# balance of system and how to size the controller

Registered Users Posts: 8
Hello,
I’m hoping to get some confirmation on my system balancing and ask a couple of questions re: controller size and panel sizes. A few months ago, I joined up to ask about charging some new AGM batteries that had been sitting for a couple of years without being charged. I followed the excellent advice and am happy to report that the batteries seem to be holding their charge.

I have been guilty of designing my system backwards, as I bought a small place with a few things already in place. Here’s what I inherited: 4 very old (+25 yrs) 75W panels (I think – hard to read the weathered specs), a 10amp Sunsaver charge controller and a fuse box. I bought four 49 ah Concordia AGM batteries, a 600 watt Samlex inverter, a 20 Amp Xantrex TrueCharge2 Battery Charger and a EU2000 Honda generator.

Estimated daily load = 270w hrs. (a minimalist: a light, some electronics, coffee grinder)
With load correction /.7 = 385w hrs. per day
385w on 12VDC = 32.1 amp hrs.

Battery capacity = 4x49ah = 196ah. Assuming the rule of not discharging below 50%, does this mean that, if my batteries were fully charged and my daily usage was around the 32.1 amps, that I could go about 3 days on a full charge? (.5 x 196=98/3=32.6) Of course, I live in the north (Yukon), so the availability of sun is a big factor.
10% peak charge current rate = 19.8 amps
19.8 amps x 12V = 237.6 W less 77% efficiency = 182 W panel?? needed

Existing panels: 4x75=300w – derated by .77 (maybe more??) = 231W /14.8 charging voltage= 15.6 amps (These panels are not currently hooked up)
Here are my questions:
How do you figure out what size of charge controller to use? Will my 10 amp Sunsaver be enough?
I am planning on purchasing some new panels. Is it okay to go with large wattage panels (say 235w) with a small system? I would eventually expand my system once there was need for water pumping, and to carry me through the darker months. If I purchased 2x235w=570w of panel, but kept my existing battery bank size, I could harvest sunlight for a longer time in the fringe seasons, right?
Any guidance on this would be greatly appreciated. Meanwhile, I'm charging my batteries using my generator as the intense spring sun is shining...
Thanks.

• Banned Posts: 17,615 ✭✭
Re: balance of system and how to size the controller

Lets start by looking at your expected load requirement vs. your battery capacity (both in Amp hours).
32 * 100 / 196 = load is 16% of total Amp hour capacity. Two days of that load with no recharging would be 48% of capacity. That all works out fine, but you really don't have the 3rd day's capacity you were hoping for.

Now let's look at recharging that 196 Amp hour battery bank. If you go for the middle 10% rate you want 19.6 Amps peak current @ the minimum 12 Volts. If you expect to push these AGM's harder (they can take 13% without worry) you'd want 25 Amps @ 12 Volts.

The single 10 Amp controller would be at its maximum for even the minimal 5% peak charge rate. You can parallel controllers if that is more economically feasible for you than replacing it with a larger unit.

But you have to have the panels to supply the power. You have four 75 Watt units now, which would probably produce:
(4 * 75) * 0.77 = 231 Watts / 12 Volts = 19.25 Amps. So what we see here is: enough panels, not enough controller. The panels themselves are likely rated around 4 Amps Imp, so with all in parallel on a PWM controller you might see a maximum current of 16. This is still more than the one 10 Amp controller can handle.

Yes you can use larger panels on 12 Volt systems, but here's the thing: they tend to be higher than 17.5 Vmp which means you must use an MPPT type controller in order to get the full benefit of the extra Watts. That can be an expensive proposition.

As for over-sizing the array and using the controller to "clamp" any excess power when available, this is not advisable. You can go a bit larger than you might expect, but not too much. Mostly you'd just be wasting panel investment when the sun did shine, and you won't pick up much more power when it doesn't. When panels aren't in full sun their power output drops drastically.

It looks to me like your existing panels & battery are compatible and what you need is more controller capacity.
• Solar Expert Posts: 3,738 ✭✭✭✭
Re: balance of system and how to size the controller
I am planning on purchasing some new panels. Is it okay to go with large wattage panels (say 235w) with a small system? I would eventually expand my system once there was need for water pumping, and to carry me through the darker months. If I purchased 2x235w=570w of panel, but kept my existing battery bank size, I could harvest sunlight for a longer time in the fringe seasons, right?
As for over-sizing the array and using the controller to "clamp" any excess power when available, this is not advisable. You can go a bit larger than you might expect, but not too much. Mostly you'd just be wasting panel investment when the sun did shine, and you won't pick up much more power when it doesn't. When panels aren't in full sun their power output drops drastically.

As panels become cheaper and batteries become more expensive I think it makes more and more sense to over-size your PV array. You will need an MPPT type of controller, otherwise as Cariboocoot pointed out you will be clamping the excess power which is not advisable. Yes, you will be wasting panel investment when the sun does shine, but on marginal days you might harvest enough sunlight to avoid running your generator. Load shifting is when you time your loads to occur when your batteries are full and the sun is shining, and water pumping (which you plan to do someday) is an excellent way to use the 'wasted' energy that you will have if you upsize your array.
--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
• Solar Expert Posts: 119 ✭✭
Re: balance of system and how to size the controller

I to am a firm believer in over paneling. I have 4200w of panel for only 450ah @48v of battery. This has allowed me to get a full charge on marginal overcast days. My generator usage has only been 12hr since I went off grid 9 months ago. I am also able to load shift and run all my power tools including a large compressor on just the panels after my batteries are charged usually before noon. There is something nice about running tools off my panels that my 3000w generator cant run.
Ned