Should a Grid Tie Inverter be Under or Over sized for the PV Array it supports?

rollandelliott · June 2012

I was messing around with the Sunny Boy string sizing software and read the help section saying most grid tie inverters are undersized in Europe? Why is this? Is it because panels are typically LESS effcient than their STC specifications indicate. Thus in the real world the extra panels make up for the power lost due to temperature and other derating factors?

By undersized I mean they might have a 5KW inverter hooked up to 5.5KW worth of panels.

Obviously you want the gridtie inverter to match pretty closely to the panels you use. 5kw GTI and 5KW worth of panels, but that is pretty much impossible to do in most cases, you're always going to be a little off.

But is it better to have the GT inverter slightly oversized or slightly undersized?

Since solar panels are often more expensive I had always thought it was better to over size the inverter. So a 5KW GTI might service a 4.9KW array of solar panels.

Thanks!

tmarch · June 2012

Re: Should a Grid Tie Inverter be Under or Over sized for the PV Array it supports?

While I can see some agreeing both ways, personally I would oversize the inverter to a degree. My 6KW of panels are being fed to a 5K inverter, NOT by my choosing, but what was installed. I'm going to upgrade the inverter to another manufacturer because it's faulty and I'm going to a 7K inverter. A little more expensive, but with panel prices where they are I may install a few more panels. It gets cold here and I want to be able to harvest the added watts (up to 125% of the name plate rating of the panels) when the days are shorter.

mike95490 · June 2012

Re: Should a Grid Tie Inverter be Under or Over sized for the PV Array it supports?

It all depends on local conditions.
I had 4.5KW of array, at non-optimum roof angle, and a 3KW inverter. I only pegged the inverter a couple of days a year.

solarix · June 2012

Re: Should a Grid Tie Inverter be Under or Over sized for the PV Array it supports?

Typically, inverters are undersized because the STC label ratings are based on ideal conditions that are rarely seen in practice. The max rating only happens with the sun perpendicular to the module (around noon on days near the equinox for fixed mounted modules) and if the ambient temperature is cool. In practice there are only a few hours per year when the array gets close to its STC rating and so an undersized inverter is justified. Even the inverter manufacturer's string size programs do so. However of course, a larger inverter ought to provide better reliability and usually costs little more to do so. Make sure you upgrade the rest of the BOS components to handle the higher inverter ratings as well.

SolaRevolution · June 2012

Re: Should a Grid Tie Inverter be Under or Over sized for the PV Array it supports?

There are some different schools of thought on this one.

-I've heard it suggested that underloading the inverter can help a little with longevity. If you think about it, how many other devices do you commonly run at full power during the hottest time of the day, 365 days a year?

Most of the other considerations point to using a larger array.

-With systems that are large enough for you to have to deal with the issue of the maximum backfeed current in an AC panel, you may want to size your inverter(s) to max out your existing load panel's AC back feed capacity and then go with the largest PV array the inverter manufacturer recommends. Having to replace an existing load panel or meter panel can really decrease your return on investment numbers.

-An array will rarely put out it's max rating and any "extra" sizing will help on days without clear skies.

Back when rebate incentives were higher here in California, you could get the most power for dollar spent by checking the list of elegible inverters for their "CEC" rating and maxing it out with a PV array's PTC rating. PTC is bases on the PV module output at 50° C instead of the STC rating at 25° C. This would often make for an array STC that was about 12% over the inverter's name plate rating. Any PV beyond that was not eligible for the rebates so it was rare to get to the manufacturer's max. It was a big deal when the rebates were up around $4/watt and panels at +/-$6/watt. This is no longer such an issue with panel costs so low.

-Alex Aragon

BB. · June 2012

Re: Should a Grid Tie Inverter be Under or Over sized for the PV Array it supports?

One other issue--House wiring. If you use a 3 or 3.3 kW inverter, you can use a 2 pole 20 amp breaker (and have a smaller Main Panel with its +20% rating for solar power connection).

So, I have a 3.5 kW array with a (currently) 3.3 kW inverter--So per NEC, a 20 amp breaker is fine.

