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Typical solar panels deliver power to the regulator at 15-18 volts, or thereabouts, depending on conditions. The purpose of the solar regulator is to translate this power into a form usable to charge 12-volt batteries. Basically, the controller is a high-quality battery charger. Some controllers can also convert voltages – taking in 24-volt or 48-volt DC from the panels and outputting 12-volt for the battery bank. Only on the smallest systems can you use a solar panel connected directly to the battery bank without risking damage to the batteries.

There are only two types of controller technology worth considering; PWM and MPPT. If you see cheaper “shunt” type controllers ignore them.

PWM (pulse width modulation) controllers are the most common. These send very rapid “pulses” of power to the battery bank – the “width” (duration of the pulse) varies depending on the state of charge of the battery bank. They are typically 3 stage chargers, sometimes with a separate equalization stage (called a 4th stage by some marketing people).

MPPT (maximum power point tracking) controllers have microprocessor technology that allows them to convert some of the “excess” voltage (when available) to amperage for more rapid charging of the battery bank. They cost more but can effectively boost the gain from a solar panel array, thus you may be able to save a panel. They are worth the extra expense, in my opinion, if you are building a system that has at least 3 panels and 400 amp hours of storage. For a good primer on MPPT technology check the following:

Northern Arizona Solar - MPPT primer

Blue Sky Solar - MPPT Technology (Acrobat format)

Controllers are rated by the amperage they can handle from the solar array. So how big of a controller do you need? First, you may need to “derate” the controller by 20-25% - although the convention in recent years is for controllers to have the derating built into their specs. If they don’t need to be derated they will tell you in the specs. Why derate the controller? Because if you bought exactly the size controller you needed for your array, you might overload it. Remember, panels are rated under standardized test conditions – they can, and do, put out more than their rated power under ideal circumstances. So you have to allow for this when selecting a controller.

Second, you want to be able to expand your system later without buying a new controller. If you are starting with only 2 panels you probably will add a third or even a fourth at some later time. You need to anticipate this, at least a little. I would advise buying a bigger controller than you think you need – it is not that much extra cost. As an example, let’s use 3 – 120 watt panels rated at 7.1 amps (21.3 amps total). You are going to need a 25-amp controller – but what about room for growth? Maybe a 30-40-amp controller would be better.

For a good solar controller expect to spend $125-$180 on a PWM controller in the 25-40 amp range. If you want a remote display it will be around $100 more. For an MPPT controller, you will spend at least $200, and as much as $500+ if you get a fully featured 60 amp Outback controller. Charge controllers represent a relatively fixed and small proportion of the total system cost. Studies show controller prices do not fluctuate much over time, like panel prices do. Bottom line is – don’t wait for that great deal, and don’t skimp on capacity.

Controllers usually have the option of having a battery temperature sensor (remember, they are really just battery chargers). This is worth the extra cost if it is not a standard feature. The reason is that the temperature at the battery affects the optimal charging algorithm. Controllers have the ability to adjust the charging algorithm dynamically, based on real time temperature information. The same adjustment can be made (independently) by your inverter/charger. So you will probably have two battery temperature sensors at your battery bank. The sensor either attaches to the battery post, or is taped to the side of a battery. It does not matter which one.

So, which controller should you get? Only you can decide. Trace C40’s are a reasonably priced PWM controller that can be expanded to accommodate almost any RV system. Heliotrope makes a good PWM controller (they pioneered them) as well. On the MPPT side, Heliotrope makes the HPV-22 available for around $240. Blue Sky Energy’s (formerly RV Power Products) Solar Boost series starts around $220 for a 20-amp version (the 30 amp 3024i is an excellent controller that will support up to 4 – 120 watt panels). The Outback MX 60 is an outstanding MPPT controller that can also do voltage conversion – allowing you to use 24-volt or 48-volt solar panels. It comes with an LCD display, and costs about $550. More on voltage conversion later.

