Current from panels when using an mppt gridtie inverter

Keng · March 2020

When using an mppt gridtie inverter and you have say 4 - 275w panels . 1100w total. The panels are connected in series and have an open voltage of 40v. So the open voltage on all 4 is 160v. Each panel has a current rating of 8+ amps. What would the current be on all 4 panel to the gridtie inverter? Would it be 1100w ÷ 160v = 6.875 amps? Max. Or less?

Thanks, Ken g

BB. · March 2020

It is a bit more complicated than that... You have Voc -- Voltage open circuit -- When no current is flowing, you have Voc~40 volts. But because Power=Voltage*Current. So if Current (I) = zero Amps, then power is also zero Watts.

There is an IV (current/voltage) curve, where you plot P=V*I or Pmp=Vmp*Imp ... There is a peak at Vmp (for your panel, around 34.4 volts)... And your Imp would be:

Pmp=Vmp*Imp
Imp=Pmp/Vmp= 275 Watts / 34.4 Volts = 7.994 Amps Imp

Now, Vmp is actually (mostly) a value based on the temperature of the solar cells/panel. As the cells get hotter, Vmp falls... A very hot panel on a windless day under full sun, So, you could have Vmp-array-hot~34.4 volts * 0.80 hot panel derating = ~27.5 votts.

Imp is based on how much sun hits the panel... Roughly, on a clear day, at noon-time you will get ~100% of the sun's energy (~1,000 Watts per sq meter). And you will get 100% of Imp. Of course, any clouds/haze and off sun angle (Ipanel = Imp (rated full sun) * Cos angle to sun).

Imp is slightly temperature sensitive (Imp slight rises as cells get hotter). However, it is a small value and typically ignored.

So, your 4x 275 Watt panels on a hot summer day would be:

4x 34.4 volts Vmp * 0.80 hot panel derating = 110.1 Volts Vmp-hot (more or less worse case)
Imp-array = 7.99 amps (under standard sun).

An MPPT based controller is designed to figure out the Vmp of the connected array--Basically figuring on the temperature derating of the panel by varying the current draw from the array (every few minutes, ramp from zero to maximum current, measure Vmp-array*Imp-array and find Pmp). Once Pmax power is found, then hold Vmp-array for the next few minutes (for another sweep) by controlling Iarray (draw slightly less current, Varray rises, draw slightly more current, Varray falls a bit). Of course, this is how holder MPPT controllers used to work. There are new algorithms that do the same thing with a different method.

This thread has some Current/Voltage curves from solar panels the show graphically the P=V*I equation over different sun and temperature conditions:

https://forum.solar-electric.com/discussion/5458/two-strings-in-parallel-with-unequal-string-voltages

-Bill

Keng · March 2020

BB. said:

It is a bit more complicated than that... You have Voc -- Voltage open circuit -- When no current is flowing, you have Voc~40 volts. But because Power=Voltage*Current. So if Current (I) = zero Amps, then power is also zero Watts.

There is an IV (current/voltage) curve, where you plot P=V*I or Pmp=Vmp*Imp ... There is a peak at Vmp (for your panel, around 34.4 volts)... And your Imp would be:
Pmp=Vmp*Imp
Imp=Pmp/Vmp= 275 Watts / 34.4 Volts = 7.994 Amps Imp
Now, Vmp is actually (mostly) a value based on the temperature of the solar cells/panel. As the cells get hotter, Vmp falls... A very hot panel on a windless day under full sun, So, you could have Vmp-array-hot~34.4 volts * 0.80 hot panel derating = ~27.5 votts.

Imp is based on how much sun hits the panel... Roughly, on a clear day, at noon-time you will get ~100% of the sun's energy (~1,000 Watts per sq meter). And you will get 100% of Imp. Of course, any clouds/haze and off sun angle (Ipanel = Imp (rated full sun) * Cos angle to sun).

Imp is slightly temperature sensitive (Imp slight rises as cells get hotter). However, it is a small value and typically ignored.

