# Convert DC to AC for Photo Voltaic Systems

## Main Question or Discussion Point

Hi there
I have an Photo Voltaic system size 141 kW DC. It gives me an about 13 000 kW/hr a month.(I'm in Hawaii)
We planning to install a new system, size ≈800 kW DC.
How can i calculate how much of kW/hr AC i will get out of new system?
Is there is any special formula to covert DC to AC for PV systems? And what parameters should i know? Inverter type? or out current of PV system?
I'm really confused...

## Answers and Replies

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The DC is converted into AC by a system called 'Inverter'.
Modern inverters have high efficiency - typically higher than 90%.
So, the DC power needed is AC power / inverter efficiency, or typically AC Power x 1.1.
The Inverter input specs need to match the solar panel specs - i.e. Vpeak, Imax etc..

Bobbywhy
Gold Member
A normal PV system used to produce AC requires a large capacity battery. (meaning large number of batteries connected in series) The PV system charges the battery and the inverter converts that DC to AC. There are many variables needed to make this system function successfully. Just for a few examples, what are the DC undervoltage and overvoltage limits? What are the inverter input current and output current limits? If any of these limits are exceeded how are the circuits isolated rapidly? Since you are dealing with a “large” system of 800 KW one error could be catastrophic…burning and damaging equipment. And there is the personnel safety issue to consider as well. Since you evidently are not qualified I recommend you hire a competent engineer to specify each component of the system.

And i did. Here is his reply in case someone need it for the future:

The formula for average daily kwh is as follows:

Average Daily KWH = (System Size in STC) x (Sun-Hours) x (Inverter/Soil/System Derate) x (Tilt/Azimuth Derate)

For Sun-Hours at Hawaii, we would use 5.81.

For Inverter/Soil/System Derate, we would use 0.811 based on the inverter we are planning on using (Power-One PVI-10.0 or 12.0).

The tilt/Azimuth derate is the derate factor we use to take into account the losses associated with not having the array at the optimal 21.5 degree tilt facing exactly south. We use PV Watts to calculate this. The derate is the predicted kwh production for the tilt/azimuth of the proposed array divided by the production for the optimal tilt/azimuth. We had assumed 8 degree tilt, and the array is 10 degrees east of due south, which would result in a 0.98 derate factor.

Hello Lexus.... the movment in the indusry is energy harvesting - and you are looking the right way Maximum KWH - not just KW and Efficency.

The larger inverters today are actually boost inverters - a boost function to get a stable DC link and then the AC inverter ( typically a Grid Tie or GTI)

The market is accepting that the boost function adds to the initial system cost AND decreases efficiency - however the system is then capable of producing MORE KWH over the course of the year.

Not to mention things like uptime - 1 day of downtime is 0.3% of annual output, and these systems typically do not get repaired in one day.

I work in the power conversion industry and since the industry is used to the price point for a straight inverter - the manufacturers are struggling to get the boost-inverters sold, even though technically that is what the market is looking for- they have not re-calibrated their price and efficiency expectations to match ( also because the government feed in - has established requirements based on the straight inverter model - etc.) it is really a PITA

Hello Lexus.... the movment in the indusry is energy harvesting - and you are looking the right way Maximum KWH - not just KW and Efficency.

The larger inverters today are actually boost inverters - a boost function to get a stable DC link and then the AC inverter ( typically a Grid Tie or GTI)

The market is accepting that the boost function adds to the initial system cost AND decreases efficiency - however the system is then capable of producing MORE KWH over the course of the year.

Not to mention things like uptime - 1 day of downtime is 0.3% of annual output, and these systems typically do not get repaired in one day.

I work in the power conversion industry and since the industry is used to the price point for a straight inverter - the manufacturers are struggling to get the boost-inverters sold, even though technically that is what the market is looking for- they have not re-calibrated their price and efficiency expectations to match ( also because the government feed in - has established requirements based on the straight inverter model - etc.) it is really a PITA
So, you saying that i should look for a boost inverter for my system?
A deal here that I'm looking for at this project more like an accountant, not an engineer. I mostly interested in "how much money i can get out of this system every month"
But yea, that 81% inverter efficiency drive me crazy. This is about how much i can get if i connect 12v motor to an alternator mechanically. Should modern inverters be more than 90% efficient?