If I was to use a 4kW inverter, then the math (as I understand NEC) gets more complex... You have to multiply the array output by 1.25 (solar "what if there is a lot of sun") and another 1.25 for wiring/breaker rating:

3,500 W Array * 1.25 NEC Solar * 1/240 VAC nominal * 1.25 NEC wiring drating = 22.8 amp NEC circuit

Round up to 25 amps, that would bearly make it "legal" for my 125 Amp service (125*0.2=25 amp max solar breaker).

A 4kW inverter would be rated for:

4,000 watt * 1/240 VAC * 1.25 NEC derating = 20.8 amp

If I got the numbers right...

So, as I understand, sometimes you can have a Larger Array with a "Nominal" inverter (say 0.77 array derating) and if you are Main Breaker Panel limited, the smaller inverter will allow you to "be legal" with a smaller main panel.

-Bill

SolaRevolution · June 2012

Re: Should a Grid Tie Inverter be Under or Over sized for the PV Array it supports?

BB. wrote: »

One other issue--House wiring. If you use a 3 or 3.3 kW inverter, you can use a 2 poll 20 amp breaker (and have a smaller Main Panel with its +20% rating for solar power connection).

So, I have a 3.5 kW array with a (currently) 3.3 kW inverter--So per NEC, a 20 amp breaker is fine.

Several manufacturers used to make a 3.8kW inverter. That was ideal. 3800/240= 15.8 amps. Two of them would fit an 8kW array perfectly into a residential 200 amp service panel.

-Alex Aragon

rollandelliott · June 2012

Re: Should a Grid Tie Inverter be Under or Over sized for the PV Array it supports?

3.8kw inverters sound cool. wonder why they are no longer made?

SolaRevolution · June 2012

Re: Should a Grid Tie Inverter be Under or Over sized for the PV Array it supports?

BB. wrote: »

One other issue--House wiring. If you use a 3 or 3.3 kW inverter, you can use a 2 pole 20 amp breaker (and have a smaller Main Panel with its +20% rating for solar power connection).

To make matters wierd;

There are residential service and load panels which come with with 100amp breakers and a buss bar rating of 125amps. You are allowed to exceed the bus bar rating by 20% so 125amps x 1.2 = 150 amps. You can add 50 amps of backfeed breakers to these 100amp main breaker panels.

The same manufacturers also have service and load panels which come with 200amp breakers and a bus bar rating of 200amps. 200amps x 1.2 = 240 amps. This makes it so you can only add 40 amps of backfeed breakers to these 200amp main breaker panels.

Cuttler Hammer AC panels are typically rated this way. I think "Square D" is similar.

-Alex Aragon

rollandelliott · June 2012

Re: Should a Grid Tie Inverter be Under or Over sized for the PV Array it supports?

interesting
i always assumed that bus bar rating would be exactly the same as main breaker rating.
do you know of any 200amp breaker boxes/load centers that have a higher bus bar rating like 225amps or something that would make them ideal for residential solar installs?

BB. · June 2012

Re: Should a Grid Tie Inverter be Under or Over sized for the PV Array it supports?

I would guess you also have to double check that the XXX rated bus bar is rated at XXX amps in model ABC box.

-Bill

rollandelliott · June 2012

Re: Should a Grid Tie Inverter be Under or Over sized for the PV Array it supports?

XXX rated!!!

I guess those boxes are decorated somehow. LOL.

solarvic · June 2012

Re: Should a Grid Tie Inverter be Under or Over sized for the PV Array it supports?

When did they quit making 3.8 kw inverters? I just bought a new Fronius 3.8 kw invertera few weeks ago. :Dsolarvic:D

SolaRevolution · June 2012

Re: Should a Grid Tie Inverter be Under or Over sized for the PV Array it supports?

solarvic wrote: »

When did they quit making 3.8 kw inverters? I just bought a new Fronius 3.8 kw invertera few weeks ago. :Dsolarvic:D

That's good to hear. I've been hiding out in off-grid and battery world so I had not seen any for a while. SMA and Xantrex were the two I had been installing as a sub a few years ago. I was under the impression that they were discontinued soon after. No?

-Alex

ggunn · June 2012

Re: Should a Grid Tie Inverter be Under or Over sized for the PV Array it supports?

