All readings are based on open current voltage and shorted load

This is just a word of warning for those who are thinking of buying modified car alternators from or

—-             I did have rather a nice photo of some alternators and parts here but Tim at Presto asked me to remove it.  Fair enough.  Click on the above links to see the nice photos and plausible sales talk.

Dear Hugh,

Please remove any pictures and content from your blog/website that are the property of Presto Wind(TM). You are free to exercise your opinion, but not to use our pictures and data, without permission. So called “car alternators” are absolutly fine and offer the customer an affordable alternative to building a RF alternator. Thank you Sir.
Best Regards,

Timothy Moeller, MSME, P.E. Ohio, Nebraska
Engineering Dept.


The specifications are not always completely clear on these web sites.

You will come across tables of figures like this

50 RPM- 18 volts x 0 amp = 0 watt

80 RPM- 24 v. x .25 amp = 6 watt

100 RPM- 32 v. x .50 amp = 16 watt

200 RPM- 72 v. x 1 amp = 72 watt


The watts figures cannot be simply used to predict the power that you will get from the alternator at a given RPM.  All readings are based on open current voltage and shorted loadmeans that these are not taken at the same time (as you would do to measure power) but under two separate artificial conditions.

In reality the alternator will not produce any power with an open circuit or a short circuit.  The useful data would be what actual power it can produce charging at 12 volt battery but you need to try and figure this out for yourself.  The results will be a lot lower than you might think.

In the above example the alternator can cut in at a very low rpm and produce 12 volts for battery charging, but its internal impedance is evidently very high which means that the output current is pitiful.  Current in amps to charge a 12 volt battery will be lower if anything than the tiny short circuit currents quoted in the above list.

These permanent magnet alternators can be useful for home-brew projects and a lot of people have fun fitting them with small blades made out of bent bits of metal.  But beware of the hype because these guys are out to sell you stuff, and not to make it completely clear whether it will actually work well for you.  A small turbine on your garage rooftop is not going to be able to catch much energy, and one of these alternators is not an efficient energy converter (any more than the bent metal blades) so don’t get high hopes of powering your house but you can have a lot of fun with them if you are realistic.

About hugh

I live off-grid in NW Scotland and have spent my life playing with wind turbines. I also love small hydros. Hands on renewable energy is my thing and I like to learn and to share my experiences.
This entry was posted in construction, products/technical, Rooftop madness. Bookmark the permalink.

17 Responses to All readings are based on open current voltage and shorted load


    Ok here is the product we have invested in to bring in the real numbers…..enspeed integrated watts view with wind and watt output. We have not posted the numbers from our former distributor presto and have tested their new claims on the 1600 watt hand crank generator–o&list=UUE9E9hMhvVtnmo3yvWnMZlg

    To set the record straight if the product does not produce as advertised we don’t want the business

  2. Also in a sense thanks Hugh for scrutinizing this as it led me to a better place so to speak

  3. By BS I simple mean our PMA is demonstrated performing in the way advertised on our you tube channel

  4. Yes see the video, as you can see this is complete BS and we have not had anything to do with presto wind since they stole over $6000 from us. Further I am still rebuilding products they sent out built improperly straight garbage but I am misleading.
    Our a href=””>permanent magnet alternators</a have used this format for some time and we openly give open voltage bu shorted amp ratings to the customer explaining they them THEY MEAN NOTHING. We also give data collected with watts view and inspeed in chart format and KW hour per month…..we have no need to deceive anyone . The truth sells. The previous charts posted on out site were from presto wind who claimed to be an "engineer" i.e. capable to provide accurate data. As part of my investigation no credentials could be found or were offered by Tim Moeller when confronted about the validity of his data. That is the truth and I have no trouble telling it. We are currently in the process of taking more data and making charts to further provide accurate and reliable data for our customers

  5. Bobby Z says:

    I have recently began really researching the idea of having wind and solar power options as a way of charging a small 12v. system, for emergency and disaster situations and am admittedly very basic in my understanding of all the power ratings and what they all mean. I get so discouraged by all the misleading information these manufacturers and distributors throw out there, just trying to dupe you into wasting your money on their product, that they know will not meet your needs. Instead they try to justify themselves by telling people to ” temper their expectations “. Knowing full well what most of our expectations are.