So, your 4x 275 Watt panels on a hot summer day would be:
4x 34.4 volts Vmp * 0.80 hot panel derating = 110.1 Volts Vmp-hot (more or less worse case)
Imp-array = 7.99 amps (under standard sun).
An MPPT based controller is designed to figure out the Vmp of the connected array--Basically figuring on the temperature derating of the panel by varying the current draw from the array (every few minutes, ramp from zero to maximum current, measure Vmp-array*Imp-array and find Pmp). Once Pmax power is found, then hold Vmp-array for the next few minutes (for another sweep) by controlling Iarray (draw slightly less current, Varray rises, draw slightly more current, Varray falls a bit). Of course, this is how holder MPPT controllers used to work. There are new algorithms that do the same thing with a different method.

This thread has some Current/Voltage curves from solar panels the show graphically the P=V*I equation over different sun and temperature conditions:

https://forum.solar-electric.com/discussion/5458/two-strings-in-parallel-with-unequal-string-voltages

-Bill
Thanks Bill, That's exactly what I wanted to know. I've been looking into getting a gridtie inverter. I've had these panels for a while using them off grid. I just got 4 more now, so I have a total of 8. I was going to connect 2 sets of 4 in series. My mppt charge controller has a max voltage of 200v. Tha gridtie inverter I've been looking at has a startup voltage of 120v. Will that inverter not work well with my setup?

Thanks again, Ken

BB. · March 2020

Hi Ken,

It really depends on the MPPT GT inverter specifications... Do you have a link?

You have some critical voltages... Voc-array-cold (not exceed 200 Volts), and Vmp-array-hot (not sure what minimum input voltage is, start up of 120 Volts). That is a (relatively) narrow range for the solar array input... Worst case, the largest range between Voc-cold and Vmp-hot works out to be around 2:1 (i.e., around 55 volts Voc-cold to 27.4 volts Vmp-hot--Sub freezing weather to something like 100F).

You can use the equations (plug them into a spread sheet makes it easier), and some companies will list what V-array voltages or at Standard Temperature (i.e., Marketing numbers of Voc-std and Imp-std).

And some companies offer a web page (or even computer program) to calculate the min/max voltages. Midnite, for their off grid MPPT charge controller has one where you can enter in your panel information and Hi/Low temperatures and get a quick answer:
9
For example, here are a couple equations for Voc (at 0F/-18C) and Vmp (100F/38C) and assume solar cells are +20C higher under full sun on windless days (note the 0.0033 and 0.0045 are "negative" or voltage drop, but too many "- -" signs is getting confusing--Just adjust the 25C - Temp order):

(Used temperature coefficients from here: http://www.midnitesolar.com/sizingTool/index.php )

Voc-cold = Voc(25C) / (1 - 0.0033(25C standard temperature - Tabient temp)
Voc-(-18C) = 40.4 volts Voc(@25C) / (1 - 0.0033 %change degree C (25C - -18C)) = 47 Volts Voc-(0F/-18C)
200 Volt max input for GT inverter / 47 volts (0F) = 4.2 panels max = 4 panels in series max (0F)
Vmp-hot = Vmp(25C) / (1 - 0.0045(25C - Tamb - 20C Trise over ambient in hot sun))
Vmp-(38C) = 34.4 volts Vmp(@25C) / (1 - 0.0045(25C - 38C - 20Crise) = 30 Volts Vmp(100F/38C/no wind)
120 Volts (starting voltage) / 30 volts Vmp(hot) = 4 panels in series

So, roughly speaking, putting 4 panels in series (assuming Voc and Vmp I guessed at), would seem to be the "optimum" for a location with a wide temperature range (something like a MIn/Max of 0F to 100F around Pittsburgh PA or similar).

I think the equations are correct... I kind of put them together on the fly here. As always, verify all information and calculations. And I made guesstimates about your panel's specifications.

