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I am looking to erect a few wind turbines to supplement my off-grid solar system as we are getting somewhat enough wind here in Southland, New Zealand and very little sun in winter (there is enough sun when the sky is clear but it is often blocked by very dark overcast). This should save lots of money otherwise spent on petrol for the generator.

A quick search for what's in store has left me puzzled. On Aliexpress you can find lots of turbines which physically appear the same (rotor diameter 1.3m, weight 9kg) but with significantly different rated power: from 400W (perhaps the most realistic) to 1500W and even more. A local NZ store sells 3.5kW turbine with rotor diameter 3m (and much much higher price tag).

Am I right assuming that if one turbine with 1.3m rotor diameter is rated 400W, then another one with the same weight and diameter physically cannot produce 1.5kW? Does anyone here have experience with wind turbines like that?

@acmotor has some experience I believe. I suspect the power ratings are somewhat arbitrary, as it literally depends on the wind. so 400 W might be a safe continuous rating? 1.5 kW peaks in gusts?

Spend a little time considering the hard facts of wind and power using a calculator like..
https://rechneronline.de/wind-power/

On a seriously windy day with average 30km/h wind for much of the day (gusts to 50km/h) my '1kw' 2.4m dia WT can produce 10kWh.
It might do this 30 days a year. Remember wind rarely blows hard all day.

In gusts the controller applies braking (load dumping) to limit overspeed.
200 days a year it will produce less than 1kWh.
The rest of the time there is no useful output. But it looks good spinning !
All this on top of a hill with open paddocks in the dominant wind directions.

So 500kWh per year. Suggesting an average wind speed over a year of 14km/h. (around 60W average power from WT)
At least some of the power is at night which helps to level things out.

Obtain a wind petal from your local weather site and or set up an anemometer at your proposed location (for a year !).

Mine is a traditional horizontal axis WT (upwind, with pivot tail wind speed limit) with fixed blade pitch.
Don't even think about vertical axis WTs.... they produce very little useable power. The physics are all wrong.

Would I put up a WT again ? Probably not unless I could rely on 30km/h for half the year, but as Chris says, 'who would want to live there ?'
Large scale, tall, variable pitch WTs at sites that nobody wants to live are best.

2kW of PVs at even only 10% output on a heavily overcast day for as little as 4 hours useful is still 800Wh with no moving parts. That is the worst case with the average
over a year being at least 5 times that even at a poor site.

Value for money and regular daily power production is better with PVs.

There is all sorts of info here https://www.thebackshed.com/windmill/home.asp if you want to build your own or improve the performance of the Chinese things with the noisy blades.

T1 Terry

They are handy for powering remote communications equipment when there are bad bush fires, lots of use, lots of wind, and no sunlight due to smoke.

I have a 40 acre property 200kms west of Sydney. It is totally off grid. I have had PV panels and a 400watt wind turbine for over 10 years.

acmotor's post above very accurately reflects my experience with PV and the turbine.

The PV works less well in winter than in summer (no surprise there!) Because of that I downloaded the weather bureau's "wind rose" for my area for the month of july. I then positioned the turbine so it would receive good wind during July.

Cheers