    They know the majority of people out there are needing products that will support a 12v. system under real world conditions. So instead of being up front and transparent when called out on the matter, these distributors like Mr. Jones instead try to defend their practices by saying, ” Well that’s just how the industry is “. Garbage.

    I’m glad I stumbled onto this blog. I had been checking into these presto wind products and had been wary of how incomplete and vague their information and videos were. Even with my limited knowledge, through online research of the supplied information, it was obvious they were trying to mislead as much as they were trying to promote a legitimate solution.

    So thank you Mr. Piggott for confirming my suspicions, for standing by your statements and calling these people and their misleading practices out. You really helped people like me out… Cheers

  6. Dale S says:

    Wow! What a load of crap! kWh per month meaningless? In the real world that is the only thing that matters, kWh power your loads plain and simple, you want to prove what you say about your machine? Put one on a tower right next to a similarly sized axial flux machine, and compare the kWh at the end of one month, simple.

  7. There you have it we agree to disagree. I loaded the chart at your request to show what was sent by a manufacturer when a loaded power curve was requested. The KW hours per month charts I am speaking of advise the consumer that they can expect a wide range of power output per month that are confusing at best. How useful is it to a consumer to see that they can expect 30 to 120 KWH per month? For me that is not very useful although I respect your viewpoint and concede such a measure is needed to calculate payback over time. If a customer had and anemometer, knew the mean wind speed yearly, and could decipher the value on the chart that would be a fruitful approach. In reality looking at a wind map is about as far as many consumer research goes. Recording data to make an informed decision is the exception and not the rule. In reference to the comment” Peak power output often sounds impressive but if you need a 40-50 mph wind to achieve it then it’s not very useful. “I agree which is why we have light wind PMA’s that will produce some useable power majority of the time rather than during an infrequent high wind event. I do find it odd that if you are critical of a product, “refer to it as a “cheap product” that you would not be open to being fair and balanced and “test” the product that the manufacturer i.e. me offered to send you for free. At the end of the day it’s your blog you win I’m out….peace

  8. admin says:

    Hi tony,

    Looking at those curves they look like power versus load resistance at a given rpm but the basic facts that should be in the caption are missing.

    I have to disagree with you (again! sorry) about the data for kWh energy per month being ‘worthless’. Actually that’s the most useful information that you can give about a wind turbine (assuming it has been properly tested and verified). The American Wind Energy Association and the British Wind Energy Association both have set standards that require manufacturers to give this information and test facilities have been set up to measure power curves so as to derive and report it.

    When people want to know how long it will take to pay back their investment they need only to know their site’s mean windspeed (not easy to measure but it’s the only scentific way to predict output) and the energy data for the turbine, and they can predict the payback and figure out whether to go ahead and buy a turbine. I also present the predicted energy production in my Recipe book so that people can have an idea of what they will get for their efforts.

    Peak power output often sounds impressive but if you need a 40-50 mph wind to achieve it then it’s not very useful. What you want is good average power, which equates to good energy production, and you get this from having a reasonably large diameter and adequate efficiency in the system overall. I’ve lived with wind generated electricity for thirty years and you learn this early on.

