Building my first solar system!

[jjphat]

Hiya folks, long time listener, first time caller here, just looking for anything that I may have left out in designing my first solar system. I've probably left out quite a bit. Here's what I've got:

(1x) 48v 100ah lithium iron phosphate battery
(1x) 48v 2000w 120v inverter
(1x) 40a 1700w @48v MTTP charge controller
(1x) 40a fuse to go between panels and charge controller
(4x) 370w 48v Renesola solar panels
Wire and connectors to hook it all together

Planning to connect the panels in 2s2p to generate 96v to feed the charge controller, as in the attached picture.
Basically, I'm trying to make one 20a 120v receptacle that I can plug a refrigerator into and have continuous backup power. So, am I missing anything?
Thanks!

 

[jjphat]

Hiya folks, sorry for the double post. Still finding my way around. Looking for anything that I may have left out in designing my first solar system. I've probably left out quite a bit. Here's what I've got:

(1x) 48v 100ah lithium iron phosphate battery
(1x) 48v 2000w 120v inverter
(1x) 40a 1700w @48v MTTP charge controller
(1x) 40a fuse to go between panels and charge controller
(4x) 370w 48v Renesola solar panels
Wire and connectors to hook it all together

Planning to connect the panels in 2s2p to generate 96v to feed the charge controller, as in the attached picture.
Basically, I'm trying to make one 20a 120v receptacle that I can plug a refrigerator into and have continuous backup power. So, am I missing anything?
Thanks!

 

[MichaelK]

Why limit yourself to just one plug? This is for power outages? Don't you want lights, and TV too while the power's out? Depending on where you are, I'd expect your system to make 5kWh of power even in winter, so you should have enough electricity to power a whole cabin. At mine I'm consuming between 1.5-3.0 kWh of power per day, depending on the season and how much the frig is opened.

Assuming you only want to consume 70% of your battery, that gives you 3360Wh of power. Assuming you want two days of autonomy, thats ~1.7kWh per day, close to a normal lifestyle. If you could double the size of your battery, I think that would allow you to reach a confortable 21'st century lifestyle.

 

[jjphat]

Hey thanks very much for the reply!
Yeah I'm kinda working backwards...make sure I have the one thing I want (the fridge) covered, then see what I can add on from there. Is 1480w of panels enough to handle two 48v 100ah batteries? (I know there is a lot of "it depends" wrapped up in that question). Otherwise, do the other numbers seem to match up? I am also going to put a fuse between the battery and the inverter.

 

[MichaelK]

You can look at your solar output in two different ways, to replace the amount of power you are likely to use, or to optimize the rate at which your batteries want to be charged. Since batteries are likely now to be your single largest expense, I like to focus on optimal battery charging.

Let's explore both pathways, outlining the math you need to make a decision. Let's assume you are likely to get as little as 2.5 sunhours in winter, and 5+ sunhours in summer. Plan your system for the worst day, not the best. Assuming you need 3000wh of power per day, 3000Wh/2.5sh =1200W of panels. So, your 1480W of panels exceeds your basic needs.

Now, the alternative methodology. Assume a lead-acid battery wants charging at 1/10th C to 1/8th C. That is 0.1C to 0.125C. Li batteries will be happier with 1/5th C all the way to 1/2C, or 0.2C to 0.5C. Check your battery specs carefully for the maximal charging rate. Assume that the maximal amperage is charging during bulk at a voltage of about 50V or so. Amperage typically drops while voltage goes up as the battery reaches full charge. Let's assume your battery will be bulk charging starting at around 50V. The math for charging at 1/5th C would work out to be....

100Ah battery X 1 battery X 0.2C X 50V charging X 1.175 fudgefactor = 1175W of panels
100Ah battery X 2 batteries X 0.2C X 50V charging X 1.175 fudgefactor = 2350W of panels

The "fudgefactor" is a correction for the expectation that your panels will almost never put out their rated output. Assuming that you are likely to see only 85% of rated output, then the reciprical of 85% (0.85) is ~1.175.

0.2C is a rather low conservative charging rate for a Li battery. Look at your battery manufacturer's recommendations for charging, and plug those values into the above formulas. Your battery might also start charging at a somewhat higher bulk voltage. Just plug in the correct value for your particular battery. Although the parameter values may change slightly, the math stays the same.

For two batteries and your 1480W of panels the charging rate ends up being (1480W/50V) X 0.85 = 25A. What works out to be 200Ah/25A = 0.125C, or about 1/8th of Ah capacity. Would the Li guys here please chime in as to how acceptable 1/8C is? If it was a lead-acid battery, then 1/8th C would be perfect.

 

[jjphat]

That is awesome. The manufacturer's recommendation is 50a for charging. Thank you for putting it into terms which I would not I have thought to think. I'll study those numbers a bit, because I wouldn't say I 100% understand it all yet, but it looks like I was right to "overbuild" it a little. Now it's a matter of working out the details. I had a bit of an epiphany last night though regarding the batteries. If were to use 2 batteries, is it "Use one while the other one is charging." Or is it "Use only one battery and the panels will charge it while also simultaneously providing power to the appliance. Perhaps a combination of both?

