Solar System Size Formula

[msobal79]

Can someone tell me what the formula for calculating solar system size?

We were sold a 11.48kW system (41 Solarworld 280 Mono black panels with 41 Enphase M250 Micro inverters), however when I look at what the system is actually producing, via Enphase Envoy & Enlightment Manager app, it seems to be maxing out at 8kW in what I would consider optimal conditions (a clear hot summer day in Southern California).

Furthermore I noticed that our Southern California Edison Permission To Operate documentation shows a CEC-AC nameplate rating of 9.931 kW.

If I run the numbers off of the solar panels (41 x 280W) I can see where the Solar company gets 11.48kW. But upon researching actual outputs of these panels they average about 253W actual peak production which means an actual system of 10.37kW.

Next when I look at the micro inverters (Enphase M250) and run the numbers off there stated actual production values 240W (250W max) (41x 240) I get a 9.84kW size which is closer to the nameplate rating of 9.931kW from SCE.

Is System size an industry term that does not actually state real (in optimal conditions) system output?

[J.P.M.]

280 W is the STC rating. Actual will be a lot less depending on orientation, shading, operating panel temps., etc.If you have a lot of shade, way off south orientation panels sitting close to flush on a dark roof, you will have penalties for those conditions. You will also probably be limited to 250 X 41 = 10.25 kW under the best of conditions. Perhaps an example of caveat emptor, know what you're buying.

[msobal79]

We have 18 East facing panels, 19 West facing Panels, and 4 South facing panels and no shading issues. panels are set 4-6 inches from roof surface.

I get that 280 W is the STC rating for the panels. My question is more in regards to terminology. It is my understanding that Solar system size is calculated by panel STC rating and that such system would never produce such amount. I calculated that I needed a 9 kW actual output system for 100% offset. So I am fine with a max output of 10.25 kW. When I purchased the system I was basing my numbers off of the Enphase data sheets at 240W per unit. It was not until I saw the system submitted to SEC for PTO that I noticed the 11.48 kW system size that it thru me for a loop.

I have friends looking into Solar and if panel STC rating is the industry standard for communicating solar system size, then I want to let them know that that number does not accurately represent the actual system output.

[J.P.M.]

We have 18 East facing panels, 19 West facing Panels, and 4 South facing panels and no shading issues. panels are set 4-6 inches from roof surface.

I get that 280 W is the STC rating for the panels. My question is more in regards to terminology. It is my understanding that Solar system size is calculated by panel STC rating and that such system would never produce such amount. I calculated that I needed a 9 kW actual output system for 100% offset. So I am fine with a max output of 10.25 kW. When I purchased the system I was basing my numbers off of the Enphase data sheets at 240W per unit. It was not until I saw the system submitted to SEC for PTO that I noticed the 11.48 kW system size that it thru me for a loop.

I have friends looking into Solar and if panel STC rating is the industry standard for communicating solar system size, then I want to let them know that that number does not accurately represent the actual system output.

Depending on the array tilts, the 18 east facing panels will produce more in the A.M., less in the P.M. , the west facing panels will produce more in the P.M., less in the A.M. and the 4 south facing panels will produce more per panel over the course of a clear day, with the greatest output around mid day, but a smaller amount than either of the other arrays due to it's small size. With east/west/south combined orientations of 41 total panels, you will not ever produce as much as 41 identical panels in an all south facing orientation.

Do this: Run PVWatts 3 times, once for each array using the actual azimuth, tilt and size of each array. Use the hourly output option. Next, add the hourly results for all three together by hour. Next, look for clear day output totals for days near the ones you want to compare to recent clear day's actual output. You will probably find a much flatter curve for your situation and actual orientations for most or all days than for an all south facing array's curve. You will also probably find that the peak output for the 3 combined arrays is closer to your 8 kW number. Note too that the combined daylong output for any PVWatts estimate for the 3 combined arrays will always be less than the output for a 41 panel array that's facing south at a tilt that's about the same as the other 3.

Bottom line: In all probability, your output is different because your panels are not optimally oriented. I suspect it's performing about as it should.

There may be some clipping from the micros, but I kind of doubt that it will be much, if any.

Also, as you note, a solar panel's rating is seldom, if ever achieved, not unlike how often an ICE's total horsepower is (almost never) developed. Or, when/how often do you pin the speedometer on your vehicle ?

When spending many thousands of $$#'s on a product, it's always good to know something about what's being bought.

Suggestion: Get and read a copy of "Solar Power Your Home for Dummies". Better late than never. Then, when done, fulfill one of your stated goals and give it to your solar seeking friends.

