Solar Panels on residential Homes

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Open Neutral

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It's just basic electrical theory. The inverter develops a voltage, and that voltage seeks a path to discharge itself.

I prefer to think of the grid-tie inverters as current sources; ones that depend on the low impedance of the public grid to limit voltage extremes.

There's a rigorous standard that prevents "islanding" - i.e. the g-t units are always followers, never leaders. When the grid fails, they shut down, period.

As for if it makes sense, bottom line; that's a different, & complex question...
 

gar

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Solar panels alone at a constant light intensity and temperature are approximately a constant voltage device up to a knee in the curve (approximately maximum power point) then become an approximately constant current source. Neither of these characteristics are seen at the output of the inverter.

In a grid-tied system the inverter attempts to transfer maximum power from the solar panels to the grid system. If the grid voltage, solar energy, and ambient temperature are constant, then the inverter looks like a current source at its output. So it is easy to see how power company current supplied varies and load changes.

For constant conditions at the solar panel if the grid voltage drops, then the inverter current increases, and vice versa. So here the conditions are not constant current output, but maximum power output from the inverter.

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Open Neutral

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If the grid voltage, solar energy, and ambient temperature are constant, then the inverter looks like a current source at its output. So it is easy to see how power company current supplied varies and load changes.

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But in a practical sense, your g-t inverter is not going to raise the grid voltage more than a quarter sneeze worth.

My point was, on a stand-alone inverter design, you spend a lot of effort making it handle various load conditions while still maintaining 120.0V {or whatever...}. A g-t unit does not concern itself with that, but does have different plates to keep spinning in midair.

(Some few units do both. The SMA SunnyBoy line will tag along not just on Poco, but via some smart tricks, also the SunnyIsland backup line.

With that team, when the grid is down, the SunnyBoy can send its excess output into the SunnyInland which uses it to recharge its batteries.)
 

tallgirl

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But in a practical sense, your g-t inverter is not going to raise the grid voltage more than a quarter sneeze worth.

That depends. I can raise the secondary voltage by way more than a sneeze, and I've got clients who swing 18 volts (reference 120) when they go from "consume" to "produce". Positive voltage gradient is the Next Big Problem coming to a grid near you.

(Some few units do both. The SMA SunnyBoy line will tag along not just on Poco, but via some smart tricks, also the SunnyIsland backup line.

With that team, when the grid is down, the SunnyBoy can send its excess output into the SunnyInland which uses it to recharge its batteries.)

The SunnyIsland is very sweet, but it has more problems (including price) than one can shake a stick at.
 

gar

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Some of tallgirl's voltage rise might be expected. It will depend upon a number of factors.

I have no direct experience with solar installations but I will make an estimate based on my home electrical system.

Assume my base load is about 1.0 KW. For 8 hours at night my consumption is about 8 KWH. I have been averaging about 40 KWH per day total usage. Thus, daytime would be 32 KWH, and the average power during the day is 2 KW.

Assume there are 6 hours during the day the solar system can supply power to the home and to the grid. The grid needs to be supplied 40-(2*6) = 28 KWH per day during the time the solar is producing. When solar is producing the grid supplies nothing and is just receiving energy. Thus, the PV system needs to put 28 KWH into the grid during 6 hours, and at the same time feed the home load. During this 6 hour period the home voltage has to be higher than the grid voltage to force energy that way. To put 28 KWH into the grid in 6 hours requires 4.667 KW into the grid during that period. At 240 V it is close to 20 A to the grid. Actually the peak current into the grid will have to be larger because solar power does not just turn on to a stead average value.

In my home with a 25 KVA pole transformer a 12 A load causes a voltage drop on the 120 circuit of about 1.5 V. I have very low drop on the service wire. It is mostly in the transformer. Thus, on 240 about 3 V. Assume we actually need maybe 36 A in the middle of the day to get the 28 KWH pumped to the grid. Also assume that linear scaling of my 12 A and 3 V goes to 9 V when the grid is being fed at this peak, then you might see a swing of from somewhat above 9 V to 18 V from peak generation to peak loading when no generating was present.

The easiest way to determine what actually happens is to do what tallgril has done and that is to measure an actual operating system. However, calculations with some reasonable assumptions can produce an idea of what will happen, and an understanding of the system.

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tallgirl

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Gar,

The largest system I've personally worked on is 11.7KW. The system I asked about in that other post is 8.8KW.

I have a client putting a 40KW system in, but it's off grid. Another client had me consult (briefly ...) on a 600KW system.

It's fun stuff, and positive voltage gradient is going to be a problem in the future. FERC says +/- 5% (6 volts, reference 120VAC) is "normal" and +/- 10% (12 volts) is the absolute limit. So, the normal practice of running the system a bit "high" -- I have 124VAC right now, at 2-mumble AM -- means folks running wind and solar are at risk of being disconnected for high grid voltage.
 
