Grid Tie inverter question

raydias · August 2011

I have seen that most of the Grid Tie inverters have a start voltage of 150+. The question is wouldn't that mean that early morning and late evening when the voltage drops below the start point you no longer have power going into the grid. Wouldn't that mean you are missing out on power that could be used?

I notice that the lower start voltage is one of the advantages of micro inverters. Enphase has even made their current offering start at a lower voltage (16 for the m215 vs I believe 22 for the older models)

thoughts?

solarix · August 2011

Re: Grid Tie inverter question

It doesn't take long to get the string up to the inverter's minimum voltage.
PV panels are pretty much a current source meaning the voltage comes up quickly and then the current increases as the sun comes around. Plus if the string voltage is that low, then you have miniscule current and very little power. Not much is lost really. The trade off is that by limiting the input voltage range, the inverter's circuit can be better optimized for the real power zone and be more efficient / less costly as a result. You'll be amazed at how dark it is when your GT inverter starts up each morning.
And that Enphase spec is for one module so is 16V out of maybe 30Vmp
The GT spec was 150V for the string of maybe 400 - 550V so would actually start before the Enphase

SlimDiesel · August 2011

Re: Grid Tie inverter question

what solarix said...

My 4kW GT system goes on the grid before the sun is above the horizon and stays on until after it sets (at only a few watts, but still 'on').

rollandelliott · August 2011

Re: Grid Tie inverter question

some of the bigger grid tie inverters 6kw and higher start up around 300v and then it takes a lot of planning to get the string size correct.

mike95490 · August 2011

Re: Grid Tie inverter question

It takes a lot of planning to get any of this right. Each inverter mfg, has their own string calculator, so 90% of of the work is done for you.

rollandelliott · August 2011

Re: Grid Tie inverter question

http://stringtool.power-one.com/

is the only good string tool IMHO. most of the others do not give you various rowsxcollumns options and having to manually figure out how many rows/collums you need is time consuming.

raydias · August 2011

Re: Grid Tie inverter question

Thanks for the information. From the outside looking in it just seemed like you would get more power if the start voltage of the GTI was lower. Wonder if there are any studies on this out there?

BB. · August 2011

Re: Grid Tie inverter question

We have had lots of discussions about this...

My array has a Vmp of 300-400 VDC (hot to cold), and my GT inverter would start around 200 VDC... I have never seen it successfully start below 300 volts (unit would fault with insufficient array current to support operation), and was usually higher in the evening when the panels are cool and no direct sun on panels.

The amount of energy you can receive from the array at 1/2 Vmp-cold late in the evening from a 3.5 kW array is not enough to even power the input stage electronics (maybe 30 watts required from array?). Or enough to supply any meaningful power to the grid.

My inverter would run just fine at 300-400 volt Vmp suppling 0-10 watts to the grid... If it could have gotten more power at 200 VDC, it probably would have with as it was in its MPPT tracking range.

-Bill

System2 · August 2011

Re: Grid Tie inverter question

Here are voltage and current readings from one of my Enphase converters for a 24 hour period. You can see that there is almost instant voltage at very low light but no power until the current starts. Even if the inverter were to start working at 1 Volt there is simply no current available to make useful power.

raydias · August 2011

Re: Grid Tie inverter question

Thanks Jeff

it would be interesting to compare the readings from a centralized inverter. From the graph it looks like you start producing power right from the start and accelerate as the sun comes up. May not seem like a lot by over time I would think it would add up.

Reason i ask is to help me decide on which inverter to go with (centralized or micro). The fact that you can get to this granularity is a plus to me.

thanks

mike95490 · August 2011

Re: Grid Tie inverter question

A central inverter has the same charcteristics, only the voltage of the array swings more throughout the day as the panels heat & cool.
here is a link to a spreadsheet that lives on one of my pages, the spreadsheet has 3 tabs at the bottom, one has the volts/amps/watts of the array listed every 90 seconds. (from my log file)

A sample of my daily PV output charted, from a spreadsheet ( was a partially cloudy afternoon )

System2 · August 2011

Re: Grid Tie inverter question

Ray, There is so much more. Although you can only show two variables at a time on the graph you can pick any two from Power, Temperature, DC Current, and DC Voltage, and remember this is by individual panel. I know which panel has the bird poop on it!

I have a 27 panel 4.95KW DC array and am producing just over 34 KWH on a good day and just over 4KW at noontime.

at 27 panels I would have spent slightly less with a central inverter but going the Enphase route allowed me to slowly build up the number of panels on the roof. There is some value in being able to simply add one panel when you have the cash, and shading becomes essentially a non issue if just one or two panels get shaded for a while you do not lose an entire string.

Grid Tie inverter question

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