Oversizing the system

[khanh dam]

Kwh/KW = hours right? I just do not understand the units. not being petty. spelllllling police, have areldy tickedted me. LOL, when someone says you aer petty and you tell them there(their?) grammer and spelling is badd, thats just icing on the cake! LOL. Couldn't have proved my point better!

[Ampster]

My two cents on the topic of this thread is that higher DC to AC ratios, as @Solarix mentioned, are becoming more common. Mine is 1.5 to 1 and that may be on the high side. However, even at a latitude of 38 degrees North, I am consistently getting a ratio of1500 kWhs/kW on annual results. That is based on DC panel capacity. Based on inverter capacity my ratio is over 2300 kWh/kW of inverter capacity.

[J.P.M.]

seriously, your comments always amaze me in their ability to become more and more petty. I dont' see you making this "correction" on any one elses' posts, just mine. wonder why?

Seems like your petty comment is like the pot calling the kettle black.

Since it looks like you're asking a question (while ironically making more grammatical and spelling errors as you go), I'll try to enlighten you.

1.) My option.

2.) In this case, to point out what looks to my like your hypocrisy and your attempt to get pissy with someone who wrote something you didn't like.

3.) As for what you claim you don't see, look around for some of my prior posts if you're interested. I, and several others around here I'd note, point out unit and other (mis)usage of technical terms and units on a semi regular basis. FWIW, kW and kWh are often incorrectly used. Not pointing such things out can only increase the confusion.
This obviously isn't a peer reviewed journal - and the way I see it, not much more than a lead generator masquerading as an an unvetted chat room, but some of us try to maintain some aspects of professionalism. Sue me. Your stuff is nothing special. Just more ignorance to be pointed out in an effort to avoid confusion to others who may be reading your stuff. If you can't reconcile that within you brain, consider the correction a public service from a condescending prick.

4.) If you're going to get pissy, don't be surprised at that comes back to you.

If you want to look ignorant, suit yourself, but this place would be better and more professional if you didn't stink it up so much with your crap.

If/when folks point out my errors (which certainly happen more than I'd like), and if I agree that they are (errors), I thank them, apologize for my screwup, make note/correction and move forward. Look hard enough and you'll find a few of my mea culpas.

I'll withhold comments about your grammar and spelling above so as not to embarrass you more than you seem to be doing to yourself.

I learned a long time ago that not everyone who tells me what I may not like to hear or read about my actions is trying to hurt my feelings.

BTW, if you don't like my posts, you can always ignore them, or exclude (block) them from showing up.

The last word on the subject is your if you want it.

[SunEagle]

you dropped the unit of years in your calculation how did 9000kwh/year just become 9000kwh?

Getting a little pissy about what I post.

Whenever you run a pv watts calculation it provides and estimated production in kWh/year. So the "annual" or "yearly" output is pretty much not always mentioned.

But it still comes down to how many kWh will be produce for each kW of solar panels you install based on the amount of sunlight, dirt, panel angle, efficiency, etc.

[khanh dam]

And as long as your separating fly frass from pepper and seeming to be attempting to start a pissing match, why didn't you capitalize the "W" in "kWh" as is customary for abbreviations of S.I. units named after people ?

Just like correct grammar, spelling and syntax in writing, if you want to be taken seriously, conventions and rules matter. Otherwise, fair or not, you'll probably look ignorant to those reading what you write and who do know the subject being dealt with. That can make it more difficult to be taken seriously.

seriously, your comments always amaze me in their ability to become more and more petty. I dont' see you making this "correction" on any one elses' posts, just mine. wonder why?

[SunEagle]

I have another question: when using PVWatts, if the nearest data centre to my location is 53 miles, will it give a useful approximation? Or should I use another application?

Whenever you use PVwatts you are still going through some type of estimate. If the nearest location is at the same or close to the same latitude then your calculation should be the same or close.

If you are at a much different place based on latitude you can adjust your calculation by changing the panel angle in the calculation.

[Sami Najjar]

I have another question: when using PVWatts, if the nearest data centre to my location is 53 miles, will it give a useful approximation? Or should I use another application?

[Sami Najjar]

The specific solar yield is in kWh/kW/year but I added "annual" to the sentence that's why I ommited the "year" in the units. Not a big deal, sorry for the complication.

[J.P.M.]

you dropped the unit of years in your calculation how did 9000kwh/year just become 9000kwh?

And as long as your separating fly frass from pepper and seeming to be attempting to start a pissing match, why didn't you capitalize the "W" in "kWh" as is customary for abbreviations of S.I. units named after people ?

Just like correct grammar, spelling and syntax in writing, if you want to be taken seriously, conventions and rules matter. Otherwise, fair or not, you'll probably look ignorant to those reading what you write and who do know the subject being dealt with. That can make it more difficult to be taken seriously.

[khanh dam]

1500kWh/kW)

you dropped the unit of years in your calculation how did 9000kwh/year just become 9000kwh?

[SunEagle]

1. your bill is based off AC usage, so any calculation based off that is also based off AC usage
2. the cost difference between a 5kw and 7kw inverter is pretty small. Always better to use a bigger inverter so that it does not heat up as much and lasts longer IMHO.

"the annual specific solar yield is 1580 kWh/kW."

your units are wrong. it is 1580kwh/year

if you use pv watts and put in a solar array size of 1kw DC then the website will give you your annual specific solar yield.

Actually some calculations from PV watts can be used to show a kWh/kW production.