    I am beginning to think we’ll need to agree to differ on many aspects of wind energy and electrical power but I shall keep the link to your site on my old page so that people can find your site and hopefully pick up a cheap alternator that they can have fun with.

    all the best,


    Here ya go…..and you are not going to see “loaded power curves” on their sales page which is my point. I received that graph by e mail from the sales representative after asking for an output chart under a load. I am not here to pick on effsun, this is simply how wind generators are advertised on the commercial market 500 watts on your sited chart “100 watts under a load”. We can site numerous other examples outside the “car alternator” PMA realm. I have noticed that Southwest Wind Power in many ads advertises KW hours per month with a wide range depending on wind speed which in my opinion is pretty worthless as well. This was just an example from a generator that they “Effsun” to my understanding recently stopped making. Simply placing the term “rated” in front of power does not mean that you are not receiving open voltage times shorted amps sales charts. I am not here to throw mud but the fact is many of the vendors in today’s market simply fabricate output charts, we do not. If your goal is simply to inform readers that they need to be aware of loaded power output, “rated” power output for battery bank environment i.e 12, 24, 48 volts and or resistance as programed by a direct grid tie as the load at a particular cut in. I tip my hat good luck and I spend a lot of time personally educating our customers in kind.


  10. admin says:

    Hi tony ,
    please could you quote a link for that?
    I searched for Effsun 500W alternator details and got this page :-

    It’s pretty straightforward. Tells you the rated voltage, rated power and rated rpm. That’s all I am asking you to do. Power into the battery is measured as voltage times current in the circuit. Open circuit voltages and short circuit currents are of interest no doubt, but the data we need for battery charging is the data that Effsun give on the above page.

  11. Pertaing to your comment “As far as ‘mainstream’ using open voltage/ short circuit current, this is a habit exclusive to your small community of car alternator modifiers, and it is nonsense, which is why I wrote the blog post.” I can provide numerous examples” Outside “The car alternator PMA community to support my point therefore you viewpoint is nonsense from my perspective” For example “all of the PMA’s exported from China which account a majoruty of the market. Effsun for example sold a PMA rated for 500 watts that has a “loaded” power curve that of less than 100 watts. After your comment I wonder if you know how to measure current….This will be my last comment good day.

  12. admin says:

    Hi tony,

    To find out who Wendy is, read her comment above and her website. This is an open forum in which you have your say equally same as me and her.

    As far as ‘mainstream’ using open voltage/ short circuit current, this is a habit exclusive to your small community of car alternator modifiers, and it is nonsense, which is why I wrote the blog post.

    I have suggested what needs to be done and it is simple. Publish the output power that your alternators can produce at different speeds and voltages. That’s a simple request and it would allow people to begin to get an idea of what they can do. If you can measure the input mechanical power as well that would be brilliant. This approach is the best to ‘clean up the business’, and would win you plenty of customers and positive comments. Meantime the practice of quoting short circuit current and open voltage are the actions of people who are motivated by profit ahead of helping their customers and I will not shrink from saying so.

    I suppose you guys do know how to measure power output? Measure the DC voltage and multiply that by the actual DC current into the battery at the same time. Go ahead and do that with honesty and I shall make an announcement on this blog about that.

  13. Ok, I too am an enthusiast and can appreciate that point of view, wind is a lot more fun to watch than a solar panel. I have no idea who Wendy is nor does she have inside information about who or where I get my products. I cannot remember having any complaint from anyone named Wendy nor has she asked for a refund. I do appreciate your support in the past and have reciprocally linked to your site as a partner and as well as supported you in the past as an authority on wind mills. This it is particularly hurtful to see a posting warning people about”Hurricane Wind Power”. I have no issue with debating the merits in open forum as I am very familiar with how our products are designed and work as I work on them myself. The particular problem in my opinion is the fact in the mainstream all wind mills are advertised in open voltage multiplied by amperage giving a whopping unrealistic number. Some companies spin a PMA to 10000 rpm and data is placed in a chart open voltage. If you have feed back about how to clean up the business put it on a even playing field and advertise under a load I am all ears. Admittedly you have a larger following than I, Make a proposal. i.e wattage as a measure of work being done. We do custom build our PMA’s to customer specific need accounting for wind zone , voltage cut in, wind, Hydro or other driven application i.e how much power does the application make that can be converted to torque for the PMA. Wire gauges, rotor magnetic strength, and airgap can be adjusted to get the most out of a product. I am personally annoyed with the assumption that everyone is charging a 12 volt battery bank. This ignores the input range of 12V to 300V for battery banks and grid tied set points so we are not in a one size fits all world. A PMA that does not provide the 50 volts needed to cut in an Aurora Power One is useless to the person who hooks up any number of PMA’s up to it that will not make the 50 in voltage. If you want power…….we have it. We continue to build grow and bring new and innovative products to market…..
    Perfect Storm and we will continue to build bigger and more powerful units. Because the rest of American Wind Power with the exception of importers are stuck inside a Delco casing, we are not.
    Yes it does strike a nerve when your essentially labeled as a profiteer and a “warning” does not seem balanced. …. if “Wendy” has a problem with our product have her address it with me. I do appreciate your previous support and hope that I have not upset you as well. I can accept feedback as this is an essentially part of growth and becoming better at any pursuit