Thanks for the info. I guess my question is in theory with the solar panels connected to the grid how does the resetential system know to push power out to the grid rather than pull it in from the grid? So if two sources (utility and solar panels) are both tied together then why does the home take all of its power from the solar and none from the grid if avalaible. How is it in theory that the excess power is able to be pushed out to the grid?

Does this have something to do with power system stability?
If load at the present house is less than the watts produce by the array the rest goes where ever the demand is and the demand would be the grid
 
solar panels

solar panels

Wow, suprised to see all the proffessional electricians so far off on all the answers, I which you could take one of Mike Holts classes on solar power. All these things can be easily made since of and especially installing and permiting to NEC standards. You don't want to learn this the hard way.

Solar grid tie inverters are true sine wave inverters that basically copy the power companys voltage sine wave and push back on the power supplyed to your home. It basically pushes back by having slightly more voltage then the power companys, that is why it is only recomended three inverters per voltage phase, you start to raise the voltage too much. When the presents of no voltage on the utility side it is unable to invert because the power to invert, (the inverters control relays are on the utility side. This cause the inverter on a grid tie system to shut down instantly. All inverter have a wait time when voltage is present to prevent short cycling. Good stuff
And yes solar power can reduce your electric bill and give you a good payback. You should currently pay no more than $4 per watt for solar grid tie power installed. You want to pay more, thats your business. I also find great paybacks on DC pool pumping and comming soon, DC air conditioneers. 600 watts take the place of 9 panels and an inverter converting to ac power. The new wave will be solar direct DC systems for pools, air conditioning and ventalation, and DC appliances. Plus solar panels are going down in price. Guess what, energy is going up. learn all you can about the new efficient products and chose wisely green products that work. This is the answer to our crapy ecconmy. See sunpowertechs.com for good solar pricing. PV
 

greenie

Member
In Wisconsin renewable energy systems are exempt from property taxes by state statue.
Other states should do this as well. But the greeds are always looking out for the big guys first.
 

don_resqcapt19

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That depends. I can raise the secondary voltage by way more than a sneeze, and I've got clients who swing 18 volts (reference 120) when they go from "consume" to "produce". ...
How is that possible with a properly designed and installed system? That implies an excessive voltage drop on the conductors between the inverter and the utility.
 

gar

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Ann Arbor, Michigan
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EE
101027-2348 EDT

Don:

Do you have some real world examples of voltage drop at a main panel for a loading of 50 A across a 240 V source? What are typical minimum and maximum values for residential installations? What transformer, wire, and length sizes? Doubling this drop would be an estimate of the total swing from sourcing to sinking.

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don_resqcapt19

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Gar,
It varies, but most I have looked at have been less than 5%. In Illinois, the utility is required to provide voltage between 113 and 127 at the customers service point. A 9 volt drop, assuming normal operation at mid range, would put the utility in violation of the Commerce Commission rules and subject them to administrative penalties.
 

electrichi01

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Hawaii
Solar

Solar

Solar panels and systems are more economical in states that have excellent incentives and ways to get faster returns on the investment.
www.dsireusa.org is a great site where you can learn about the incentives in your area.
On the business side, we believe not just knowing the incentives and how to, code wise is important, but also the business tools in job box are equally as valuable. Check out the latest here at the Solar Business Blueprint and learn how to optimize your business and not chase revenues, but profits.
 

PaulWDent

Member
Q: How does an inverter know what to do: Answer:

Q: How does an inverter know what to do: Answer:

Thanks for the info. I guess my question is in theory with the solar panels connected to the grid how does the resetential system know to push power out to the grid rather than pull it in from the grid? So if two sources (utility and solar panels) are both tied together then why does the home take all of its power from the solar and none from the grid if avalaible. How is it in theory that the excess power is able to be pushed out to the grid?

Does this have something to do with power system stability?

All a grid tie inverter does is push power back to the grid. It cannot power your home at all, if there is no utility. Your home is dependent upon the utility.

How it works is, briefly: The inverter is like a generator that syncs with the utility. If you try to speed it up, it will deliver more power to the utility; if you try to slow ot down, it will deliver less power. The inverter controls it own "speed" (phase) such that power it sucks out of the array and delivers to the utility causes the array voltage to drop (and the array current to rise) to the "Maximum Power Point", which is where the array is most efficient. This has nothing whatsoever to do with your house's consumption.

There are a couple of different types: (1) Dual Metering, and (2) Net Metering.

In dual metering, the PV inverter is connected through an "Export" meter direct to the utility drop though a breaker/AC disconnect. It does not go though your service panel nor your normal "import" electricity meter. You pay one price per KWH for the electricity you consume, as indicated on your "import" meter, and the utility pays you a different (lower) price for the KWHrs you have fed to the grid, as indicated on the "export" meter.