When I ran a calculation a few years ago for a 6kW system PV watts showed about 9000kWh/year. That calculated to about 1500kWh/kW (9000kWh/6kW = 1500kWh/kW)

[khanh dam]

1. your bill is based off AC usage, so any calculation based off that is also based off AC usage
2. the cost difference between a 5kw and 7kw inverter is pretty small. Always better to use a bigger inverter so that it does not heat up as much and lasts longer IMHO.

"the annual specific solar yield is 1580 kWh/kW."

your units are wrong. it is 1580kwh/year

if you use pv watts and put in a solar array size of 1kw DC then the website will give you your annual specific solar yield.

[Sami Najjar]

Thanks so much for all the help, I truly appreciate it. Very glad to be here.

[J.P.M.]

Sami, Welcome to the forum.

I'm not sure where the 1580 kWh/kW rate came from. Perhaps it is a specific model for your region and installation. If you want to calculate the correct ratio for your specific situation, many of us like a web tool called PVWatts. It does much better than a "rule of thumb".

Bob: I'm with you on PVWatts for Sami's modeling. Seems to me that Sami is more savvy about some solar stuff than the average bear, but if nothing else, getting up to speed on PVWatts will at least confirm or refute the 1,580 kWh/yr/STC kW. I use the specific yield number all the time and in various ways. FWIW, my system's logged actual 7 yr. specific annual yield is 1,726 kWh/(yr*STC kW). PVWatts comes up with 1,732 kWh/(yr.*STC kW). It can be used as a preliminary sizing tool. It has other uses as well.

Sami: Welcome to the neighborhood.

1.) Your methodology may work, but I'd get familiar w/PVWatts. Almost too easy not to use to get a bit more info. Read ALL the help screens a couple of times before you make any runs. The model is pretty easy to learn with that intro. Use 10 % system losses. The 14 % default value seems to underpredict output for many locations as confirmed by many folks around this forum and elsewhere.
2.) PV systems are generally rated by STC size. That is, (the panel's STC wattage) * (number of panels in the array). If so, for your numbers, that 5.7 STC kW number is the D.C. array rating of 5.7 STC kW before the inverter losses. If your number of 1,580 is the modeled # of kWh/yr. that 1 STC kW of panels will produce in a specific location and orientation, then the array size needed to produce 9,000 kWh/yr. as a long term average is probably something close to 9,000 kWh/yr./ (1,580 kWh/yr*STC kW) = 5.7 STC kW or so. The caution here is to make sure that your 1,580 number is the modeled specific annual output in units of kWh/yr. per installed STC kW.
Example from my array above : System size : 5.232 STC kW. Annual modeled output = 9,062 kWh/yr. Specific output = (9,062 kWh/yr.)/5.232 STC kW = 1,732 kWh/(yr.*STC kW).

2.) It's probably not necessary and it's probably overkill to have an inverter that has a larger rating than the array - or for that matter even as large as the array.
Reason: In actual operation, an array will only produce it's STC rating a few times a year, and then only for short periods of a few minutes or so. Unless you're in a cold climate and more than a couple km. above sea level, under good conditions of clear skies, low(er) temps. and a good breeze, about the best steady output you'll get out of a well designed and sited system is maybe 90-92% of an array's STC rating, perhaps a bit more, but if more, only for short periods of maybe a few minutes or less and only a few times/yr. The value of what's usually that small increment in annual output may not be worth the cost increase of a larger inverter. Example: I live near San Diego with close to a goldilock's climate. My array is 5.232 STC kW. My inverter is 5.0 kW. The highest continuous output of more than 10 minutes duration I've measured and logged has been 4.91 kW out of the array and into the inverter. array output is clipped by the inverter at 5.032 kW about 1X/month or so for maybe a minute or less.

So, if you want to oversize the array, and using your 1,580 specific output #, to meet a 20 % higher load, you may well need a (9,000kWh/yr*1.2)/1,580 kWh/(yr*STC kW) = 6.84 STC kW array. But you probably won't need a 6.84 kW inverter. For a lot of reasons too complicated to thoroughly explain here, mostly because I'm a hunt & peck typist, If your array will be split or a lot off south, the inverter can often be smaller yet.

If you haven't done so already, try this: More PVWatts. To reiterate, read all the help screens, particularly the parts dealing with DC/AC size ratios. Run up the array size until you get to whatever annual output you want. If you want, say,10,800 kwh/yr., and if the modeled array size comes out to be ~ 6.84 STC kW or so., the inverter will probably be less than 6.84 kW. Then, vary the DC/AC size ratio while holding the array size constant and see what happens to the annual output as the DC/AC ratio increases.

Note: An array's annual actual output (or maybe more accurately an array's running 365 day modeled output) will vary by maybe +/- 10 % or so from the modeled output mostly due to irradiance fluctuations from clouds, with very sunny and very cloudy climates often having less variation (example: After the first year's operation, my array's 365 day running variation seems to be ~ +/- 4.0 % or so, but I'm in a pretty sunny location).

While you're at it, download a PDF of a slightly dated copy of "Solar Power Your Home for Dummies", or get the revised hard copy at bookstores/Amazon for ~ $25 or so. A good primer on residential PV.

[RichardCullip]

Thank you very much Bob. And for the 1580kWh/kW, it is the Peak Sun Hours multiplied by the losses factor of 0.8 and by 365 days (for annual calculation), this is how it is done here in my region.

Interesting calculation. Not sure what area of the world you are in but down here in sunny San Diego I'm getting better performance out of my solar array. I have a 4kW (dc) array on my roof and since installation in 2019 I'm averaging ~7,600 kWh (ac) of yearly production. That puts my yield at 1900 kWh(ac) per year per kW(dc). As others have said PVWatts should get you in the ball park.