 

[jjphat]

The sun hours here are very good. I'm on top of a hill and 2 panels will face east, with their western edge slightly elevated, and the other 2 will face straight up.

 

[jjphat]

Oops. 50V for charging, not 50A.

 

[MichaelK]

The sun hours here are very good. I'm on top of a hill and 2 panels will face east, with their western edge slightly elevated, and the other 2 will face straight up.

Sunhours is exposure independent. It assumes you have full unobstructed sunlight all day long. A sunhour is NOT an hour of sunlight. It is a estimation of the total watthours your panels can produce in a day, and is a way of accounting for low production in the early morning, and late afternoon. You multiple the # of sunhours at your location by the number of panel watts to get the total Wh of power your panels are likely to make in one day.

Are you in the tropics? I'm assuming so if you have a set of panels pointed straight up.

I would not divide battery capacity up. Wire them so they are all at exactly the same state of charge 24/7. That way they will be charged equally, and discharged equally.

 

[jjphat]

Southeastern USA (GA). It looks like we average 4.99 sun hours here, and plus being on the eastern facing side of the hill. Just starting with the one battery for now, but I see how having two could also work. Still digging into those numbers you posted up there...thanks a lot for that!

 

[Hedges]

More panels. Have 2 more facing west. Multiple angles broadens curve of power production. More panels gives more watt-hours and helps make up for overcast days.

If your LiFePO4 can take 0.5C at around 25 degrees C, probably only 0.05C to 0.10C just above freezing (or approaching high temperature limits.)
Determine max C rate PV and SCC will deliver, and adjust BMS charge cutout temperature accordingly.
Keep the battery at a comfortable temperature so BMS never has to cut out, but the temperature limits provide protection.

 

[12VoltInstalls]

I like the multi directional arrangement after I tried it. Midday peak is missed, but it keeps ahead of the fridge later in the day better so the hours of overnight battery use is less.
I’m thinking, though, is flat for the panels ideal in GA? A little bit of slope might be better, plus it helps with ‘self cleaning’ when it rains.

 

[jjphat]

Hm...HOA says no panels facing the street, so the western side is not really feasible. Basically I'm going to be using the panels as a ceiling for an upstairs deck that is uncovered now.

 

[jjphat]

The flat ones won't be totally flat either. Enough for water to run off obviously.

 

[tglozano]

Hiya folks, long time listener, first time caller here, just looking for anything that I may have left out in designing my first solar system. I've probably left out quite a bit. Here's what I've got:

(1x) 48v 100ah lithium iron phosphate battery
(1x) 48v 2000w 120v inverter
(1x) 40a 1700w @48v MTTP charge controller
(1x) 40a fuse to go between panels and charge controller
(4x) 370w 48v Renesola solar panels
Wire and connectors to hook it all together

Planning to connect the panels in 2s2p to generate 96v to feed the charge controller, as in the attached picture.
Basically, I'm trying to make one 20a 120v receptacle that I can plug a refrigerator into and have continuous backup power. So, am I missing anything?
Thanks!

Why not use a All-In-One charge controller? Like the MPPSolar.

 

[jjphat]

They look nice, but maybe a bit pricey. The MakeSkyBlue controller was about $100 and I got the inverter for about $250.

 

[MichaelK]

The flat ones won't be totally flat either. Enough for water to run off obviously.

You would be somewhere around 33-34 degrees North? Nearly flat would not be optimal. Here's what you can do. Cut a 12" piece of PVC pipe and hold it in front of the panel. When you can see sun shining through the central hole, the pipe is pointed directly at the sun. A straight stick, or even a pencil will work almost as well.

What you will find is the a flat orientation will only be close to optimal right around noon or so (your noon, not the time). I'd say an angle that perfectly perpendicular right around 11:00am or 1:00pm will be what produces the most power over the course of the day. That angle though will change, with the changing seasons.

I designed my mounts so they are seasonally adjustable, ranging from the summer angle to the winter angle. Because my mounts can rotate East to West, when I'm not striving for maximal power, I keep some of the panels pointed SE, and others pointed SW, to maximize output over the course of the day. I also have another array on the other side of the cabin facing WNW. That's for power late in the afternoon around 6pm, when my primary south-facing arrays start to shade.

 

[jjphat]

Got everything all hooked up and connected to the freezer. Of course on the day I got it all together, we're scheduled for like 9 days in a row of rain. "Plan for the worst days, not the best" I guess! Anyway, it all seems to be running. I set the MakeSkyBlue controller according to the directions and I think the numbers all jive correctly. The battery hasn't even lost one bar after 3 cloudy days of powering the freezer!

The MakeSkyBlue directions say to basically just set it to "lithium" and it takes care of the rest, so that's what I did.