# Solar & Wind Power Excesses / back to the grid

• posted on September 10, 2010, 4:12 am

I was told that if a person operates a solar or wind generator, and if they produce more power than they need, this power is put back into the grid, and the producer is actually paid for the energy they make.
Ok, this all makes sense, but I'm a bit confused how this works. First off, I assume that a home solar panel or wind generator produces either 120VAC or 240VAC. And if I'm correct, the solar panels actually produce DC which is converted to AC.
Now, here comes the confusing part. If there is a surplus of power, it backfeeds into the grid and makes the persons electric meter go in reverse. But there is a pole transformer. On the input side of this transformer there is a high voltage of 15K (or other amount of volts).
So lets say I'm back feeding 120V into my pole transformer. Does this transformer actually work backwards and increase the voltage? Or does the person need some special transformer? And lets say that this back flow is putting 20amps of 120VAC into the grid. Don't the much higher current from the power company just sort of fight against this low current, or do they just blend together. I understand how power is generated using wind power, solar cells, or even a gasoline generator, but I cant comprehend how back flow into the grid works.
Thanks
James
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<%-name%>
• posted on September 10, 2010, 7:18 am
On 9/9/2010 9:12 PM snipped-for-privacy@myplace.com spake thus:

Short answer: yes, transformers work both ways. So if you put power into the secondary side (the side that normally feeds power to your house, for example), it flows out of the primary side into the rest of the grid. Of course, it's a little more complicated than this, but it does work that way. And you don't increase the voltage on the line; the voltage remains constant--120-0-120 from your house to the service drop. You increase the *current* flowing through the line (amps).
Another detail: the inverter (the part of the solar system that converts the DC from the photovoltaic cells to AC) must be synchronized to the exact power line frequency, so that the power put back into the grid is exactly in phase with the power coming out. Otherwise, the power you put into the grid would, most likely, be fighting against the power company's power.
By the way, it's not always true that the power company has to buy your power. This is what we've been told for many years, but here in California, for example, it turned out that PG&E didn't pay people who generated power back into the grid. This is now being addressed by the various regulatory agencies who are in charge of this area.
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The fashion in killing has an insouciant, flirty style this spring,
with the flaunting of well-defined muscle, wrapped in flags.
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<%-name%>
• posted on September 10, 2010, 2:59 pm

Good point. In many places the power companies do pay you for what you put back. But at the very least, how they pay you, the amount, etc varies widely. NJ for example is supposed to have the most generous payback in the US. You can get paid several thousand dollars a year for a typical home system. A small part of that is that the power company does pay you once a year for the excess you put back to the grid. But 90%+ of the thousands is actually from renewable energy credits. You get one credit for every so many Kwh of solar energy your system generates REGARDLESS of whether you use the energy or it goes back into the grid. In turn, NJ utilities by law have to acquire a certain percentage of their electricity from renewable sources. To meet that requirement, they buy these credits from homeowners and that is where the thousands come from. Of course the rest of the poor saps are paying for it in higher electricity bills.
In other areas where they pay you for the electricity, it's at some wholesale rate, not the retail rate you pay on your bill when you use it.
Bottom line, how you get paid, or if you get paid varies widely.
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• posted on September 10, 2010, 1:14 pm
On Sep 9, 11:12 pm, snipped-for-privacy@myplace.com wrote:

Depends.
Yes.
Yes.
No.
The same way the power company's generators "back flow" into the grid.
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• posted on September 10, 2010, 1:40 pm
On Sep 10, 12:12 am, snipped-for-privacy@myplace.com wrote:

To do this you would have a spcial system that syncronizes your local power exactly to the grid power. Allowing the gird and your local power source to be connected together. Then when you are using power your local source contributes slowing the meter. If you are not using power then your loal source would backfeed the grid. Transformers work in either direction. They are just two coils. The step up or step down is determined by the number of windings on each coil. So a pole transformer can be used in either direction. But unless you are in a rural area the odds are your local power source would be used by your neighbors before it ends up back on the hv part of the grid.
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• posted on September 10, 2010, 3:24 pm
On Thu, 09 Sep 2010 23:12:37 -0500, snipped-for-privacy@myplace.com wrote:

The grid tie inverters are driven by the grid and then allow excess power to flow back into the grid. If the grid is down the inverter shuts down. That is how they stay in sync. The voltage they generate is slightly higher than the grid so it will flow backward through the meter. If you have the standard meter, you will get 100% credit for the power you are feeding back. Most utilities would rather pay you less but this would require a special meter and for most installations the cost is not justified by the net amount you would be selling them. There are probably not a dozen residential installations in the country that actually generate more than they use over time. The trend is going toward micro inverters that simply plug into a regular outlet. This will not really require much in the way of permits or even telling the utility you are doing it. This is totally scalable. You just buy more as you can afford them and plug them in.
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• posted on September 10, 2010, 5:35 pm
snipped-for-privacy@aol.com wrote:

i've had mine installed for 3 months, and each month i've generated surplus power. i get a credit for future use, and get paid at the end of the year for excess credits.
i expect to generate surplus power every month, given that summer is the highest usage in my area (phoenix, az).

