Am I Crazy?

[sandiegolloyd]

I plan on designing and installing a solar PV system on my roof. I contacted two solar equipment wholesalers and got strange responses regarding how many panels that I need. They stated that the greater power producing panels that I buy the less of them I need. This does not make sense to me when the micro-inverter that I like has a max output of 250w.

Here are my specific:

San Diego Ca.
South facing roof
Would like to generate 13,500 kWh ANNUALLY.

Their calculation
13,500/12/30/24hours = 1.562
1.562 x 5 = a system size of 7.8kw

7.8kw/ 250w panel = 31 panels
7.8kw/305w panel = ~26 panels

How can this be when the micro- inverter is max 250w so it seems like I would always have to buy the 31 panel system to get my annual output. Any comments would be appreciated!
Thanks!

[Naptown]

Check the Enphase site regarding compatibility between the panels and inverters.
If they are compatable you might be better off with the larger panels.

[slopoke]

I plan on designing and installing a solar PV system on my roof. I contacted two solar equipment wholesalers and got strange responses regarding how many panels that I need. They stated that the greater power producing panels that I buy the less of them I need. This does not make sense to me when the micro-inverter that I like has a max output of 250w.

Here are my specific:

San Diego Ca.
South facing roof
Would like to generate 13,500 kWh ANNUALLY.

Their calculation
13,500/12/30/24hours = 1.562
1.562 x 5 = a system size of 7.8kw

7.8kw/ 250w panel = 31 panels
7.8kw/305w panel = ~26 panels

How can this be when the micro- inverter is max 250w so it seems like I would always have to buy the 31 panel system to get my annual output. Any comments would be appreciated!
Thanks!

The output of the micro inverter is 250w AC. The recommended input is 210-300 watts DC. The output of the panels is DC. I have the M250s on my 300w LG panels.

[Naptown]

Then save a few bucks and use a solar edge system. all the advantages of M250's monitoring wise, will meet the 2014+ NEC and costs less.

[Wy_White_Wolf]

....
7.8kw/ 250w panel = 31 panels
7.8kw/305w panel = ~26 panels

How can this be when the micro- inverter is max 250w so it seems like I would always have to buy the 31 panel system to get my annual output. Any comments would be appreciated!
Thanks!

Because you are only thinking about instintaous outout instead of total daily production.

A 250W panel with a 250W MI will only put out out 25W under ideal conditions. While a 305W panel on a 250W MI will put out the maximum that the inverter can handle for hours on end. At the end of the day the larger panel would have provided about 25% more then the smaller one.

WWW

[sandiegolloyd]

Starting to understand

Ok I think I am beginning to understand the instantaneous versus total. So what is the formula to calculate the number of panels required for a desired annual power output? I was using the following:

Total desired annual 13,500 kwh

divided by

365 days

divided by 24 hours

equals 1.5411

multiplied by 5 (hrs of good sun in my area)

equals 7.7055 kw system

I then divided this number by the power output rating of the panels 250 versus 305 and get a different number of panels depending on which panel output I use . It seems like this formula is not accurate since the inverter output is a gating factor and I don't see where I would take into account that with a higher output panel you get 25% more "total" output. thanks!

[russ]

for your area's sun hours and potential production try http://pvwatts.nrel.gov/

[sandiegolloyd]

Sun Area

Yes I got the 5 hours from the site that you suggested. Did you happen to have a better formula for calculating the number of panels based on using a different panel output even though the inverter remains the same? Thanks!

[Ian S]

Yes I got the 5 hours from the site that you suggested. Did you happen to have a better formula for calculating the number of panels based on using a different panel output even though the inverter remains the same? Thanks!

"5 hours" is a rather meaningless number. Use PV Watts to do an actual calculation that will give you expected monthly production over the entire year. PV Watts uses averages that account for weather variables as well as physical layout. Just change the derate factor to something more typical such as 0.83-86. Just vary the DC power number to wind up with what you want in total annual production. Once you have the DC power value, then you can figure out the number of panels assuming you have the power specs for the individual panel.

[J.P.M.]

Yes I got the 5 hours from the site that you suggested. Did you happen to have a better formula for calculating the number of panels based on using a different panel output even though the inverter remains the same? Thanks!

Around here (N.C. San Diego) for reasonably well oriented, reputable equipment that is installed by a reputable vendor, you can expect about 1.5 to 1.7 kWkrs./yr. per nameplate (D.C) Watt of solar panel, pretty much regardless of panel manufacturer.

So, using 1.6 kWhr./yr. per Watt installed: 13,500/1.6 = 8.4 kW system size, +/- some, first cut. Your method got you close, but the logic was a bit off. I'd Respectfully suggest you spend some time w/ basics of solar and then use PVWatts - scrap the default rerate factor and use something like .82 to .86, depending on your shade situation.

[sandiegolloyd]

starting to sink in !

