Help please....

AndrewC · March 2017

Bill: This site explained it pretty well....thanks!

http://rredc.nrel.gov/solar/calculators/pvwatts/system.html

AndrewC · March 2017

It appears to me that along with the solar I BETTER have a generator ready for powering my batteries

AndrewC · March 2017

These are the batteries I am looking at:

https://www.batteriesplus.com/productdetails/wkdc12=100p

BB. · March 2017

Yep... You are learning the details now.

12 AGM batteries--I am not sure I would suggest that configuration. In any case, have you figured out your loads?

-Bill

Horsefly · March 2017

@AndrewC:

I'm a relative noob too. Welcome to the club. I've learned most of what I know from these guys, so you should listen. They are awesome. I don't think I saw a post of the 52% derating from Bill until I had managed to come up with my own:

Solar access - 97%. If there are times that some shadows or whatever impede the sun hitting your panels during the "peak sun hours". For where we are putting our panels, it should be 100% (no derating), but I used 97% just in case. Probably lower than I needed.
Charge controller efficiency - 98%. PWM controllers tend to be less efficient than MPPT controllers except at the margin voltages. I didn't know what to use, so I used 98%. Probably higher than we needed.
Battery efficiency - 85%. Bottom line is that it takes more to go in than you get out. After reading and reading and reading, I decided on 85% not knowing better.
Inverter inefficiency - 85%. You've seen this and already understand it. Converting DC to 120VAC includes some losses, and 85% is a good estimate. My inverter (Schnedier SW4024) only specifies a optimum efficiency of ninety-some percent.
Wiring - 97%. No matter what, you are going to lose some of the power your panels generate in wiring. Wiring to from the array to your charge controller is the main one, but wiring to/from the battery will still have some loss. This adds up quick, and 97% efficiency is pretty good.
Solar panel efficiency - 75%. Panels are rated under STC "standard test conditions" (25 deg C, 1000 kW per square meter), which is pretty unlikely to exist in most cases. Panels also include a NOTC, which is more realistic. I used 75%, but Bill is far wiser, so maybe 81% is OK. ;-)

So if you multiply all mine together, you get 50%. Nice round number, even if it is disappointing. My system is all designed, purchased, and ready to go. We are just waiting for the snow to melt up at 9,000 ft so we can go install it.

Steve

Photowhit · March 2017

AndrewC said:

Bill: This site explained it pretty well....thanks!

http://rredc.nrel.gov/solar/calculators/pvwatts/system.html

This looks like a very poor explanation to me. The chart shows Standard Test Conditions, so doesn't take into account the greatest loss!

Panels are flash tested at a high light value and never allowed to heat up, this is call Standard Test Conditions (STC) Many even publish the Normal Operating Cell Temperature values (NOCT) I do bargain shop, but I have not seen a NOCT value above 75% of panel rating in a long time. I'll now hunt for yours!

I couldn't find NOCT values/ratings for your panels, but STC temps are 25 degrees C and the NOCT is 46 degrees. Your information will tell you that for every degree above 25 you will lose .335 volt and gain .047 amp for a net power loss of .42 % per degree above 25. This is compound so I don't want to figure out how to do the math right now...lol

I did NOT hunt down the same values, but here are the values for Canadian Solar's 340 watt mono crystalline panels;

Solar Panel Model Canadian Solar MaxPower CS6U-340MSTC DC Watts Maximum Power Pmax: 340 WPTC DC Watts Maximum Power Pmax: 311.8 WNOCT DC Watts Nominal Power Pnom: 245 W

Not the normal 245 watts is about 72% of the 340watt panel rating. PTC is yet another standard used for some California incentives. The Suniva is listed as 306 watts under PTC.

http://www.gosolarcalifornia.ca.gov/equipment/pv_modules.php

Clear as mud?

Photowhit · March 2017

If you go back and read through Bills earlier post, he does a good job of explaining the 'total cross system' loss involved. AGM's tend to be batter than Flooded Lead Acid (FLA) but charging batteries to store energy comes at a loss as well, and is likely the 2nd largest loss in using stored energy (if you count panel derating as a loss, and greatest if not) FLA batteries use voltage about 15% higher than the batteries resting voltage to transfer current into the battery. Also as the battery near completion it will accept less current and there will be greater losses.

Bill uses 52% total loss, I like 50% (easy peasy).... Inverters list there 'best efficiency' and often their idle with minimal loads is huge. My 8 watt fan at night means my inverter never goes into search mode, it idles at @20 watts. Other things would keep it out of search mode as well. interesting things happen when you do bounce in and out of search mode. Very minimal/hard to read loads, such as diodes on power strips, will burn out. I'm not sure if it's from the tiny line charge voltage or the popping on and off.

Photowhit · March 2017

AndrewC said:

These are the batteries I am looking at:

https://www.batteriesplus.com/productdetails/wkdc12=100p

AGM batteries are a bit more susceptible to user damage, Many/most 1st timers will make a mistake with their first system.

FLA batteries are a bit more 'bullet proof' the energy density for 6 volt golf cart batteries is about the same and the cost is less than a 1/3rd of the $280 at Sam's Club or Costco (about $80). At least Sams club has a very liberal exchange/core charge. I've had them take a lawn tractor battery at 2 different locations! You might check with them then hit small lawnmower repair places for junk batteries (might not be legal in some states to buy/sell used batteries) In Missouri they would only get 4-5 dollars in scrap for a lawn tractor battery. If that much. I think the core charge was down to $15 last time, early last year. but I suspect this varies from state to state as well.

If you or a friend doesn't have a membership, you can often buy a gift card online that will allow you a one time entrance.

AndrewC · March 2017

THIS IS SO MUCH TO TAKE IN!!!!

mike95490 · March 2017

AndrewC said:

THIS IS SO MUCH TO TAKE IN!!!!

But it's much better to get it over and done with, before you start spending $$$

Estragon · March 2017

FWIW, I spent about 2yrs lurking here and using a couple of golf cart batteries and generator for power at my cabin before designing my system. Even so, there are a few things I would do differently with the benefit of hindsight.

oil pan 4 · March 2017

Panel derating is what you paid for versus what you actually get.

Help please....

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