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I am currently purchasing an off the grid property with two buildings (a house and a shop). This property has a few small solar panels and a small wind generator with some battery storage and separate inverters for each bldg. Instead of purchasing additional solar or wind power capacity (along with storage capacity) I am investigating the purchase of a PTO powered generator to provide the energy I envision needing to operate a future wood shop and potential greenhouse(s). Existing utility service is available but is not close enough to acquire without some additional expense beyond the 100 ft the utility will provide for free. I have an opportunity to purchase either a 25 KW single or a 25 KW 3 phase PTO generator. My understanding is that I can operate (balanced) single phase equipment and systems within a 3 phase service but not necessarily the other way around. My questions are:

1) Is 3 phase power more efficient to generate/use?

2) How difficult or complicated is it going to be to draw a balanced single phase load(s) from a 3 phase service?

3) If I decide to get connected to the utility at some point, will I be able to sell my excess power back to the utility if I am generating 3 phase power?

4) Is there anything else I should be aware of when determining whether to generate single or 3 phase power for my use?

Thank you in advance for any input anybody may provide.

Thanks for the responses. After more research I realize that since I live in the country and there is no 3 phase available from the utility lines that would service my property, I would have to convert any 3 phase power I generated before the utility would allow an inter-tie to my generation. That would require additional expense for a converter and I assume there would be some loss(?) in the conversion?? Anyway, to better understand my questions and concerns (as well as highlighting my electrical ignorance) I would like to expand my story and circumstances.

Here is my scenario. I am purchasing a 2 acre property that has two buildings; a 16'x24' insulated, wired and plumbed single story steel pole bldg (converted dog kennel/house) and a 20'x24' plywood clad, uninsulated, unwired pole shed (workshop). There is no electric service on the property as the original service was provided by a neighboring parcel which was subdivided and sold. The original electrical supply lines were removed and currently the closest transformer is approximately 300' from the closest bldg. There is propane service and wood stoves together with a few small solar panels and a small wind generator hooked up with battery banks and inverters that supply a minimal amount of energy and heating for the bldgs. I have the capability to make biodiesel and clean veg oil and a couple diesel engines that can be used as power plants. I am looking into getting a PTO driven generator to attach to the diesel engine which then can power the workshop and home and potentially a greenhouse in the future. I envision also reclaiming and utilizing waste heat from this setup for space and/or water heating/cooling. Looking forward as I start to layout electrical service and outlets in the workshop and contemplate a separate aquaculture greenhouse in the future, I am trying to determine if my needs would best be served by providing 3 phase or single phase power for the tools and equipment I will be utilizing. I currently do not own any 3 phase equipment but I am aware of the ability to acquire such at considerably reduced cost via auctions and used equipment sales and am cautiously leaning in that direction. My concerns however center around how costly and complicated it is going to be to separate and provide my single phase needs with a 3 phase generator. My cursory understanding is that the loads need to be balanced. As it is my intention to start with a 25kW generator, I am also concerned about not wasting the excess capacity of the generator as my initial needs are considerably less than the 25kW potential maximum. Therefore, I want to investigate if it would be cost effective to pursue a grid intertie and dump my excess capacity to the grid as opposed to just dumping it or before designing an alternative storage/utilization capacity somehow (e.g. batteries, water pumping, alcohol distillation, thermal storage, greenhouse lighting, etc. etc.) I assume that a future aquaculture venture would likely benefit from 3 phase equipment (lighting, pumps, ventilation, etc.)

In desiring self sufficiency and having unutilized waste of any kind as a pet peeve, I guess one of my main questions is; Is 3 phase a more efficient method of utilizing electrical energy and would it be smart to start with 3 phase generation and step down to provide single phase and possibly grid intertie requirements or should I generate single phase and step it up for specific 3 phase applications? Although I have a philosophical commitment to renewable and life-affirming systems thinking I am not independently wealthy (just a poor carpenter) and cost is definitely a factor in this decision-making process. Therefore, I would like to know if installing 3 phase distribution infrastructure (wiring, transfer switch(s), inverters/converters, etc.) would be smarter than installing single phase distribution infrastructure (wiring, transfer switch(s), inverters/converters, etc.)? I know that without specific equipment specs and firm usage applications it is impossible to arrive at precise cost estimates but in generality, which is better and why, 3 phase or single phase? Opinions anyone??

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    \$\begingroup\$ I'm voting to close this question as off-topic because ask a lawyer, law maker, or your local utility company \$\endgroup\$
    – Passerby
    Dec 5, 2016 at 22:17
  • \$\begingroup\$ Selling back power should be in POLITICS stack exchange \$\endgroup\$
    – Autistic
    Dec 6, 2016 at 11:00

3 Answers 3

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To answer the other part of your question.

You cannot simply "sell power back to the utility", they have, in most jurisdictions, no immediate obligation to accept it. The way is usually works is that you must apply for a "Net Metering Contract" with them, meaning that you will be connected to the Grid, and drawing FROM the grid when you are not producing, then supplying TO the grid when you are, the bill you pay them at the end of the month is the net difference.