BB. wrote: »

One other issue--House wiring. If you use a 3 or 3.3 kW inverter, you can use a 2 pole 20 amp breaker (and have a smaller Main Panel with its +20% rating for solar power connection).

So, I have a 3.5 kW array with a (currently) 3.3 kW inverter--So per NEC, a 20 amp breaker is fine.

If I was to use a 4kW inverter, then the math (as I understand NEC) gets more complex... You have to multiply the array output by 1.25 (solar "what if there is a lot of sun") and another 1.25 for wiring/breaker rating:
3,500 W Array * 1.25 NEC Solar * 1/240 VAC nominal * 1.25 NEC wiring drating = 22.8 amp NEC circuit

Round up to 25 amps, that would bearly make it "legal" for my 125 Amp service (125*0.2=25 amp max solar breaker).

A 4kW inverter would be rated for:
4,000 watt * 1/240 VAC * 1.25 NEC derating = 20.8 amp

If I got the numbers right...

So, as I understand, sometimes you can have a Larger Array with a "Nominal" inverter (say 0.77 array derating) and if you are Main Breaker Panel limited, the smaller inverter will allow you to "be legal" with a smaller main panel.

-Bill

The extra 1.25 multiplier for excess insolation you mention is for sizing DC conductors and has no bearing on your AC wiring or breaker size. The size of your array isn't a factor, either. Your AC wiring is determined from the maximum output current that can be produced by your inverter at the line voltage at which it will be interconnected, subject to deratings from temperature and conduit fill, if any, and whatever voltage drop you can tolerate. Your backfed breaker size is the inverter maximum current multiplied by 1.25, rounded up to the next standard size irrespective of how much PV you are feeding the inverter.

BB. · June 2012

Re: Should a Grid Tie Inverter be Under or Over sized for the PV Array it supports?

My theory, if the design/NIC is consistent, if you up size the DC side, you must upside the AC side to the maximum rated GT inverter output or uprated DC side.

Basically, if I have a 3.5 kW array, and a 3kW inverter, then the output is limited to the current regulation point of the "smaller" GT inverter.

If, however, for example I have a 3.5 kW array with a 5kW inverter, then the output wiring/breakers can be sized either on the output of the GT inverter (5kW/240vac * 1.25) or by the array (3.5 kW/240 VAC * 1.25 * 1.25)---Which ever gives the smaller result.

The only two reasons this matters is 1) the maximum 120% rating of the Main AC Panel or 2) possibly less efficient (larger inverter, higher losses when driven at less than "full power").

3,000 watts * 1/240 VAC * 1.25 = 15.6 amps (GT Inverter limited)
3,500 watts * 1/240 VAC * 1.25 * 1.25 = 22.8 amps (Array Limited per NEC calcs)

Of course, I may be entirely wrong about how NEC "understands" the problem--But this is what the "engineering" would require (you cannot "uprate" the DC side without taking the AC side limitations into account when designing/installing the system).

But it would not be the first thing I would disagree with the NEC about (especially on the DC side).

-Bill

PS: Again, this was in discussion with the original question of should a GT inverter be over or undersized vs the supported array.

ggunn · June 2012

Re: Should a Grid Tie Inverter be Under or Over sized for the PV Array it supports?

BB. wrote: »

My theory, if the design/NIC is consistent, if you up size the DC side, you must upside the AC side to the maximum rated GT inverter output or uprated DC side.

No. Per code, the AC side must be wired and OC protected to accommodate the maximum nameplate output current of the inverter, no matter how much PV you connect to it. The maximum output is just that; if an inverter is putting out its maximum, adding PV to it will not increase its output. It goes into clipping mode.

As to the optimum overloading of an inverter, that depends on the location (latitude and weather/climate conditions) and orientation (azimuth and tilt) of the array, as well as any intermittent shading issues. Figuring out that number is a non trivial exercise, and overloading by 40-50% is not uncommon in higher latitudes. It's all about maximizing kWh/kW, and some clipping can be tolerated if the loss to clipping when the array is producing its maximum is more than offset by the added production under less ideal conditions.

I would never underload an inverter; that would be a waste of money.

Should a Grid Tie Inverter be Under or Over sized for the PV Array it supports?

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