How to Size a Charge Controller

[Canine]

So you decided to set up your own system. You have purchased your solar panels and now you need a charge controller. How can you tell if a charge controller is big enough to handle the panel(s)? This can be somewhat complicated, so I'm going to try to keep it simple as possible for a person designing their own system for the first time. Different controllers have slightly different methods, so this won't cover every controller. I will use the Blue Sky Energy 3024i since that is what I use and am most familiar with.

1) When you look at the specs, make sure you are using the correct column. If you have 36 cell solar panels hooked up in parallel, look in the 36 cell solar panel column. If you have 60/72 cell panels or have two 36 cell panels hooked up in series, look in the 60/72 cell panel column.

2) If you have a 12 volt battery bank, look at the specs for a 12 volt system. If you have a 24 volt battery bank, look at the specs for a 24 volt system. The panel amps, panel voltages, and overall panel wattage will be different. It's easy to become confused and look at the wrong section or column. Many controllers will work only with a 12 volt battery bank.

3) Make sure you are looking at input limits and not output limits. The output limits don't matter- only the input limits.

Now that we know where to look for the specs of the charge controller, we can now learn how to gather information from the solar panels.

4) Look at the wattage of the panel or add the wattage of both panels if you have two connected together. Write this down.

5) Look at the max PV VOC of the panels; it will be the highest voltage number. If you have two panels in series, multiply the VOC by two. If you have two panels in parallel, the VOC won't change. Write this down.

6) Finally, Look at the Isc (Short Circuit Current also shown as STC); it will be the highest number for current. Current is also called amps, so look for either word. If you have two panels connected in parallel, multiply the Isc by two. If you have two panels connected in series, the Isc won't change. Write this down.

Now that we know how to find the information, let's do two examples using the 3024i with one 60 cell solar panel connected to a 12 volt battery bank.

LG makes the strongest 60 cell panel that I know of, the LG365Q1C-A5, so let's use one of those panels.
1) The panel has a max wattage of 365 watts, so the wattage is within the 400 wattage limit of the 3024i charge controller. (You will see a spec for 540 max pv panel wattage, but that is for 36 cell panels, so don't use the 540 watt spec. You will also see a 800 max pv panel wattage, but that is for a 24 volt battery bank, so don't use that one either. Easy to be confused here.)
2) The panel has a VOC of 42.8 which is within the 45.6 max VOC of the charge controller. We are good here, too.
3) The panel has an Isc of 10.8 which is within the 24 amp input of the charge controller. We are good here, too. (I had made a mistake earlier today and thought the max amp input was 12, but the 12 amp spec is for 72 cell panels, not 60 cell panels, so you can see how easy it is to get confused.)

We have a winner: The 3024i will work with the LG panel. The panel is big, but it will work.

Let's do one more example.

Sunpower makes what many people think is the best solar panel available to the general consumer. The strongest 60 cell panel they make is the SPR-X21-345 with a max wattage of 345 watts. It is as powerful or maybe a tad more powerful per square inch than the LG we just used, but because it is a bit smaller in surface area, it has 20 fewer watts.

Let's see if it will work with the 3024i. The max wattage is 345, so that is good. The max amps (Isc) is 6.39, so that is good. The max volts (VOC) is 68.2, so that is no good. It is higher than the max 45.6 volt input limit of the 3024i charge controller.

While the Sunpower panel has less power than the LG panel, it won't work because it has too high of voltage. We need to find a different 60 cell or 72 cell solar panel or combination of 36 cell panels, but that's another thread.

One last comment. If you have a panel or combination of panels that makes 400 watts, don't use the 3024i. Use a bigger charge controller. Yes, 400 watts is within spec, but when you continually use any equipment at its max it will fail quicker. Also, there are some conditions where a panel will make more watts than it is rated. If it's cold and sunny, it won't instantly burn up your controller, but that is putting a lot of stress on the controller. Quality MPPT controllers are expensive, so I like to do what I can to take care of mine.

 

[Redbearded]

I ran into a spreadsheet from Victron that looks like it would do a good job getting you close.
https://www.victronenergy.com/support-and-downloads/software
You can add your own panels in the system and then use their specs for their products to get an idea of the voltages and currents you will need in a controller. Also it looks like a lot of the smaller controllers from them are pretty reasonable on amazon (link is a 75V 15A controller for ~$90).