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• posted on September 10, 2010, 7:40 pm

Tell us what kindd of a system you have.
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• posted on September 10, 2010, 8:11 pm
hr(bob) snipped-for-privacy@att.net wrote:

standard grid tied pv cell. total of 7.5kw panels.
http://s587.photobucket.com/albums/ss312/chaniarts/House/?action=view&current=IMG_7618.jpg
http://s587.photobucket.com/albums/ss312/chaniarts/House/?action=view&current=IMG_7618.jpg#!oZZ2QQcurrentZZhttp%3A%2F%2Fs587.photobucket.com%2Falbums%2Fss312%2Fchaniarts%2FHouse%2F%3Faction%3Dview%26current%3DIMG_7617.jpg%26
in the 3 months, i've already had \$500 off my power bill from last year. payoff is expected to be in 4.5 years just from savings with current rates, although my electric company has already filed for rate increases. if i get any \$ back from the power company, it will be less than 4.5 years.
there should be no almost no maintenance except washing them off. warrantee for the converter is 10 years, and 25 years for the panels.
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<%-name%>
• posted on September 11, 2010, 9:10 pm

I don't understand how this could be paid off in 4.5 years. Assuming \$500 saved every 3 months, that's \$2K a year. Around here, NJ, a \$7.5KW system costs about \$50K. The federal tax credit can bring it down to \$35K and there can be other incentives, but still seems a long way from 4.5 year payback. ???
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• posted on September 13, 2010, 4:05 pm
snipped-for-privacy@optonline.net wrote:

well, you're not here :)
in a word: rebates. in my area at the time i placed the reservation with my power company, the total rebates (federal, state, and power company) were about 80%. due to the popularity, that's now down to about 40% in my area. also, the manufacturing of panels is done in my city, so they're a bit cheaper as my installer is the largest in az and buys local in large bulk.
further, note that the savings from above are in the summer, my high peak a/c season (phx, az). i should generate higher savings during the winter, unless i fire my kilns more. i actually sized the system to power my kilns (8kw), but i don't fire them constantly.
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• posted on September 13, 2010, 4:59 pm
Smitty Two wrote:

why? shortness of days? the data logger i have on it shows actually a decrease in production as temperatures rose during august and are just now showing a decrease due to day length. however, it's very gradual since the daytime temps are also starting to fall under 115. during june i was producing ~50kwh/day and it's down to maybe 45kwh/day now.
i doubt i'll see much more of a decrease, but we'll see. my normal winter power consumption is about 25% of summer consumption (gas heat) at most. even if i get a 50% drop, the savings will be more in winter than summer.
furthermore, we don't get snow or more than 4-5 days frost/winter, and don't get any rain so 99% of the time the sky is clear. we probably get 340+ days of sun/year. it's also cooler than summer, increasing power production.
peak power production here for pv is feb-june.
remember, location counts.
regards, charlie phx, az
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• posted on September 14, 2010, 8:13 pm

How are you going to generate higher savings when you use LESS electric? I guess you mean you'll be getting credit because you'll be generating more electricity than you use. But usually the solar system is sized to provide Kwh over the total year to be about equal to your total usage. So, usually what excess you get credited for in winter you wind up using yourself in summer, no? Also, what rate do the pay you for any net excess you have, if any over the year?
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• posted on September 14, 2010, 8:22 pm
snipped-for-privacy@optonline.net wrote:

it's sized to run my kilns, or oversized to my total year kwh if i chose not to run them as much. the excess generated is sold back at the cost i would pay for (actually, it's a kwh credit 1 for 1, which i draw down during the night or at high usage day time, depending if i generated that power credit during peak or non-peak time). so the payment for excess is what i would have paid for it if i were a net purchaser.
furthermore, i try to use more power at night or weekends, when i'm purchasing power at a lower rate (time differential billing), and generate during the day, when my reselling profit is more than my nighttime cost.
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• posted on September 14, 2010, 11:14 pm

You have a good deal on the electricity buyback. In many states, if they pay you for net generation, it's at the wholesale rate that the electric utility would buy power from it's regular sources. That amounts to a fraction of the price they charge retail customers.
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• posted on September 10, 2010, 8:27 pm
wrote:

I looked into a solar system but from what I understand there will be a MUCH cheaper solar cell available soon.

I have the perfect roof for a solar cell system. Faces south with full sunlight in Central TX.
I figure it's like computers or plasma TVs (we paid over \$700 for our 1st VCR); wait awhile and the price will drop dramatically. The quotes I got on available solar cell systems are not in my budget.
Jim