Estimating Generator Size

Before you can get started building, buying, or installing a wind generator electrical generation system, you're going to need to know how much system you're going to need to do what you want it to do. There is more than one way to go about this estimation and some ways are less accurate than others.

We'll focus on the methods that are accurate enough for most people as well as simple enough to do yourself without needing a big knowledge base or expensive equipment. Just remember that the numbers you generate here will not be set in stone and could change with your situation, budget, or the availability of equipment. We'll start with this one first, as it is by far the easiest to use, to estimate consumption for a Utility Grid-Active Connected System. Since you're already connected to the utility's power grid and are therefore getting billed every month, you've already got the numbers you'll need to work from to estimate your wind system's requirements. This method also works for currently grid-connected buildings or housing that you plan to disconnect from the grid. Using your most current electric bill, you can usually get everything you need without any more research. Most electric utilities now include not only the current month's usage (usually measured in kWh or kilowatt hours), but also your average per month, for the past year. If your bill doesn't include this information, you can either ask the power company to supply you with it (often this comes with a fee) or go through your files of past bills and put the information together yourself. Once you know what your average monthly power usage is, you'll want to go through those months and see what your peak, or highest usage per month was. Make a list of these numbers for handy reference. You'll need to know: your average monthly usage, your peak monthly usage, and your lowest month's usage. If you want to do cost estimation and a Return on Investment (ROI) crunch, you'll want to write the current cost per kilowatt and average monthly costs as well. It wouldn't hurt to create a column next to each to write down a per-day estimate. Just take the number and divide it by thirty, so if you’re average monthly kWh usage is 30, then you use 1kWh per day. Now you have an idea how much power you need to meet you’re lowest, highest and your average monthly requirements. This gives you a best case scenario to work from, as replacing all of your energy needs with wind would be ideal. Now, hopefully you've already got your budget range for how much you're ready to spend on your new wind system. Since you now have all the numbers you'll need, you're ready to get started designing a wind generator system that's right for your situation.

90% of you will most likely utilize the Utility Grid -Active Connection as it is the simplest and cheapest of the group. This is most recommended for everyone that lives on the power grid presently. The system is based on this power delivery setup so that you don’t need to spend hundreds or even thousands on charge controllers, system monitors and batteries. The average American home uses about 10,000 kWh a year, which equates to 830 kWh per month but refer to your power bill to be accurate.


In this chapter, we'll be looking at the various components involved in a wind generator portfolio, and how they work together to make a complete electrical generation system. We'll also look at what size of generator, tower, etc. will best suit your needs and what options are available for both tower-mounted and rooftop-mounted generators and accessories.

What Size Generator Do I Need?

Generators are measured in two different ways: average monthly or average yearly output and often have numbers representing their peak or maximum performance ratings as well, all of which are measured in kilowatts (kW). They are often grouped into categories such as micro/small (under 500kw per year), mid-sized or medium (501kw to 999kw), and large (1 megawatt and larger). The smallest, useful enerators are usually around 20kw/month units used for very small applications such as lighted signs and electrical water pumps. The output you can expect, however, is a combination of the limitations of the generator itself and the average wind speeds received that year. The numbers manufacturer's give are usually based on averages (about 14mph of wind) and maximums attainable. This is why they usually use two numbers to describe their generators (both average and maximum). For most homes and small farms, units in the micro range are usually what are used, producing up to 500 or so kilowatts per year. Most homes in the United States use an average of about 10,000 kilowatt hours (kWh) of electricity per year, which translates to about 830kWh per month. A kilowatt hour is one kilowatt of electricity multiplied by the number of hours as a unit of measurement. This is the unit most utility companies used to measure and charge for electrical power usage. Since there is 1 kilowatt in one kilowatt hour and there are 720 hours in a month, using only one kilowatt per hour would result in you’re using only 720kWh per month. Most households, obviously, use more than this. Reversing the math, if your Generator puts out an average of 500 kilowatts per month, you’re producing 500kWh of electricity, or roughly 60% of your energy use. That could result in pretty significant savings per month! Before you order, most reputable wind generator manufacturers and dealers will be happy to supply you with numbers specific to your chosen model and location. There are many resources on the Internet to tell you what your area's average wind speeds at various elevations are and the maps below are an example.

Click here or on the picture for more Global information.

Click here or on picture below for more US information.

pWind Classes for US-DOE Wind Maps

Each wind power class should span two power densities. For example, Wind Power Class = 3 represents the Wind Power Density range between 150 W/m2 and 200 W/m2. The offset cells in the first column attempt to illustrate this concept.

