What Size Charge Controller Do I Need For Solar Panels?

What size charge controller for a 100 watt solar panel? Or 200w, 300w, or 400 watt solar power system? Or more than that?

(A charge controller installs between the solar panel array and a battery bank)
(description below)

Here’s how to figure out the size charge controller you need (its rating).

I have completely revamped my original post on this. I used a simple hypothetical 300 watt solar panel example to charge a 12 volt RV battery. The concept of how to figure it out will be similar for other solar panel wattage values.

Ready? Lets go…

Every make/model charge controller has their own unique specifications. I know that sounds pretty obvious. But it’s important to match your solar array (which has its own unique output specifications) with the proper charge controller. And it’s important to match your charge controller with the battery or battery bank.

The primary factors that matter most while choosing the right charge controller are:

  • the solar array output voltage (volts) under peak conditions
  • the solar array output current (amps) under peak conditions
  • your battery bank voltage (12 / 24 / 48 ?)
  • your battery bank charge amps requirements

The charge controller input specifications (from the solar array) need to be at least able to handle the maximum voltage and current (amps) from your given solar array.

The charge controller output specifications (to the battery or battery bank) need to match the battery voltage, and enough current (amps) to properly charge the battery (batteries).

There’s no one size fits all answer for this. You need to understand the specifications.

I’m going to give you an example, albeit a fairly simple one. This will give you the concept of what we’re doing…

I will use one 300 watt solar panel (actually I have 12 of them in my array, but I don’t want to get complicated here).

Size charge controller for 300w solar panel

Lets say I bought one 300 watt panel to mount on the roof of an RV. This will help charge the 12 volt battery in the RV while off grid or boondocking.

First I choose a 300 watt solar panel. For the sake of this example, I will choose the following. Why? Because while searching for a single 300w panel, I noticed this one which is bendable. Might be ideal for an RV roof!

Bendable Solar Power Module
(view on amzn)

Okay, now lets check the important output specifications:

  • Output voltage: 18V
  • 300 (watts) / 18 volts = 17 amps

Now I know that I need a charge controller that can handle at least these input specifications.

But what about the charge controller output specs?

I already know one of them, the battery voltage. We will be charging a typical 12 volt deep cell battery. I happen to use the Trojan AGM-31. A typical battery of this particular type is rated for 100 AH (amp hours). In other words, it is the battery energy capacity.

This capacity rating is often specified as the C20 rating. “It is the energy a battery can deliver continuously for 20 hours at 80°F without falling below 10.5 volts.”

In other words, a typical deep cell 12-volt battery that has a C20 rating of 100 AH, means that this battery can deliver 100 amp hours of energy over a 20 hour period.

Why do I say all this? Well, for one, it’s good to know! But secondly, a typical recommended charge rate (amps) for this battery is 20% of C20 during its bulk charge phase. How do I know this? I looked up the specs. on their website.

So this means that ideally the charge controller output would be able to supply 0.2 x 100 amps. Or 20 amps.

Note that this would be (20A x 14.4V), or about 288 watts. (14.4 volts happens to be the recommended bulk charge voltage). So our 300 watt solar panel is perfect for this!

Also note that although in this case a 20 amp charge controller will work, adding at least a 25% margin is even better. Like most systems, you don’t want to run them with “the pedal to the metal” or 100% all the time <grin>.

I would advise a 30 amp charge controller for a 300 watt solar panel system. This provides plenty of margin.

>> 30 Amp units on amzn

Yet another note… If I had two of these exact 12 volt batteries in parallel (for more capacity), the 20% C20 charge rate would now become 40 amps. So I would want a bigger charge controller, and I would also need another solar panel!

That said, it’s not absolutely necessary. If you don’t beef up the system, the charge time would just be longer instead. This may or may not be a problem for you.

10 amp, 20 amp, 30 amp charge controllers are widely available, commonly used on 12 volt battery systems.
>> Amzn has lots of them

Most common solar pv charge controllers come in 12, 24 and 48 volts.

High power systems will typically utilize 48 volt battery banks to advantage smaller cables and losses than otherwise on lesser voltage battery systems.

