Intertie question

Sep 4, 2004 09:37 am
Intertie question

Hi there I'm a newbie so I'm sorry if these seem like dumb questions. I've been researching installing a grid intertie system for about six weeks now and I've found a lot of good information and some misinformation. If some would be so kind to help me answer these two questions I would appreciate it.

Hypothetically, If I have one 120 Watt Panel working at 80% efficiency = 100 Watts for 10 hrs per day. This equals 1 K/Hr per day and 30KW/H per month that I would not being buying from my utility. Is this correct?

As far as components go for a interty system I know I need solar panels and a intertying inverter such as the Sunnyboy SMA 1800. What else do I need?


Charles @

20 Posts
Sep 4, 2004 10:13 am
Re: Intertie question

Yes, but if you live somewhere that ahs 10 hours of sun a day that is amazing.  Typically the US ranges from 3 to 6 sun hours a day, 3 being in the NE and 6 being in the SW.  Also to that 20% loss in the panel you will get another 10-15% in the inverter itself, although some batteryless systems are above 80% overall.

For a batteryles system an intertying inverter such as the Sunnyboy SMA 1800 and solar panels are all you need.

Sep 5, 2004 11:24 pm
Re: Intertie question

3 Posts
Sep 8, 2004 11:11 am
Re: Intertie question

That's right about the hours.  5.5 in San Diego (33 deg N.)

Parts:  Depending on where you live you'll need DC and AC disconnects to comply with code (to get the rebate) and you'll have to make sure the electrical panel is adequate for the additional current being added (although it is subtracted from some parts of the circuit).

4 Posts
Sep 10, 2004 10:55 am
Re: Intertie question

I have a Bergey XL-1 wind generator, 2 80watt solar panels, a Trace DR3624 invertor/charger and a Trace C35 12/24 charge controller. I live outside the US in a country where code is the last thing on anyones mind. I am in the tropics, lots of sunshine and on a hilltop with wind virtually year-round. I want to hook everything up to charge batteries (for the frequent power outages), use the power generated to augment my power needs and return excess power back into the grid during low power needs. I have heard that the meter will run backwards during this period. My questions are:

1. Is it possible to do this? (from a technical standpoint)
2. If so, how? A simple schematic would be helpful.


20 Posts
Sep 10, 2004 01:40 pm
Re: Intertie question

Well with a DR inverter you can't feed back in to the grid.  That inverter has no way of matching what is comming in so you would have to upgrade to a grid tie inverter to do this.

I would suggest just using as much of the power that you generate as you can, maybe adding a dump load such as a water heater element to use up any extra power, or maybe a window AC unit  Wink

4 Posts
Sep 11, 2004 12:17 pm
Re: Intertie question

Perhaps I also should have asked what does "matching what is coming in" mean? I would think that 120AC 60Hz is 120AC 60Hz? What does a grid tie inverter do differently?

A dump load would only run when the batteries were charged and there wasn't sufficient load to use all the power that is being generated..correct?

Thanks for your answers and help!

351 Posts
Sep 11, 2004 04:06 pm
Re: Intertie question

What does "matching what is coming in" mean?
A. It could have been a caveat that since you are in another country, you may not have 120vac 60hz power on the grid.
B. Verbal shorthand for you have a stand alone inverter.
C. both of the above.

A stand alone inverter works under the logic that it will only accept generation for the amount of load on the system, whether that is battery charging or the house load. They will work without being plugged into the grid. If plugged in, they will only accept power from the grid to charge the batteries or carry load. They do not put power out on to the grid, under any conditions. THIS is what you have.

An intertie inverter has a different logic that says I will accept all of the generation available, and put the excess at a higher potential than the grid, so that it will flow outward.
So, to do what you want, you need a new inverter. (Make sure it is an intertie with batteries, some intertie inverters are designed to function W/O batteries and have no provision for them)

Your other choice is to install what you have, and forget the intertie until you see how much excess you actually have. Since you have frequent grid outages, I doubt that you will have much (if any) excess.

"A dump load would only run when the batteries were charged and there wasn't sufficient load to use all the power that is being enerated..correct? "

Partially correct. A dump load is not designed "to use all of the power" that could be generated.  It is designed to put a base load on the wind turbine when the inverter is not accepting enough power from the wind turbine (medium-high winds and low electrical load). This prevents the wind turbine from going into a "no load" condition and overspeeding.

