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!

[ecomodded]

Are the 305w panels more efficient or just larger ?

If the panels are no more efficient just larger, i would stick with your preference as you must like that 250w inverter for a reason.

[pinballlooking]

Go to PV watts
PVWATTS v. 1
this will help you figure out what you need.
Are you going with Enphase 215? They max out 225watts and pare nicely with 250 watt panels.

[sandiegolloyd]

Thanks very much for the responses. I still think I am not getting it. If the max output is 250w of the micro-inverter why does it matter what panel that i buy as long as it is at least 250w? (It is enphase 250 btw). If I take the Annual volume and divide by a panel that produces 350W or 250w does it really give me more annual power since the most the inverter can produce is 250W? Thanks!

[pinballlooking]

Look at this white paper. It is for a M215 but it also applies to the M250.
http://enphase.com/global/files/Enph...ightsizing.pdf

[jeff5may]

Lloyd,

The main idea with buying larger panels is to maximize the power going into the inverter module. A 300 Watt panel will put out 20% more power than a 250 Watt panel, given the same lighting conditions. On an average day, that 300 watt panel is not going to max out for long. So the whole time the panel is at 86% output or lower, you'll be gaining that extra 20% worth of power going into the inverter. It adds up fast.

Yes, on those bright, sunny days, you will generate more power than the inverter can digest for maybe an hour. But that's OK. In 10 or 15 years, that 300 Watt panel will only be putting out 250 watts on the same sunny days.

[stevehull]

Originally Posted by 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!

Sandy, this calculation of needed power of 7.8 kW (to get to an annual amount of 13,500 kWhrs) is just wrong.

Look up PVwatts and put in your specific information.

Also note that the Enphase 215 is a lot cheaper then the M250. I think they are trying to get you to buy the most expensive panels and not to look at your most inexpensive option.

Go to wholesalesolar for some specifics on Astroenergy panels.

But as others have mentioned, do the PVwatts calculations first.


Steve

[MN Renovator]

13,500kwh annually. OMFG! Reduce your usage FIRST, it's a much cheaper way to go. Not sure what the San Diego 'rule of thumb' is but in MN 7.8kwh nameplate would get about 9,360kwh annually if on a fixed position south facing ground mount where the snow could be cleared off with each snowfall. I'm looking to cover my annual electric usage with 12 275 watt panels(60 cell) with 300 watt micros, that's 3.3kw nameplate. My PVwatts has me about 3,900kwh. My usage in 2012 was just under 1800kwh, the remaining 2000kwh will be my electric car once I'm done converting it.

How are you using 13,500kwh? Is it a huge amount of air conditioning, do you have resistance heating, or do you not turn off the lights or something?

[pinballlooking]

He does not seem that bad for usage
“In 2012, the average annual electricity consumption for a U.S. residential utility customer was 10,837 kWh, an average of 903 kilowatthours (kWh) per month. Louisiana had the highest annual consumption at 15,046 kWh and Maine the lowest at 6,367 kWh.”
How much electricity does an American home use? - FAQ - U.S. Energy Information Administration (EIA)


We are higher but we have a Chevy Volt that we are driving about 20,000 solar power miles a year. We have a pool. We will make about 18,000 KWH this year and not cover everything.


But it is MN Renovator is 100% right reduce usage is much cheaper than adding more solar to cover it.

[Daox]

It really all depends on what other energy sources you use. Last year, I used 3400 kWh which may seem low (or high depending who you are). However, I used 18,900 kWh of natural gas! The gas is only for home heating and my range (which accounts for less than 1% of the usage), everything else is electric (including hot water).