  14. admin says:

    Hi Tony,
    I am sorry if I upset you. You need to take the rough with the smooth. I have advertised your site for free these last 2 years here because I wanted people to know the options.

    The above is just a little balancing commentary. Wendy tells me that you get your alternators from Prestowind. You don’t even offer specifics. Prestowind offer the above misleading numbers. I just want to be sure that people are aware of the pitfalls. I may have been a bit rude, but my impression has been all along that you were out to sell people stuff, and I can’t object to that. I just want people to know that they have to use their own brains and perception just as they need to when buying any product.

    I am sure that your products do what they are designed to do. What you don’t actually tell us is what that is. You ought to offer curves of output at 12 volts (if that’s the design voltage, or a family of curves on a chart for other useful voltages) over a range of rpms. Then people could make an informed decision.

    Keep on doing what you do, but if you want credibility then publish some real numbers.

    All the best,

    and PS I don’t make any profit from my blog – I do it because I am an enthusiast. (And I hope it encourages people to buy my books)

  15. In response to your “story” about “these alternators”…..Do you own one? Have you tested it? Are you aware that is pretty arrogant at minimum to judge someone’s site as “nice photos and plausible sales talk”? Maybe we should point our high powered perception at you in such a manner? As you must be aware from the implied site review our generators come with different windings and will make more amps under a load with a heavier winding which is what should be used in the case of most 12 volt systems unless the site is in a “low wind” zone? You are generalizing an entire line based on your “presumption” the world is charging a 12 volt battery with our light winding PMA? I hope this is not Hugh’s commentary I would expect much more as all I see is arrogance, ignorance and judgment. We are not the guys advertising 1500 watts out of a delco alternator in the title line of the Ad as many do in venues such as ebay. Our products do what they are designed to. My Cat 5 provides power my rural property and I believe in our product. I constantly speak to people about realistic expectation and feel very much that many customers see that we have a dual PMA, others unscrupulously advertise delco PMA’s with outputs in excess of 1500 watts which is complete BS. They then assume our PMA’s output 3KW in a 12 volt battery environment because of dual rotors and stators. Let me go on record here for everyone…NO…..that’s not realistic. I certainly would not go and place a story on my blog warning everyone about Hugh Piggott’s Axial Flux Wind Turbines without testing one. I would hope you are not doing as such. You imply we are merely in search of the evil profit and you are doing everyone else a public service as a non-profit?
    Regards Anthony Jones
    Owner Hurricane Wind Power

  16. Wendy says:

    Many thanks for this explanation Hugh. It makes a vast amount of sense … now … with hindsight … We too were caught out by this. We watched the YouTube videos, were impressed by the power curves and naively thought these alternators would suit our hydro situation. They didn’t. And the company (Presto Wind) were not pleasant to deal with. It was a costly mistake: they refused to refund us our money and being outside the US, we had no effective means of pursuing a complaint. Our story with these alternators is here.

    I’m happy to say though that this experience sent us down the road of constructing one of your axial flux alternator designs and this DOES work.

  17. Audun says:

    Thank you for this post.
    I think many people should read this before they invest their money. I have bought a Hurricane cat 4 myself and I’m not satisfied with its output at all.
    Audun M.

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