Ok here is what I think I understand now based on the great responses:

13,500 A/C annual kwh (Desired output)
1.6 DC annual Kwh (effective panel output per rated system watt)

8,437 DC watt annual system size requirement

250W DC panel yields 33 required panels

305W DC panel yields 27 required panels

Now the second part of my question is still stuck in my head... if the inverter max AC output is 250W how do I know the real impact of switching from a lower panel output to a higher? Thanks folks!

[Ian S]

Ok here is what I think I understand now based on the great responses:

13,500 A/C annual kwh (Desired output)
1.6 DC annual Kwh (effective panel output per rated system watt)

8,437 DC watt annual system size requirement

250W DC panel yields 33 required panels

305W DC panel yields 27 required panels

Now the second part of my question is still stuck in my head... if the inverter max AC output is 250W how do I know the real impact of switching from a lower panel output to a higher? Thanks folks!

You rarely will see full rated output from a panel. The power rating is for ideal conditions that will rarely be seen in real life. That said, you may still see "clipping" where the panel will try to output a higher power than the inverter can handle. The inverter will in that case simply output the maximum it can. That could result in the loss of a tiny percentage of what your system is capable of producing. In the majority of cases, that won't likely amount to enough to offset the additional cost of the higher power inverter but it's very hard to predict that beforehand.

[J.P.M.]

Ok here is what I think I understand now based on the great responses:

13,500 A/C annual kwh (Desired output)
1.6 DC annual Kwh (effective panel output per rated system watt)

8,437 DC watt annual system size requirement

250W DC panel yields 33 required panels

305W DC panel yields 27 required panels

Now the second part of my question is still stuck in my head... if the inverter max AC output is 250W how do I know the real impact of switching from a lower panel output to a higher? Thanks folks!

It sounds like you are (assuming) using micro inverters. That is certainly a choice, but may or may not be the wisest choice. Again respectfully, more reading and study will help answer your question and unstick your head.

[Datacruncher]

Formula to calculate panels n production

I have been looking at a DIY system vs leases for a month now n have learned much in the process. I am always open to corrections

System Size = (# of panels) * (kw/panel) = kw

Estimated Production = (sys size * 5.5 solar hours/day * 365 days) - Loss = kWh/year

5.5 is the solar hours/day for my area. Typical pitch is 22-26 degrees for my area.

I have found through quotes that LOSS is 25-30%. (over 5 diff quotes) This is from solar contractors. I thought it is high but after seeing multiple quotes n reverse calculations, this is the number I'm seeing.

So then; 13500 kwh * (1.3) = 17550 kwh annual production before 30% loss

Sys size = 17550/(5.5 * 365) = 8.742 kw system. = 8742 watts.

1). 8742 watts/ 250w per panel = 35 panels. 35x250w= 8.75 kw system

2). 8742 watts/ 305w per panel = 29 panels. 29x305w = 8.85kw system

8.75 * 5.5 * 365 = 17565 kwh/year , - 30% loss (5269) = 12295 kwh/year est production

8.85 * 5.5 * 365 = 17766 kwh/year, -30% loss (5330) = 12436 kwh/year est production.

U now have the equations n can play with the what ifs.

[J.P.M.]

I have been looking at a DIY system vs leases for a month now n have learned much in the process. I am always open to corrections

System Size = (# of panels) * (kw/panel) = kw

Estimated Production = (sys size * 5.5 solar hours/day * 365 days) - Loss = kWh/year

5.5 is the solar hours/day for my area. Typical pitch is 22-26 degrees for my area.

I have found through quotes that LOSS is 25-30%. (over 5 diff quotes) This is from solar contractors. I thought it is high but after seeing multiple quotes n reverse calculations, this is the number I'm seeing.

So then; 13500 kwh * (1.3) = 17550 kwh annual production before 30% loss

Sys size = 17550/(5.5 * 365) = 8.742 kw system. = 8742 watts.

1). 8742 watts/ 250w per panel = 35 panels. 35x250w= 8.75 kw system

2). 8742 watts/ 305w per panel = 29 panels. 29x305w = 8.85kw system

8.75 * 5.5 * 365 = 17565 kwh/year , - 30% loss (5269) = 12295 kwh/year est production

8.85 * 5.5 * 365 = 17766 kwh/year, -30% loss (5330) = 12436 kwh/year est production.

U now have the equations n can play with the what ifs.

Before you start with sizing estimates, most folks start by knowing/determining how much electricity they want to produce. Also, many are of the opinion it is not a slam dunk that offsetting 100% of an electric load is cost effective.

Go to and use PVWatts with a .84 correction factor. Before you do however, read the info, FAQ's and other stuff associated with it. Respectfully, it looks like you're not on firm knowledge ground. It'll be faster, easier and more accurate with PVWatts. Don't shoot me, I'm only the messenger.