It is best to decide if you are going to use a MPPT controller during the design stage. This may influence your choice of solar panel. The reason is that MPPT controllers, by the nature of their operation, work best with higher-voltage solar panels. Solar panel ratings vary some – choice of higher voltage panels would be appropriate for an MPPT controller. You have to trade off the value of the higher voltage with the potentially higher cost per watt of the high voltage panels. Any panel rated 16.9 volts or higher would benefit with use of an MPPT controller – the higher the voltage the better. There are some panels on the market that are 12-volt panels that put out (around) 22 volts. These typically have 44 cells, instead of 36, and in combination with a MPPT controller a 100-watt panel can put out about 7.5 “boosted” amps. They also cost a lot more than a 36-cell panel (about $150 more than a Kyocera 120 watt panel with a 7.1 amp rating). You can do the math and see if you think the cost is worth the gain. Just remember that most of the “ratings” that are either optimistic, or are based on “perfect” conditions. Expect an average of 10-15% gain from an MPPT controller used with a 36 cell panel, as compared to a standard PWM controller. Under ideal conditions (cold ambient temperature, and a depleted battery) you may achieve as much as 30%.

Solar panel prices fluctuate a fair amount. There are deals to be found online. Once you design your system you can look for those deals, knowing that you have the information to make the tradeoffs in price/performance. Personally, I would go with an MPPT controller with the “best-deal per watt” panels I could find. But I would make sure the panels were rated for at least 16.8 volts. Kyocera 120’s can usually be found for $500 or less. (Well, that was true up to the begining of 2005, but now panel prices have jumped, and availability is becoming limited in some areas.) When doing this assessment, take a look at the 44-cell panels sold by http://www.amsolar.com/ . Depending on the street price of conventional 36-cell panels these may be worth the extra cost. You have to run the numbers.

Using 24-volt and 48-volt Panels

You can buy panels in 24 volts, or you can connect two 12-volt panels in series, just like you do with batteries, to increase the voltage to 24 volts. Why would you do this? Because voltage loss over distance is reduced as voltage is increased. You can send 24 volts twice as far as 12 volts over the same wire size (at a specified rate of loss). This means you can use smaller wire, or perhaps use the wire your manufacturer all ready ran in the “solar prep” package if it is otherwise too small. Or, if the solar regulator is far from the solar panels you can minimize voltage loss by sending 24-volts to the regulator instead of 12 volts. Use the table below to calculate drop, or this interactive calculator: http://nooutage.com/vdrop.htm.

Chart accounts for both wire runs.
wirelengthchart.jpg

To do this you need a solar regulator that can convert 24-volt input to 12-volt output. There are a number of regulators that can do this. They tend to be higher-priced controllers with advanced features, so you have an overall better package. I particularly like the Blue Sky Energy 30-amp 3024i MPPT controller, about $279.

 

Obviously, you need to decide if you want to go 24-volts during the design stage. If you go this route, look for 24-volt panels. They will make installation a little easier. You can easily use pairs of 12-volt panels in series, but the wiring will be a little more complicated. Use of 12-volt panels also means you have to have pairs of them (three panels won’t work). The price per watt is always the determining factor for me. If the price was close I’d go with the 24-volt panels.

What about 48-volt systems?  It is the same as principle as 24-volt, with the same benefits. If you have long wire runs, consider stepping up to 48-volt panels and regulator. Almost all the solar regulators that handle 24 volts also handle 48 volts. Just make sure that the 48-volt regulator will output 12 volts. Some will only output 24 volts. This is because residential alternative energy systems often (usually) run on 24-volt batteries. This is not convenient in most RV’s, because your DC house systems are all 12-volt. (Unless you have a bus, then you are usually 24 volts.) The disadvantage of 48 volt panels is that you often pay a higher cost per watt. And it is much harder to find a regulator that will take in 48-volts and output 12-volts. Only you can make the tradeoffs required.

Solar Regulator Summary

  • Size: make sure you buy bigger than you need. You want to be able to expand your system.
  • Type: MPPT controllers are the best, and are worth the extra expense if you are building a larger (bigger than 200 watts) system.
  • Wiring: Make sure you have big enough wires. Use the charts - do not guess. It is unlikely that an RV manufacturer supplied heavy enough wires for a proper solar installation. Make sure you check. 
  • Voltage: consider the benefits of using panels rated at 24 volts, or series combining to get 24 volts. You need to buy a regulator that can handle voltage conversion if you go this route.
  • Features: A remote display is nice, but not required if you have a good battery monitor. Solar regulators operate fine without any attention. Temperature compensation is a feature worth paying for. By all means get this. Equalization can be handled by most inverters more efficiently so is not worth paying extra for.