Solar Charge Controller Sizing, Part 1: PWM & Shunt

All renewable energy (RE) systems with batteries should include a charge controller. In this article we'll principally be referring to solar charge controllers . Charge controllers prevent battery overcharging and also prevent the batteries from sending their charge back through the system to the charging source (i.e., the solar panels). Think of a solar charge controller as a battery nurse—its job is to monitor the battery bank, feeding it what it needs and checking its vital signs. Since a solar controller does its work in line between the solar panel array and the batteries, it would make sense that its selection and sizing would be influenced by those components. And that's exactly the case.

Voltage and amperage (or current) are the parameters we use in solar charge controller sizing. The solar  controller must be capable of accepting the voltage and current produced by the DC source (usually solar panels) and delivering the proper voltage and current to the batteries. This situation might make you think that the DC source, charge controller and batteries must all share a common voltage. While that is one system design strategy used in many installations, it's not the only one. More on the alternatives later. For now, it's one voltage for everyone!

Technically speaking, the DC source must always have a higher operating voltage than the battery bank in order for current to flow from one to the other. A handy way to remember this fact is the statement, "Curent flows downhill.” For the purpose of this discussion, we'll use nominal voltage which means common battery voltages. Nominal voltage in this sense is synonymous with battery voltage. Since batteries (where they are used) are in many ways the heart of an RE system, we can call the bank's voltage the system voltage. The system voltage selected for any given installation is usually, though not always, determined by the battery bank required by the application; the inverter, if one is used, will also influence the choice of system voltage. 

Sizing comes down to this: there's the quick method, which will very likely give an acceptable, if perhaps oversized result, but won't describe the why ; and there's the more detailed method, which will reveal all the steps for greater understanding and precision. We'll look at both methods.

Short Method:

The Detailed Method (with mysteries revealed!):

In systems using the same-voltage-in-and-out solar charge controller type, all three main components must match nominal voltage throughout the system: the solar panel (or photovoltaic, PV) array, the solar charge controller, and the battery bank. Such solar controllers come in either of two types: shunt or PWM (Pulse Width Modulated). So for these situations, the first step in solar charge controller selection or sizing is voltage selection:

1. A shunt or PWM solar charge controller must match the system voltage.

The next step in sizing involves current . You might think we could simply match a given solar panel/solar module (or array) output current, as it appears on the module's label, with a solar charge controller that is rated to accept at least that current; and we could do that, but there's a risk involved. A solar modules' current output varies with changing conditions of temperature and sunlight intensity, so we must add a prudent safety margin to ensure that the charge controller is not subjected to potentially damaging amperage. Zap! = bad. Adjusting the solar controller's rating values is called "de-rating”. This step is a matter of a simple calculation based on the solar array's rated output current.

That rating is easily obtained for a single-panel (module) system: just look at the label on the back of the solar panel. Current output is listed there in at least two ratings: Imp , or current at maximum power, and Isc , or short-circuit current. Isc is always higher than Imp and sizing calculations are always based on Isc even though a short circuit in the system may seem like an unlikely occurrence. The important point here is that there are conditions under which a module can produce more than its rated current. We must protect the solar charge controller against those circumstances; hence the de-rating.

We use a standard factor to account for all Photovoltaic (PV) output-boosting circumstances (read more about the specific conditions in Solar Charge Controller Sizing, Park 2) [link to that article]. That factor is 1.25 or 125%. So our formula for current safety margin (de-rating) might look like this:

2. Module Isc X 1.25 = minimum charge controller amperage (A) rating.

This value is the minimum needed charge controller amperage rating. Of course, there may be no charge controller available that offers that exact amperage rating value. Common ratings include 5A, 6A, 8A, 10A, 12A, 15A, 20A, 25A, and on up to 70A. Select a solar charge controller with a rating equal to or higher than your de-rated value. Always "round” upward.

If the solar panel array consists of more than one module, we must be sure to take the array wiring scheme into account as we calculate voltage and amperage, since the wiring method affects these figures. wiring produces additive voltage and constant amperage; wiring produces additive amperage and constant voltage. Calculate total array current accordingly. Our Step 2, then, would properly read:

2. Array Isc X 1.25 = minimum charge controller current (A) rating.

A second de-rating factor may be required. For systems in continuous operation, additional protection must be included, according the National Electric Code (NEC), to allow for heat and equipment stress. Continuous operation is defined as three hours or longer of continuous use, which would include most PV systems. This factor is also 1.25 or 125%. We can either run this second calculation after the first, like so:

 

Or we can combine the two calculations into one, using the following close approximation:

2. (Alternate calculation) Array Isc X 1.56 = fully de-rated min. charge controller (A) rating.

Why use two different multipliers?

The reason we point out these two ratings separately is that many solar charge controller manufacturers already include this 2nd de-rating (for continuous use) in their products' rating figures. Case in point is the Morningstar brand. Their charge controllers are essentially already rated for continuous use. With a Morningstar solar charge controller, a total de-rating factor of 1.25 is all that's needed for a given system. If you're unsure whether the controller you're considering has already been de-rated for continuous use, err on the side of caution and use the 1.56 factor in your calculations. Turn the page for a summary of the method.

A detailed method steps go like this:

Sizing Solar Charge Controllers (Short Method)

Example (Short Method)

A shunt or PWM solar charge controller must match the system voltage. Array Isc X 1.25 = (A) de-rating for 1st safety margin. If needed, include continuous-use de-rating: (Result from Step 2) X 1.25 = fully de-rated min. charge controller (A) rating. Select controller rated as equal to or greater than result in Step 3.

Two 125W, 12V nominal modules, wired in parallel (12V system voltage); module Isc = 8.0A: 12V nominal voltage requires 12V nominal solar charge controller. 8.0A X 2 = 16A 16A X 1.25 = 20A 20A X 1.25 = 25 Some solar charge controller options: GoPower! 25A or Morningstar SunSaver 20A*. (You could still use the C35 or Morningstar Prostar 30 as well) * The 20 amp Morningstar is an option because it is already "derated" and accounts for the additional 25% increase.

Notice that the second (more detailed) calculation has led to a lower amperage value and therefore different charge controller options. You can see that, in this case at least, the short method has resulted in a bigger margin of safety than the detailed method. Is this a problem? Not really—all the controllers mentioned will be perfectly fine in this system. So unless you're on a very tight budget and the difference between one charge controller price and the next one up will break you bank, use the method you're most comfortable with.

Alternatives

In some cases, it makes sense to have a PV system with a higher-voltage array to reduce voltage drop over long distances; yet you may need a lower-voltage battery bank to run some appliances (e.g., a 48V array and a 12V battery bank).  In those instances you can utilize the special powers of a different class of charge controllers. They represent the alternative strategy I hinted at earlier: it's a charge controller technology that can make them "smart” enough to handle DC sources with substantially higher voltages than the batteries they're feeding. The smart ones are called MPPT (Maximum Power Point Tracking) charge controllers. You can read about them in Part 2 of this article.

Plan Ahead!

What you don't plan for can come back and bite you in the behind! Ambient conditions of sunlight intensity and temperature can boost your PV array voltage and current output and put your charge controller at risk. Save yourself money and hassle by planning your RE system carefully, making appropriate allowances for conditions—however unlikely—that can affect component sizing calculations. Over time, the money you save can go toward expansion of your RE system.