-Bill

BB. · March 2020

Because hot panels produce less Vmp voltage... We kind of assume that solar panels+GT inverters will output (on average) about 77% of the solar panel name plate rating (the highest typical Power output into your home's electrical system). For example, if you have 4 panels connected to your GT inverter, then the minimum suggested rating of your GT inverter would be (for best power harvest):

4 x 275 Watts (standard temp) * 0.77 panel+GT inverter deratings = 847 Watt inverter (suggested GT inverter minimum size)

Of course you could go with a larger inverter (1,000 or 1,500+ Watts)... And typically you can go with even a smaller inverter (MPPT systems, and GT inverters will normally/safely limit their output current/power to max rated output--Even if you have "more solar panel" power).

At times during the year, the solar array may output more than ~847 Watts, but it will only be on cool/clear days, by a little bit, and for not that long of time, so you do not lose significant harvested energy.

-Bill

Keng · March 2020

BB. said:

Because hot panels produce less Vmp voltage... We kind of assume that solar panels+GT inverters will output (on average) about 77% of the solar panel name plate rating (the highest typical Power output into your home's electrical system). For example, if you have 4 panels connected to your GT inverter, then the minimum suggested rating of your GT inverter would be (for best power harvest):
4 x 275 Watts (standard temp) * 0.77 panel+GT inverter deratings = 847 Watt inverter (suggested GT inverter minimum size)
Of course you could go with a larger inverter (1,000 or 1,500+ Watts)... And typically you can go with even a smaller inverter (MPPT systems, and GT inverters will normally/safely limit their output current/power to max rated output--Even if you have "more solar panel" power).

At times during the year, the solar array may output more than ~847 Watts, but it will only be on cool/clear days, by a little bit, and for not that long of time, so you do not lose significant harvested energy.

-Bill

BB. said:

Because hot panels produce less Vmp voltage... We kind of assume that solar panels+GT inverters will output (on average) about 77% of the solar panel name plate rating (the highest typical Power output into your home's electrical system). For example, if you have 4 panels connected to your GT inverter, then the minimum suggested rating of your GT inverter would be (for best power harvest):
4 x 275 Watts (standard temp) * 0.77 panel+GT inverter deratings = 847 Watt inverter (suggested GT inverter minimum size)
Of course you could go with a larger inverter (1,000 or 1,500+ Watts)... And typically you can go with even a smaller inverter (MPPT systems, and GT inverters will normally/safely limit their output current/power to max rated output--Even if you have "more solar panel" power).

At times during the year, the solar array may output more than ~847 Watts, but it will only be on cool/clear days, by a little bit, and for not that long of time, so you do not lose significant harvested energy.

-Bill

Hi Bill,
I really appreciate all the great information. I'm getting older and on disability so I can't afford a high cost GT inverter. I've looked at so many and this is the only one that fits my price range. It has a lot of good reviews and a few bad. It's the ABB Aurora Powerone PV1-3.0..

Hopefully this one will work for me. Like I said I have a total of 8-275w panels now. I guess I could put all 8 in series for higher voltage, or do 2 strings of 4 in series. The 2 channels on the inverter can be separate or in parallel.

Thanks again,

Ken

BB. · March 2020

Ken,

For this inverter, 8x panels in series on one channel would seem to be the right configuration. And you could put another 8x panels on a second channel if you ever wanted. It would be a little oversized (8x 275 Watt panels * 0.77 derating = 3,388 Watt recommended minimum).... But it should work fine and not really lose much harvest. Or you could put 7x panels x2 parallel and be at 2,965 Watts derated.

4x in series is too low of voltage to run the system (may not even start).

As to the brand--I am not in the business, so I really do not have much to add about this.

GT Solar power is pretty cheap these days (panels are cheap, GT inverters are not expensive). HOWEVER, you are generating your own power, so you are responsible for maintenance and following up on warranty and out of warranty work.

In general, I always suggest looking at conservation first. Replace old/inefficient appliances, insulate walls/ceilings/HVAC ducts, installed LED lights, turn off appliances not being used (computers, entertainment systems, lights, etc.). If you have never done any conservation work, it can be possible to reduce your energy usage by upwards of 50% without really crimping your lifestyle.