 

[John61CT]

For Victron you just need to make sure the panels Voc totals well under their max rating for the SC, the first number in the model name.

e.g. for 75/15, I'd stay under 65V, so two 36Voc panels in series would be too close, you'd need to move up to 100/X

The second number is Amps **output** they build in a lot more tolerance for overpanelling so few would hit the max there.

So for 15A, that works out to around 220W if you never wanted to discard peak output, but say you got a great bargain on a 300W panel, that would still be safe, and give a huge boost in overall average output in suboptimal conditions.

Note all these numbers double with a 24V bank, actually based on amps, watts is derived.

The Victron line doesn't care about number of cells, lots of flexibility

various different cell technology types, past or coming out in future

use of high-voltage including grid-tie panels, and

serial vs parallel configurations.

Also tying in multiple SCs together, and to their various logging/monitoring gadgets, also open-source data comms for DIY.

 

[tx2sturgis]

Add up the power (or amperage or voltage) of your solar panels.

Multiply by 2.

Buy that size controller.

 

[John61CT]

That approach could get expensive if your talking quality gear.

Wasting money due to lack of knowledge is what I think most are here to avoid.

 

[Trebor English]

My charge controller is 5" wide, 3" high, 1" thick.  Is that the right size?   : )

 

[tx2sturgis]

Ok....multiply by 1.5

Better?

We buy cars and trucks and vans with twice the horsepower we need.

We buy power tools with 3 times the batteries we need.

We buy sound systems with 5 times the wattage we need.

We buy beer with 5 more beers in the six-pack than we need.

We buy computers with 8 times the CPU speed and RAM we need.

We buy houses with 10 times the room we need.

We....oh never mind...



If you size the solar power production and controller and batteries (and other components) according to your power needs, financial budget, and space limitation, with some thought given to balancing the system, then an oversize controller will be a benefit in the long run. 

Yes, you can overpanel a controller, and that is another topic for more advanced users. This thread, I think, focuses on solar beginners.

 

[jimindenver]

The Blue Sky certainly interesting in its approach to sizing. I have dealt with a handful of different MPPT controllers and their ratings say it all. Each will tell you how many watts they can handle at certain battery voltages, usually 12 and 24 volt but some go up to 48 volts. Some controllers will say that they can take more watts but you will not get the extra power. So if you have a 60a controller that can take 800w and put 1000w of panel on it, you will still only get 60a/800w of power max. The excess panel is good early when your panels are not putting out full power but you will never see the peak that you would with a controller designed to take 1000w.

I also agree with not pushing your controller to its limits all of the time. I have 750w on my 60 amp controller rated for 800w. 435w on my controller rated for 600w.

The second critical rating is the Voc limit be it by a single panel or multiple panels in series. Unlike over paneling which some manufactures say is okay, exceeding the rated Voc of a MPPT will damage the controller in all but a few cases. In the few exceptions the controller cuts the power from the panels when the Voc is exceeded so you are not getting anything by doing so.

Here again I believe in giving a buffer to the rated acceptable Voc because if your panel says its Voc is say 39v, that does not mean it will never exceed 39v. I have seen my 435w panel rated for 79v exceed 100 Voc on a normal day. I have seen a 220w panel rated for 36v exceed 40v on a cold clear day when exposed suddenly. Bob speaks of someone frying their controller driving over a high altitude pass in Colorado so a buffer is a good thing when it comes to the Voc rating.

So in most cases it just is not that complicated to find a controller that is big enough for the panels that you are putting up. Once you know the sizing you can start looking for other features such as the ability to set variables, temperature compensation, voltage sense, etc.

So look for a controller that will accept the total number of watts and handle the total Voc of your panels.

 

[Canine]

When I bought my controllers, I got the next size up for $45 more a piece. I spent $90 more for both of them. Solar panels are getting cheaper and stronger, so I spent what I consider a modest amount of money to make room for growth should I need to replace a panel with a new, stronger one due to damage or just because I want more solar. (Grunts like a gorilla.)

 

[MotorVation]

Oh good, my brain is bleeding again. Math does that to me..   :huh:

 

[Canine]

Oh good, my brain is bleeding again. Math does that to me..   :huh:

It sounds like a lot, and it is if you try to juggle all of that in your head. Best to take it one step at a time. Write down each piece of information. If you take your time, in 5 minutes you will have your answer. Not all controller specs are this convoluted, either.

 

[Oneleggedcowboy]

My brain is bleeding and overheated! LOL