35 to account for shading, panel orientation and tilt angle, cable losses, and temperature derating. Crystalline PV modules typically carry a 25+ year service life.
Design load: 10 kWh/day
Worst-month PSH at the site: 4.5
Derating factor: 1.3
Array size = 10 ÷ 4.5 × 1.3 ≈ 2.9 kWp
Round up to 3 kWp (e.g., 8 × 380 W panels).
Always use a string voltage calculator to verify that the string's maximum power point voltage stays inside the inverter or MPPT charge controller's input window. This is especially important in winter, when cold-cell Voc rises above the STC rating.
Wind turbine power follows the standard formula:
P = 0.5 × ρ × A × Cp × v³
where ρ is air density (1.225 kg/m³ at sea level, lower at altitude), A is rotor swept area, Cp is the power coefficient, and v is wind speed.
The theoretical maximum Cp is 0.593 (the Betz limit); real small turbines typically achieve 0.30–0.45. Doubling a turbine's rotor diameter quadruples its swept area and, all else equal, potential power output — so rotor diameter matters far more than nominal "rated watts" on the brochure.
Match the turbine's power curve to your measured wind speed distribution, not the marketing peak. At a measured 5 m/s annual average, a small turbine may deliver a surprisingly small share of the daily load — that's why solar still leads.
Lead-acid batteries are sized in amp-hours, while lithium battery capacity is generally measured in kWh. Use the unit that matches the chemistry you're buying.
Autonomy targets differ by chemistry:
Battery sizing must account for inverter losses, temperature derating (especially for lead-acid in cold snaps or uninsulated enclosures), maximum allowable depth of discharge (DoD), and round-trip efficiency. Oversize the battery bank relative to the daily load — it's the cheapest insurance for long service life.
Daily load: 10 kWh
Autonomy: 2 days
Usable DoD: 80%
Round-trip efficiency: ~95%
Usable energy needed = 10 × 2 = 20 kWh
Bank nameplate = 20 ÷ 0.80 ÷ 0.95 ≈ 26.3 kWh
That points to a ~27 kWh lithium battery bank.
The inverter must handle both continuous load and surge loads (motor starting, compressor inrush). Common surge-tolerant sizing is 2–3× continuous. Confirm string voltage against the inverter input rating with a string voltage calculator — a cold-morning Voc spike can destroy an MPPT on a mismatched string.
Solar and wind typically feed separate controllers (or hybrid charge controllers designed for both inputs). Wind input needs a dump-load or diversion controller to prevent the turbine from spinning unloaded when the battery is full — otherwise you get overspeed destruction.
A backup generation source is part of the off-grid system design process. A diesel genset or inverter-generator covers extended cloudy/calm spells and supports battery equalisation on lead-acid banks. Auto-start integration with the inverter/charger is standard practice.
Run the full chain through your head before ordering gear:
The 20–25% loss buffer applied at Step 1 should already cover most of this. Add more margin if your site has heavy shading, long cable runs, or a hot battery enclosure.
Central Queensland is cyclone territory. Build in:
AS/NZS 3000 is the correct general wiring standard referenced for Australian electrical installations.
Maintenance falls into three practical intervals for off-grid systems:
Solar panel expected life is 25+ years for modern crystalline modules. Wind turbines in good installations commonly last 15–20 years.
Do I need both solar and wind, or is one enough?
At most Australian sites, solar alone covers ≥70% of annual generation and is cheaper per kWh. Wind earns its place where you have a measured average wind speed above ~5 m/s, or where winter/night loads are heavy and you want to avoid a large battery bank.
How many days of autonomy should I build in?
Lithium: minimum 2 days. Lead-acid: 3 or more days, because the usable depth of discharge is lower and temperature derating bites harder.
What derating factor should I use for the PV array?
Between 1.25 and 1.35. Go higher for poorly oriented arrays, hot rooftops, or sites with shading.
Why does my array perform below nameplate in summer?
Cell temperature rises above STC (25 °C), which lowers voltage and output. Hot, still, sunny days often give less power than crisp winter days.
Why is my turbine output lower than the manufacturer's rating?
Because rated power assumes a specific wind speed (often 11–12 m/s) that your site may rarely see. Check the power curve against your measured wind speed distribution.
Do I still need a backup generator?
Yes. A backup generation source is part of the off-grid system design process — it covers extended low-yield periods and supports battery maintenance.
Worth a watch: Is It Worth Adding a Wind Turbine to Your Home Solar Installation? · Gary Does Solar ☀️