There’s more to it than what I’ve described (about choosing a charge controller). However the calculation above is the most determining factor for the rating.

Other factors include:

  • MPPT (Maximum power point tracking – the most common) or PWM (Pulse width modulation)
  • Temperature compensation
  • Ability to charge varying types of batteries
  • User adjust-ability of charge parameters

Most manufacturers have sizing tools to help the decision process.

What is a Charge Controller?

MidNite Solar Classic Charge Controller

My current ~4 KW solar system charger is the Midnite Solar CLASSIC 150 (96 Amps / 48 Volt Battery Bank)
>> view on amzn

It is a device which most importantly properly charges a battery or battery bank, enabling as long a life as possible.

A solar charge controller is designed to receive power from a solar panel or array of panels and use it to charge a battery or bank of batteries.

The charge process is performed in a ‘smart’ or ‘intelligent’ way. This involves varying charge stages, voltages, and current — depending on the type of battery and other factors.

Not to be confused with an inverter, a charge controller is specifically designed to charge batteries in a controlled fashion.

HIGH POWER INVERTER / CHARGE CONTROLLER

If you are considering a higher powered system, such as for an off-grid cabin (or whatever), I highly recommend that you contact Sol-Ark (a sponsor here) who makes excellent Inverter / Chargers! They will be happy to help determine your needs. If you contact them, mention you came from Modern Survival Blog :)

Sol-Ark

[ Read: The Four Essentials of Off-Grid Solar ]

[ Read: How to measure power consumption – designing a solar system ]

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21 Comments

  1. Thanks Ken, good photo. Nice assortment of ‘solar’ power parts…. We have a garage where the roof faces south. Are considering some solar.

      1. Joe c,
        The rifle (an old Marlin 880, .22 caliber), sets at its resting place (as shown) which is near the back door. It’s for dog-potty duty at night. Coyotes. My Sampson is just a little feller. Though he has no idea he’s little.

  2. Note that the example numbers listed above are simply the “ratings” (the max that a given charge controller can do).

    It’s not the actual charge profile parameter(s) — which will vary depending on one’s setup.

    For example my ‘MidNite Solar’ brand ‘Classic-200’ MPPT charge controller (as shown above) has a multitude of user-selectable settings and tweaks. This allows me to perfectly adjust the charge parameters for my particular battery bank.

    Just because my charge controller is rated for 79 amps and up to 48 VDC, doesn’t mean I’m going to push that through a single battery! (for example). Rather, it means that it’s able to pump out that current — which in my particular series/parallel combo 48 volt battery bank of 24 batteries — is perfectly fine.

    Make sense? Someone contacted me, apparently confused about it, so this is my attempt to clear it up.

  3. As mentioned by Joe C:

    Do guns and solar systems go well together?

    Neat set-up Ken. Is your system off-grid? Does your state allow or grant permits for an off-grid system?

    I know some states like California did not allow for off-grid systems and now PG&E is bankrupt so depending on your location, solar systems may make financial sense.

    1. Calirefugee,
      No special permits required here where I live. And yes it is off-grid in the sense that I have installed individual transfer switches for most of my electrical panel circuit breakers from the grid. This enables me to choose which circuits I want to be solar powered rather than grid. During the summer I can run whole-house on solar. During the winter, the essential systems (unless a long period of clouds – thus draining my batteries over several days). So I play with it throughout the year.

  4. Ken,
    Very nice set-up. Mine is in an outdoor building which is just attached to the side of the house. It’s not really a building, as there is not room for a person to enter it. Matching hardi-plank siding so it looks OK. Gotta keep the wife happy. When the power goes out, she keeps her smile, for a while at least.

  5. hi, and thank you. would 2 separate controllers be a good idea for 4 100amp hr lifeion batteries to split the loads

  6. Does anybody have experience with the Lithium Iron Phosphate batteries for your solar setup?

  7. Timely updated article. I’ve taken my first plunge into solar and ordered a Renogy premium 400w kit from Amazon (I know, starve the beast, but I want what I want….).

    I also ordered and received 4-100AH AGM batteries to go with it. It comes with a 40Amp Charge controller.