Now some states rightly thought it was so unfair to consumers that they should have to pay 10c/KWH for power they used but only get 5c/KWH for power they generated, that they mandated that utilities should offer the second system - net metering.
In Net Metering, the PV inverter is conncted to a breaker in the main service panel and feeds power back to the grid through your normal meter. So your meter registers the net difference (power used minus power generated) and you pay only for that. So you are effectively getting the same price for your own generation as you pay the utility. In theory, you could have enough panels on your house to gurantee that the meter went net backwards over a 12 month period. However, utilities have negotiated sweet deals with State Utility commissions that say they get to keep the excess over a 12-month period without having to send you a check.

It would seem feasible to have two service connections to your property e.g. one to the house and another to an outbuilding (barn or workshop) and ask for net metering on one and dual metering on the other, if you wanted the best of both worlds.

The above is for grid-tie. There is a whole 'nother type of system which can power your home in the absence of utility power called a "standalone" system, which requires a big storage battery charged by the array.
 

PaulWDent

Member
Property tax on solar panels: Not in NC at least

Property tax on solar panels: Not in NC at least

North Carolina exempts the value of a solar installation from being included in the value of yoiur property for tax purposes.

Gee those guys in the Capital did the right thing for once!
 

gar

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Ann Arbor, Michigan
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EE
110322-1448 EST

Don:

Your post 52 did not include a current level at which the 5% drop is allowed. Further if the power company wants to use marginal transformers relative to peak load, and this does make sense, then they can adjust the system to provide a nominal voltage higher than 120 and this allows for a larger % drop and not fall below 113. With their voltage regulators they can avoid overvoltage.

The important point is that a home with a large solar array, and the owners not home during the day (here these are called dog houses because only the dogs are home in great big houses), thus light load when the solar array needs to pump lots of power into the grid is likely to cause over voltage.

One place or another I have indicated that my noload voltage is generally in the range of 123 to 127 V. My normal load, 2 KW, only brings it down a little. My panel rating is 48 KVA and my pole transformer is most likely 25 KVA. If I ran a resistive load of 48 KW, and the transformer and service lines have a 5% impedance then the voltage drop would be about 10% at the main panel because the panel is rated twice that of the transformer.


PaulWDent:

You do not speed up an inverter to force energy on to the grid. You raise the house voltage. If the house voltage is greater than the grid, then some energy flows from the house to the grid. If the voltages are equal, then there is no transfer to or from the grid. If the house voltage is lower, then energy flows from the grid to the house.

.
 

iwire

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Location
Massachusetts
Don:

Your post 52 did not include a current level at which the 5% drop is allowed.

I always took it to mean the current level you provided to the power company when you applied for a service.

In other words if I applied for a service and told them my calculated load is 78 amps it is up to them to make sure they are in the voltage guidelines anywhere from 0 to 78 amps.
 

mull982

Senior Member
You do not speed up an inverter to force energy on to the grid. You raise the house voltage. If the house voltage is greater than the grid, then some energy flows from the house to the grid. If the voltages are equal, then there is no transfer to or from the grid. If the house voltage is lower, then energy flows from the grid to the house.

So in order to be able to pump excess power back onto the grid you have to raise the voltage coming out of the inverter to make it higher than the grid voltage? Is this done with an adjustment at the inverter.

Is it possible by adjusting the inverter voltage higher than the utility that you can supply the whole house load as well as feed back to the utility? (Assuming you have enough solar capacity)

So I asume that the only way to control what we put back on grid is through voltage control since we cannot control the source impedances?

If the inverter was set to the same voltage as the utility then I guess the utility and inverter would share the house load but what if inverter source impedance was much smaller than the utilities. Would any excess power created by solar inverter flow to utility or no since they were at the same voltage? Where would any excess power generated by solar array go?

So this sounds like this would be the same by controlling the voltage output on an On-Site generator in order to share the load. Is this correct?
 

gar

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mull982:

View the inverter output as a constant current source. The grid and its distribution transformer is a relatively low impedance source relative to the impedance of the inverter.

I believe way back in this thread I described distribution of current, therefore power, using batteries. If not here it was somewhere else.

If the inverter does not have enough power at any instant to supply all that is required of the house, then all of the inverter power goes to the house, and the remaining needed is supplied from the grid. This is all automatic based on the electrical circuit sources and impedances.

If by chance the exact amount of power required by the house is supplied by the inverter, then of necessity the house voltage exactly equals the grid voltage, and no current flows from the grid because the voltage difference is zero.

If the house requires less power than the amount being produced by the solar system, then the house voltage will be somewhat greater than the grid and power will flow to the grid.

Take two adjustable power supplies, use one in voltage mode and the other in current limit mode. Pick a suitable resistance for an internal impedance for the voltage supply. Some other resistances for the load. Simulate the system. Vary the load and observe what happens.

What your grid tied solar system does is to extract maximum power from the solar panels. The the inverter performs the maximizing operation, and feeds this power to the house load and when this power is greater than the house needs the excess automatically flows to the grid.

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