Classes of wind power density at 10 m and 50 m (a).
Wind Power
10 m (33 ft) 50 m (164 ft)
Wind Power Density (W/m2) Speed (b) m/s (mph) Wind Power Density (W/m2) Speed (b) m/s (mph)
1 0 0 0 0
100 4.4 (9.8) 200 5.6 (12.5)
150 5.1 (11.5) 300 6.4 (14.3)
200 5.6 (12.5) 400 7.0 (15.7)
250 6.0 (13.4) 500 7.5 (16.8)
300 6.4 (14.3) 600 8.0 (17.9)
400 7.0 (15.7) 800 8.8 (19.7)
1000 9.4 (21.1) 2000 11.9 (26.6)

This is not the motor that was used to build the wind generator. You will notice that no wattage rating is given. However, it does state that it is good for ¼ horsepower.

1 Horsepower = 745 Watts divided by 4 = 186.25 Watts at 1725 RPM

Safety Considerations: IMPORTANT!

Several safety considerations need to be addressed before you choose a Generator and tower system to put up on your property. These includes, but obviously aren't limited to:

Clearance in a radius around where the tower is to be built.

Generator high-wind and rotor control mechanisms.

Wiring safety and lightning protection.

Cutoffs and electrical breakers for disconnecting the generator from the electrical grid (if attached).

Generator rotor brakes or stops for safety during maintenance, installation, or removal. Choose the location for the tower carefully and be sure that nothing and no one can be hurt should the tower fall.

Give extra added space to the movement area of the wind generator (movement of the blades and spin of the tail end).

For example:
So if you have a tail fin and blades that span and move in a one meter circumference, make sure that at least two meters of clearance is available in a radius on all sides.

Most generators come with control and safety mechanisms that automatically function in the event of very high winds or winds that are higher than the generator's rated safety speeds. Generators have automatic braking to slow the rotors, they turn into the wind to reduce "drag" and friction, fold in half to reduce their ability to spin, and utilize any number of other techniques to protect themselves and those who use them during very high wind conditions. Most generators are now equipped with built- in grounding connections for protection in the event of a lightning strike and have breakers and other safeguards installed to keep the energy from flowing to the grid or building they're connected to. Many generator towers also include "break-off" or shear points where the tower will intentionally fail to safely fall into a smaller space in the event of gale - force winds or other problems. Most qualified electricians, installers, and dealerships will also show you how to use cutoff switches to manually disconnect the unit from the grid, battery bank, etc. during maintenance and in an emergency. Finally, most wind generators have a brake, stop, or physical way to keep the rotors from turning that you can safely engage in order to perform maintenance, installation, etc. on your generator safely. These are not necessary as you can simply "unplug" the system and if the generator you're considering purchasing does not include a feature like this, it’s ok.

Getting Parts

DC or AC Motors:
These can be found on eBay or in power tool departments. Although using the motor from old power tools is inexpensive, be aware that they don’t produce a lot of power. DC motors can create power instead of just using it by turning the motor in the other direction. The energy that is created travels using the wires that would be utilized if power were coming in. When deciding what DC motor to use, there are some things to keep in mind. For starters, it must have low RPM's because when the motor is generating power as opposed to using it, it spins much quicker than what is rated. The DC motor for your wind generator should also have high current and voltage of at least 12v. An example of the perfect motor is one that is rated less than 400 RPM at 30 volts because when used as a generator it will create 12v at low RPM. Another thing to keep in mind when choosing a DC motor is wind speed. The lower the wind speed, the lower the RPM rating necessary in the motor. However, the ideal condition for the wind generator is high wind.

To make your tower you will need a stable structure that is between 6 - 12 feet tall. You can use old satellite towers or even 3 inch steel pipe. The tower can be secured to the ground using cement or use a basketball standard with a water base. For those of you who are handy, you might want to create your own tower by welding pipes together.

See the pictures below for example of towers:

Old Satellite Tower:

Portable Basketball standard:


It is recommended that the blades be made out of PVC or ABS pipe and are coated with UV inhibiter to increase the life span of the generator. The ideal diameter for a blade is 4 feet. Don’t go over-board with the length of the blades. The bigger you go, the more stress you put on the structure.
At this size and with wind speeds of 20 miles per hour minimum you can generate the desired 200 watts of energy. However, too large of diameter can be too big for many backyards. A quick fix is to cut the blades to a size that better suits the size of your property making sure to maintain the shape of the blade.

If you know of or know someone who has an old ceiling fan that doesn’t work, you can take the blades off of that and install it on your system.

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