If you think you got the wrong end of the sick, looks to me that your own solar ignorance may have had a hand in it.

[huge]

You're "system" is rated at DC power. Edison looks at AC. DC power is higher than AC power, so your system looks right

[sunnyguy]

Yes DC nameplate is the standard way to describe the "size" of a solar system. It is a consistent way to describe the system based on the materials used not output. Otherwise it would be impossible to set and compare pricing.

It's not even that useful to know that your system produces 8kw peak in august. What is useful is to know the average kWh/W output per day, month, and year for your specific installation. This can vary widely even for neighbors. And even that might not be enough information if you are on a time of use plan and your net metering period is not a traditional calendar year or doesn't use a true-up. You may need a monthly analysis based on hourly data.

Fortunately tools like pvwatts give reasonable estimations for this data. But if you don't like numbers and spreadsheets you will get frustrated quickly. This is why you see people try to find a salesman they trust instead of actually understanding what they need. It often ends poorly.

Your 11.48kw system should produce at least 17,000kwh/yr so hopefully that is how much you need.

[msobal79]

Here is the system as represented thru Enphase enlighten program. I noticed in the settings that I could get estimated generation from integrated PVWatts calculator. My installer left the tilt, do array size and systems losses fields blank, surprise surprise, which are what is needed to get generation estimates. I know my roof tilt is 17 deg. from the system plan prints, and the dc array sizes is pretty straight forward. The only two fields I wasn't sure about are the system losses and module type. When I look up system losses on PVWatts it looks like it's talking about system loss due to soil shading etc. which I have none of so I just put 0.0%. Am I correct on these fields?

[osagieik]

Hi all,
I am sorry I have ask this under this topic because I can't seem to find anywhere else to ask.
Please I need a guide on how to setup may 300watts/24v solar panels:

I have a battery bank of two (2) 12volts, 200AH connected in series to make a 24v / 200AH system because the input of my inverter requires 24volts. I have two (2) 300watts / 24volts solar panels with the parameters below:
Optimum operating voltage (Vmp) = 35.8v
Open circuit voltage (Voc) = 45.2v

I am considering two option below but I need suggestions on which is better and advisable:

1). Connect the two 300watts / 24v solar panels in parallel through a 24v MPPT charge controller. This is to enable me achieve a solar array of 600watts, thereabout 35.8v at peak operation and double the current from one panel (i.e. two panel wired in parallel) hence, more power for my batteries and load.

2). Wire the two solar panels in series, which will give me a 300watts array of about 70volts and same current output as one panel (peak period) - This is just to ensure I get good charging voltage even when the light intensity is low.

Considering the use of MPPT controller, which of the alternative above (Parallel or Series) is more efficient and practicable?

Thank you very much for you help.

[littleharbor]

You get 600 watts either way. Your panels are probably about 8.38 amps each. Parallel wiring doubles your amperage at 35.8 volts. Series wiring doubles your voltage while your amperage stays at whatever your panels are outputting at that time. In your case, if you have a "REAL" MPPT charge controller you should series wire your panels and let the MPPT do the conversion of high voltage into your battery bank.

[osagieik]

@littleharbor:Thanks a lot. You have cleared my doubt and I am going to use the Tracer4210 EPEVER 40A MPPT charge controller (I hope this is a true MPPT charge controller).

Thanks again.

[littleharbor]

Yes the Tracer MPPT controllers are real MPPT

[J.P.M.]

Here is the system as represented thru Enphase enlighten program. I noticed in the settings that I could get estimated generation from integrated PVWatts calculator. My installer left the tilt, do array size and systems losses fields blank, surprise surprise, which are what is needed to get generation estimates. I know my roof tilt is 17 deg. from the system plan prints, and the dc array sizes is pretty straight forward. The only two fields I wasn't sure about are the system losses and module type. When I look up system losses on PVWatts it looks like it's talking about system loss due to soil shading etc. which I have none of so I just put 0.0%. Am I correct on these fields?

Forget what the installer did with respect to PVWatts. You can do this yourself and learn a lot along the way. Read the PVWatts help/info screens and get an understanding of what's going on after you download and read the book I referenced (free for a slightly dated but still useful version). FWIW, many users have found the 14 % default for "system" losses that the program uses gives estimates for system output that are often lower than observed. Try 10% and use standard panels. Or, once you get more comfortable with the estimator, try the system loss calculator after you understand what each of the terms represents as described in the PVWatts help screens and have a reasonably accurate way to get/estimate a value for each of those terms . 10% seems to be a good first approximation.

[Sunking]

Watts = Watt Hours / Sun Hours.