As such, it should be wired in so that it dumps excess generation from the wind turbine, not the solar panels.

If you do decide to go with the intertie system, you need to check with the local utility on their requirements for an intertie. Although most utilities accept running the meter backwards, some do not and require two meters, one incoming and one outgoing.

You kinda blew off the comment about AC & DC disconnects with your "code is the last thing on anyones mind" comment. Although most intertie inverters will recognize a grid outage and disconnect from the grid for the duration, some utilities will require a visible disconnect that can be physically opened and locked out by them during outages, so that they know their personnel will not be harmed by your system.  DC disconnects (particularly fused disconnects or breakers), should be installed, both for your safety and convenience.

4 Posts
Sep 13, 2004 06:37 pm
Re: Intertie question

Thanks for pointing out the differences between an intertie and stand alone invertors and dump loads. I don't want to go to the expense of buying another inverter and will stick with the one I have.
I wasn't trying to be indifferent to electrical workers' safety with the code comment (I didn't know how an intertie worked). I live in Central America and while there may be codes defined somewhere, they are not put into practice by anyone. Things here are pretty much jury-rigged and you are on your own with most everything.
Sep 14, 2004 04:40 am
Re: Intertie question

 If some would be so
>kind to help me answer these
>two questions I would appreciate it.
>Hypothetically, If I have one 120 Watt
>Panel working at 80% efficiency =
>100 Watts for 10 hrs per
>day. This equals 1 K/Hr per
>day and 30KW/H per month that
>I would not being buying from
>my utility. Is this correct?

No. PV modules typically only convert the energy availible from sunlight into electricity with an efficiency of around 14% to 23% at standard test conditions. When it is at its greatest intensity sunlight strikes the Earth at a rate between 600 to 1000 watts per square meter. Most 100 watt PV modules measure about a square meter give or take. Solar thermal panels, which rely on the infrared spectrum, can reach efficiencies of 80%.

Something more important to consider is the number of hours of equivilent full rated power output per day which is subject to varibles such as; a given region, the seasons, the manner in which the PV module is oriented in relation to the Sun, the number of hours it is in full unshaded sunlight and, the big one - weather.

So to put this into some kind of perspetive. Where I live in eastern NC. solar south is between 7 and 12 degrees west of south on a compass. A 100 watt PV module; mounted in such a way so that a 2"x4" block cut square (about 8" long), placed with its end against the face of the PV module at 12:00 noon on the shortest day of the year (Dec. 21) will cast no shadow and, the PV module is exposed to full sunlight on a cloudless day with no haze, from 8:00 am to 4:00 pm, will produce approx. 350 watt hours.
During the summer on the longest day (June 21) if all I did was lay the PV module down to compensate for the suns higher angle and, weather conditions were the same, the 2x4 block would cast no shadow at 1:15 pm and the PV module would produce approx. 600 watt hours.
The trick is to "tune in" the PV module as close as possible to being perpendicular to the sun at all times of the day but, trackers are not cost effective for all regions or locations. In some case's it would be more cost effective to spend that money on more PV modules.

>As far as components go for a
>interty system I know I need
>solar panels and a intertying inverter
>such as the Sunnyboy SMA 1800.
>What else do I need?

Start by contacting your electric power supplier. Each one has different rules or policiy's concerning how an RE system is "tied on" as well as how they compensate for the power provided from the RE system. You will need disconects between the PV module and the inverter and between the inverter and the meterbase. There may be a need to add a transfer switch as well. Oh! it wouldnt be a bad idea to spend the $40 and get a the most recent NEC handbook and study it, Article 690 in particlar.

Did any of the links I gave you before help? I thought for sure the rendered schematic in the Homepower article about "Solar Cheese" would be helpful. How is your research going by the way?

462 Posts
Oct 12, 2004 09:12 am
grid intertie

cycle, from the sound of your situation, I can't understand why you would want to tie into the grid, especially if the grid is not very reliable. Seems you would be better off getting a regular inverter and some batteries and keep the power you generater for yourself....

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