(Notice to reader: Please note that there are Microsoft Excel spreadsheets embedded in this web page so the reader can try different scenarios. If you have Excel loaded on your local computer the page will come up very quickly. If you do not have Excel it will still load and be usable but it may take 15-30 seconds. I am working on alternative software for the page.)
Typically people only have three forms of truly renewable energy available to them: wind, solar, and geothermal. On this page we will deal with the second most potent form available: wind.
One of the first things to notice about wind is that it is very non-uniform:
So it is important to realize from the beginning that your power output will be irregular at best.
The next item is a turbine issue. Power from a windmill or wind turbine can be calculated using the equation:
Power=(efficiency)*(air density)*(Spinning blade disk area)*(wind speed)^3
For those of you that aren’t as geeky as I am, there is something subtle here. When you double the wind speed, the power output increases 8X! When you think about that and the chart above, you realize that when a windmill makes power it comes in constantly changing surges and you are likely to need overload protection.
Below is a calculator that let’s you see what the real output from a wind turbine can be. First we will look up the average wind speed for our location from the National Renewable Energy Lab’s website (link to map). For our example let’s use 4 meters per second or 8.9 miles per hour as the average wind speed. Typing that value in the interactive spreadsheet below we can see that output from a good turbine will be:
267.4 kWhrs per year. OK..That’s not tiny but my farm house with an electric stove, dryer, in law apartment, and hot water heater uses 1200 kWhr’s per MONTH. That six foot windmill isn’t going to get me off grid any day soon.
If you play with the calculator the numbers say I need a windmill 45 FEET in diameter to be off grid with wind alone.