    Very interesting that the first shipment was never shipped but I received the batteries. I reached out and they said the first kit was lost and they would send a replacement. After telling them that I needed the kit and that I already ordered the batteries, the rep actually credited me for the batteries as not returnable and sent out a new Panel kit. It’s supposed to arrive tomorrow.

    I’m going to order 4 additional batteries since they credited me for the original 4. I’ll have 800AH (400AH usable)

    Since the 40Amp Charge controller should handle about 7 panels, I’m wondering if anyone has ever connected 2 sets of panels and 2 charge controllers connected to the same battery bank? I’m thinking I should maybe just get a charge controller like the one referenced in the article. Any thoughts? Again this is my first “experiment” with Solar and I’m thinking I can at least keep a freezer or two going.

    1. That’s great that you’re getting a system!

      You should not connect 2 charge controllers to the same battery bank.

      I’m a little confused about what you’re going to end up with, given the problem you had with shipment… Sounds like you will end up with 1 kit and 8 100AH batteries?

      Anyway, You could simply connect all 8 batteries in parallel (which sounds like your plan) and connect to the 40 amp charger from the kit. The 8 batteries will just take longer to charge (compared to 4 batteries).

      1. Exactly, the 8 100AH batteries in Parallel Giving me 400AH usable with initially 4 100Watt panels. At 5 Amp output per panel I can probably hook up a total of 7 panels to stay under the controllers max. I was thinking that if I expanded beyond the 7 panels I would use another controller, but I think I’ll just get a bigger controller and put the 40A in the F -Cage.

        As I tend to over-engineer things (don’t get me started on my Chicken compound :-) , I’ll be building a frame to mount the panels on and then making it tilt-able, then mounting that on a roll-able stand on wheels in case I need to move it in or out of a garage stall.

        I did talk to the folks over at Iron Edison and they were very informative. Figured I’d start small and tinker a bit before looking into a bigger, more permanent & expensive solution. At least I’ll have something, as who knows what lies ahead.

    2. Sure enough, the 2nd Amazon shipment was a fail. I talked to them and then saw that the kit was now $120.00 less than when I originally ordered it. They refunded my original pymt and had me order again at the lower price. Now I have to wait until next week, if it even shows up….. At $520 per kit, I’m tempted to order a second and just mounting them on the garage. I just want to get something in and start tinkering…..

      1. Double Tap. Good job getting that deal. I just looked to find it’s 799 now! I hope your project goes well.

        1. Thanks. We’ll see if this third order actually gets shipped. The tinfoil hat is whispering to me that maybe TPTB don’t want people to have any energy self-sufficiency……

  8. Ken,
    Thank you for running this again. We have decided to move into solar slowly. Buying components as they become available too us. So I will be laying out a system based on a recent purchase of 6500 watts of panels, through a friend that bought two pallets of panels for his stand alone system. He was able to up his order and get a better price per watt ( $0.54/watt). I’ll pay him back and start picking up other components as I get more $$. Big investment for us, so want to do it right. Thanks again for these articles.

    1. Minerjim,
      Wow! What a great price per watt!
      My system only has ~ 3800 watts of panels. You will be doing great with 6500!

      I do plan on posting a lot more on solar powered systems and such… I do enjoy it.

      1. Ken,
        Yes, it was an offer too good to pass up, and got them while they are still availible. This is with a Good friend who has had an off grid system for over 20 years in the Rockies. He is now going full solar for a new place in the desert. 6500 watts of panel should allow us to be fully independent of the the grid at some point, if we can get the $$ for the battery storage. That area is rapidly changing also, so maybe by waiting and gathering all my other components i can get into some good deals for battery storage. I look at it as a long term learning project. My thought is putting together a completely independent power system in parallel with the grid system, and slowly migrating too it. thanks again for the continued solar articles.

        1. What would be really cool if there was a “desktop/tabletop” ATS. For instance having a freezer/fridge plugged into some ATS device with the device plugged into both the wall for grid power and also to a battery bank, either directly or through an inverter. I know it should really be done at the circuit panel, but for small experiments/appliances, etc…… Will have to look for this mythical beast…….

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