Reality check: it's NEVER going to be that you get paid more than they sell to you, they totally control the transaction. The main reason is, you must provide what is called "Utility Interactive Equipment", meaning the equipment that you use to connect to the grid must allow THEM to control it. If not, then it can be DEADLY for their line workers. Let's say a utility line goes down and they have to send a lineman out to repair it. Before he starts working, they kill all of the power to that line. If you were uncontrolled, you could all of a sudden stat putting your power on the grid WHILE he is handling the wires and kill him! They cannot allow that to happen so regulations require that YOU provide that interactive equipment that they CAN control. When you do that, and you approach producing more power than you have used from them, they simply shut your connection off. They are NOT going to cut you a check at the end of the month...

But certainly you CAN effectively reduce your net consumption to a bare minimum.

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  • \$\begingroup\$ Thanks for the reply. I am aware of the need for safety for line workers but can see how your comments may in fact be a reality under specific locations and circumstances. Since I hope to never need utility power and here in WI will only receive about 1/3 the price for generation compared to cost for consumption, it may be a real stretch to justify the costs and hoop jumping needed just to tie in and utilize my excess generation capacity. Cheers! \$\endgroup\$
    – T Hud
    Dec 6, 2016 at 21:08
  • \$\begingroup\$ @THud You may be more interested in setting up a battery bank so you can use your excess generation capacity at times when you aren't generating. \$\endgroup\$
    – user253751
    Dec 7, 2016 at 0:42
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Three-phase is not inherently better for single phase loads, although true 3ph motors tend to run smoother and quieter.

I see no reason why you couldn't sell power back to the grid, provided you electricity supplier was ok with it. (if you have an older mechanical power meter, then as long as you used more than you exported, they'd never know ;)

As for balancing the 3ph loads... are you aware that all power stations generate 3ph power anyway? If precision balancing was an issue for them, all houses would be 3ph.

Although, if you are the majority user of a local transformer and said transformer is not a Wye type (i.e.delta wound) then significant voltage imbalances can occur, whereas the Wye transformer will shunt the current imbalance through the neutral (which is not really ideal) but at least you'll still have pretty balanced supply voltages.

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  • \$\begingroup\$ Thanks for your reply. It is my intention to not ever need power from the grid except perhaps during a system maintenance or catastrophic failure situation so I am left with determining whether or not the hassle and expense of tie-in with the grid is worth it. Since I will be starting with a 25kW generation capacity and presently only need a fraction of that for my own use, I am investigating means to utilize what would otherwise be lost or wasted capacity. I will expand my circumstances shortly in an answer to my question and hope that people can add comments or respond to it. \$\endgroup\$
    – T Hud
    Dec 6, 2016 at 20:49
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I will try my best to answer all your questions

1) Yes, three phase power should be more efficient in generating power. The main reason for this is quite simple actually. Three phase motor are always more efficient than single phase one. I will not really go into the technicals details of this relationship because it is a bit off topic for your question. I suppose your PTO is a kind of a gas engine powering an alternator. The alternator will be more efficient if it is a three-phased machine than a single phase one.

The same is also true when powering equipement for your wood mill. Three phase motor are not only more efficient, they also don't rely on a capacitor to provide the starting torque. A three phase motor can generate torque at zero speed. The problem I can see is that I'm not sure of the scope of your wood mill. If you need industrial equipement, you definatly need to use three phase power. However, if its use is more in the scope of a residential small wood shop, most equipement you can buy will be based on single phase motor. That brings us to your second question.

2) Balancing a three phase system with single phase loads means that you need to put the same load on each phase at all time. The complexity this will generate depends a lot on the ratio of momentary load versus permanent load. The ratio is not based on the actual number of loads from each type but on the total power associated to each type.

A momentary load exemple could be a single phase motor that is occasionnaly turned on. A permanent load exemple is more like an heater or single phase motor that always runs. If you have a lot of powerful single phase momentary loads, balancing can be an issue. In your case, I would say single phase motor with power rating more than 5 HP can poses a threat but that depends on how good the PTO regulate. Balancing permanent loads is quite easy, you check the power rating of each load and you dispatch them to make all three phases push the same power. To help you balance your power grid, measuring the current from each phase is a good place to start. An unbalanced system may distabilize your PTO and make it stall.

3) Yes but you will need a three-phase to single-phase converter because you need to be in sync with the utily voltage phase. These kind of converter works by converting the three phase ac voltage to a dc voltage. The DC voltage is then inverted to a single phase voltage and you have a total control of how much power you sell to the utility. You can also have a system that convert single phase voltage to a three phase voltage if you ever need to draw power from the grid. I would you ask your utility provider about such systems as they can give you precious recommendations.

4) Remember that your power system is based on the loads it needs to provide, not the contrary. If you plan to use a lot of motor loads at significant power rating, I recommend you to go to three phase power as using three phase motor is more efficient and reliable. Based on your 25 kW max power, if you need a motor above 10 HP, I think going three phase worth it. If you only have a few motor of 1-2 HP in power rating, then going three phase might not worth the hassle to be honest...

I hope my answers to your questions were clear enough...

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  • \$\begingroup\$ Thank you very, very much for your thorough and thoughtful answers. I am going to expand my circumstances further in an answer to my own question so if you wish to further comment, I'm confident I would learn more if you choose to comment further. \$\endgroup\$
    – T Hud
    Dec 6, 2016 at 21:02

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