In general, is cheaper to conserve energy than to generate it.

Do you have an installer lined up? And you are going to work with your utility and building department to do this install? In theory, the (very rough) estimate for a GT system full turnkey install is the range of $3 to $5 per Watt (14x panel system = 3,850 Watts; 3,850 Watts * $5 per Watt installed = $19,250 or less. (I am not in the business, so these numbers are just what I have read recently--Your installation will be different).

Just to give you an idea of how much energy you could harvest:

https://pvwatts.nrel.gov/pvwatts.php

8x 275 Watt system = 2,200 Watt array (2.2 kWatt), for Pittsburgh PA, facing south. fixed array, 20 degree sloped roof, assuming $0.114 per kWH:

System output may range from 2,586 to 2,787 kWh per year near this location.
Click HERE for more information.

Month	Solar Radiation ( kWh / m2 / day )	AC Energy ( kWh )	Value ( $ )
January	2.52	142	16
February	3.50	175	20
March	4.11	220	25
April	5.13	256	29
May	5.73	290	33
June	6.03	287	33
July	6.21	299	34
August	5.97	289	33
September	5.28	252	29
October	3.84	194	22
November	3.03	157	18
December	2.18	122	14
Annual	4.46	2,683	$ 306

At an install cost of (roughly--$5 * 2,200 Watt system = ) $11,000

$306 per year / $11,000 install costs = 2.8% return on your money

And remember, the GT inverter may last ~10+ years -- So you probably have to replace it before 20 years go by... Panels should last 20+ years pretty easily (assuming no golf ball size hail storms, no rocks from kids, etc.).

My power costs in California are much higher (something like $0.20 to $0.40 per kWH).

The numbers above my not be accurate... Making lots of guesses here--But you can correct them for your location, system size, actual costs to install, etc... This is quick and dirty example of trying to "monetize" your investment.

-Bill

Keng · March 2020

BB. said:

Ken,

For this inverter, 8x panels in series on one channel would seem to be the right configuration. And you could put another 8x panels on a second channel if you ever wanted. It would be a little oversized (8x 275 Watt panels * 0.77 derating = 3,388 Watt recommended minimum).... But it should work fine and not really lose much harvest. Or you could put 7x panels x2 parallel and be at 2,965 Watts derated.

4x in series is too low of voltage to run the system (may not even start).

As to the brand--I am not in the business, so I really do not have much to add about this.

GT Solar power is pretty cheap these days (panels are cheap, GT inverters are not expensive). HOWEVER, you are generating your own power, so you are responsible for maintenance and following up on warranty and out of warranty work.

In general, I always suggest looking at conservation first. Replace old/inefficient appliances, insulate walls/ceilings/HVAC ducts, installed LED lights, turn off appliances not being used (computers, entertainment systems, lights, etc.). If you have never done any conservation work, it can be possible to reduce your energy usage by upwards of 50% without really crimping your lifestyle.

In general, is cheaper to conserve energy than to generate it.

Do you have an installer lined up? And you are going to work with your utility and building department to do this install? In theory, the (very rough) estimate for a GT system full turnkey install is the range of $3 to $5 per Watt (14x panel system = 3,850 Watts; 3,850 Watts * $5 per Watt installed = $19,250 or less. (I am not in the business, so these numbers are just what I have read recently--Your installation will be different).

Just to give you an idea of how much energy you could harvest:

https://pvwatts.nrel.gov/pvwatts.php

8x 275 Watt system = 2,200 Watt array (2.2 kWatt), for Pittsburgh PA, facing south. fixed array, 20 degree sloped roof, assuming $0.114 per kWH:

System output may range from 2,586 to 2,787 kWh per year near this location.
Click HERE for more information.
Month Solar Radiation
( kWh / m2 / day )
AC Energy
( kWh )
Value
( $ )
January 2.52 142 16
February 3.50 175 20
March 4.11 220 25
April 5.13 256 29
May 5.73 290 33
June 6.03 287 33
July 6.21 299 34
August 5.97 289 33
September 5.28 252 29
October 3.84 194 22
November 3.03 157 18
December 2.18 122 14
Annual 4.46 2,683 $ 306

At an install cost of (roughly--$5 * 2,200 Watt system = ) $11,000

$306 per year / $11,000 install costs = 2.8% return on your money

And remember, the GT inverter may last ~10+ years -- So you probably have to replace it before 20 years go by... Panels should last 20+ years pretty easily (assuming no golf ball size hail storms, no rocks from kids, etc.).