Hello Again -

Yes you should look at it as an accountant - but look at harvested energy models ( for example have an analysis based on the big picture - not just a few specs or bureaucratic stuff - I recently came across a tool that looks interesting called RETScreen ( http://www.retscreen.net/ang/home.php [Broken] ) - I have not used it however.

As for efficiency - you should beat 90% WITH the boost function - but efficiency is a very tricky spec and depends heavily on the conditions that the figures are determined. I realistic spec is actually on a cool day ( max PV efficency) but rated power for the inverter is often at max ambient temperature - the temp also affects the losses in the inverter - etc. It makes it very hard to compare apples to apples.

-- I was just at the Solar Show in Dallas - the PV Panel makers are competing for who has the best panel efficiency - we are talking 1/10 of a percent - poor install, unreliable equipment or inverter system that is not optimal - will make the best panel no better than the worst. 800KW is nothing to sneeze at - you should have an independent consultant working on the selection for Panels, Installers/Integrator and Inverter - this is a case where the Value of well informed and unbiased decisions pays off. I am in Pennsylvania and have a friend doing this in NJ - but that is a long haul to HI.

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Hello Again -

Yes you should look at it as an accountant - but look at harvested energy models ( for example have an analysis based on the big picture - not just a few specs or bureaucratic stuff - I recently came across a tool that looks interesting called RETScreen ( http://www.retscreen.net/ang/home.php [Broken] ) - I have not used it however.

As for efficiency - you should beat 90% WITH the boost function - but efficiency is a very tricky spec and depends heavily on the conditions that the figures are determined. I realistic spec is actually on a cool day ( max PV efficency) but rated power for the inverter is often at max ambient temperature - the temp also affects the losses in the inverter - etc. It makes it very hard to compare apples to apples.

-- I was just at the Solar Show in Dallas - the PV Panel makers are competing for who has the best panel efficiency - we are talking 1/10 of a percent - poor install, unreliable equipment or inverter system that is not optimal - will make the best panel no better than the worst. 800KW is nothing to sneeze at - you should have an independent consultant working on the selection for Panels, Installers/Integrator and Inverter - this is a case where the Value of well informed and unbiased decisions pays off. I am in Pennsylvania and have a friend doing this in NJ - but that is a long haul to HI.
So, any advice what questions should I ask my project engineers and installers? What brand/make of inverters i should take a closer look?
Because if we talking about 10% loss on inverter-its $2500 a month, so i want to maximize$ i'm getting back out of this system.

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Aloha Lexus - the PV system is an investment, and every (unbiased) guide on personal investing says to get an independent adviser to help with your investment.

There are a lot of issues affecting a Solar investment - crunching some rough numbers it looks like you are in the \$1 to 1.5 M range- and can not boil it down to a few questions. No offence to you or the Physics Forum - but you should be getting more professional advice than can be gained here.

My company supplies core components to SMA and other major Inverter Cos. From an investment point and the larger cos with a track record you probably can not go wrong - from a best VALUE ( Price/return) - you need a full and complete spec and place this out to bid. If the Bid package is not professionally prepared - you will not get their attention or best effort. The package is where you can define what you need, US Content ( tax implications) Support, reliability, obsolescence support ( you can not buy a 10 year old PC - the same goes for power electronics - so how will they deal with this?) etc. A good bid package will be your security blanket - at this point you can also list YOUR terms - if they will not accept they will ether negotiate or NO-Bid.
You can ALSO define a performance spec for total energy, up time etc. It can be quite involved. Basically a bid package puts YOU in the drivers seat - IMHO - there is also a corporate culture play in a good bid spec - different businesses respond in different ways. But watch out for the bottom feeders they will agree to anything and disappear tomorrow ( typical flaw in MANY government contracts - because they put ridiculous terms their bid package - and the only ones that will accept are the ones that have no intention of meeting them any way - I have seen a bunch with Unlimited Liability clause - what reputable company will accept that??)

OK sorry for the rambling e-mail - in short, there is a lot to consider, and I really have no idea where you are in the process.

OH - parting shot ...doesn't HI have a person or group supporting Solar development? Another resource.

Happy Turkey Consumption Day

A friend pointed me in the direction of Schott Solar as a maker of high efficiency panels (see link below).

http://www.schottsolar.com/us/products/photovoltaics/schott-power-poly-275-285/ [Broken]

Has anyone ever looked at these panels? How do they stack up against others from performance and cost standpoints?

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