My power costs in California are much higher (something like $0.20 to $0.40 per kWH).

The numbers above my not be accurate... Making lots of guesses here--But you can correct them for your location, system size, actual costs to install, etc... This is quick and dirty example of trying to "monetize" your investment.

-Bill

Hi Bill,

Thanks for all your information. I'm actually a retired-disabled building contractor. I did that for almost 30 years then started having some health issues. I did all of my electrical work. My father was an electrical contractor and I use to help him when I was as young as 8 years old. I only recently got interested in alternative energy, so I'm still learning especially when it comes to gridtie inverters. When I start I purchased a couple solar panels and a 2000w wind turbine which I built a 60 foot tower with a gin pole. I have it on two of the cheap Chinese 1000w gridtie inverters. The most I've seen it make is 1300w. That's few and far between though.. I kinda wish I would have put all the money I spent on that into solar. I've also been working on hydro. We have 10 acres with a stream on our property. My goal is to make at least 200w or so 24/7. I'm in Pennsylvania in the mountains.

Sorry, photos are kinda dark.

Again, I'm glad you gave me all the information you have. It don't take me long to learn. Sometimes it's hard to find the correct information that your looking for.

Ken

BB. · March 2020

Ken,

It looks like you are well on your way there... With solar electric panels, ANY SHADING on one or more panels can kill the harvest... Depending on the configuraiton and the GT inverter, just the shadow from an electrical line (or chimney, etc.) across a part of the panel, will kill the entire panel's output and could kill 1/2 the total array's output, or more (i.e., a power line shadow across the whole array).

Assuming you can find an area without shading from trees (9am-3pm or so minimum), you should be off to a good start.

Depending on your needs and desires... A DIY Wind Turbine is not bad--And frequently better than you can buy (plus you can repair yourself). Similar if you want to install one or several solar hot water panels and plumb them to a water heater for hot water/home heating (if you are into plumbing).

-Bill

Keng · March 2020

This is my first setup with the 4 panels. I'm in the process of building the second identical setup. The way I designed them I'll probably building at least a 1 axis tracking system. I can rotate the top portion. It will tilt also, but I don't think that will be necessary. I don't get any shade on them all year round. A lot of work, but very gratifying.

Ken

BB. · March 2020

Difficult to tell from pictures, and I am not a structural engineer, but I am wondering if your pole mount is strong enough to withstand high winds...

I fear that the pole may just blow over (and damage your array).

Bill

mike95490 · March 2020

Either the ground has some tilt, or your array pole is bending already !!

Keng · March 2020

BB. said:

Difficult to tell from pictures, and I am not a structural engineer, but I am wondering if your pole mount is strong enough to withstand high winds...
I fear that the pole may just blow over (and damage your array).
Bill

Hi Bill,
It's main pipe is 3 1/2" with 1/4" wall. It goes 4 ft deep. There's a 16" diameter concrete slab that deep. I drove 8 - 8 ft pieces of rebar on an angle, 2 in each direction. It's been up for 7 years and seen over 70 mph winds. My son who weighs probably close to 300 lbs can hang on the very end. I don't think it's going anywhere. The photo may be deceiving.

Ken

Keng · March 2020

mike95490 said:

Either the ground has some tilt, or your array pole is bending already !!

mike95490 said:

Either the ground has some tilt, or your array pole is bending already !!

The ground does slope a little. It's plumb and straight.

Ken

BB. · March 2020

Sounds good Ken!

-Bill

Current from panels when using an mppt gridtie inverter

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