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Solar Regulator Information

Good solar cells are very expensive and people who buy and use them want the maximum power they can possibly get from their investment. In order to make an informed decision as to which type of solar regulator you need then you must understand how a solar cell performs and what is the difference between the 3 main groups of solar regs. As with most things in life, you get what you pay for, a typical 20 A / 12 V solar regulator can vary in price from £20 – 200+, why? Is the £20 as good as the £160 or at least nearly as good or is it a total waste of money? It’s extremely important to know which technology you purchase.

How does a solar cell work and what are the important operational features? (Please note all Sterling regs are waterproof)

As a solar cell is a current making device at a given voltage, the important thing to remember is the basic formula: Power (watts) = amps x volts. Therefore, to get the maximum power out of a device you must have as many volts and amps as possible. If the voltage was allowed to reduce, then the performance and hence the power manufactured by the cells drops off, and same for the amps. Maintaining maximum volts and amps is absolutely critical to achieve maximum harvesting potential of a solar cell.

Graph showing maximum power achieved

Other key things to think about with solar regs.

Remember a solar cell only works for about ½ a day (if even). However, the solar regulator works for 24 hours regardless of whether the solar cell is producing power or not (night or cloud cover). So, the key question is, what power does the regulator use to exist (we call this the quiescent current)? With most solar regulators this averages from 25-50 mA (on a 12V system). That means, for example, a 50 mA drain over 1 year is about 430 amp hours. However, at Sterling, we try to keep the quiescent current below 1 mA, this means the loss in current over the year would be more in the region of 8 amps not 430 amps. Therefore, 422 amp hours, which would otherwise have been wasted running the regulator can effectively be harvested and passed into the battery system. Low quiescent power consumption is achieved by the use of a more expensive micro processing chip set, some manufacturers are reluctant to spend the money on this type of device. Obviously other features which are of vital important are 4 step battery charging and the ability to select the different battery types being charged. This ensures the best charging curves for the battery and also prevents damage to battery bank.

3 main types of solar regulators

Power curve for a standard solar cell

Basic regulators

These are simple switching devices which switch between 2 voltages, typically they come on at about 12.9 volts at the battery and switch off at about 14 volts (x 2 for 24 V). This is effective at covering power loss from the battery but will not charge well or effectively use the maximum power potential form the solar cell, which for a lot of applications is absolute fine. Ideal for low cost 10-100 watt low quality solar cells.

The ideal use for this type of reg is, for example, if you had a motor home or a small boat on a swinging mooring and a small solar cell whose only job was to keep the starter battery topped up when vehicle / boat is not in use, then this product is perfect, a low cost cell and low cost switching regulator will do the job. However, if you have a good quality, expensive solar cell, whose job is to effectively and efficiently contribute useful power into a system then this type of regulator would be a very poor choice indeed, as such we limit the range of this product as to use this technology on larger cells simply makes no sense.

Advantage: very low cost. Suitable for: Low cost cells where the objective is simply keeping a relatively inactive battery topped up (such as a starter battery) as opposed to contributing useful power into a system. Disadvantages, very ineffective at maximising the harvest from a solar cell. (Please note the Yellow section on the Power curve graph)

PWM: Pulse Width Modulate regulator

(mid range cost, mid range performance) This type of regulator is still connected to the battery bank and is still under the influence of the battery voltage. However, the pulse width modulation allows the cell voltage to rise a bit further but it is still restricted because of its direct connection to the battery bank. Its performance is better than the basic regulator type but still falls short of achieving the maximum available power. The lower the battery voltage (i.e. a battery could drop to 11 volts), where maximum power is required, the less efficient this product is. This can be seen in figure 4, whereby as the voltage drops the power does also. Other weakness with PWM controllers is the fact that solar cells open circuit voltage changes with temperature. The colder the cell is, the higher the open circuit temperatures and vice versa, this is important to ensure that the large voltage swings from the solar cell are within the rating of this type of product. This type of regulator is not recommended for use with ‘on grid’ systems as there is still significant inability to achieve maximum power. They are best used on ‘off grid’ systems.

Advantage: Mid range harvest return from the solar cell, works fine especially in good sun conditions, Disadvantage: no boost ability so not good in lower light (Northern European use) , Please note on Power curve graph, the power harvested would be the Yellow plus the Green section.

MPPT: Maximum Power Point Tracking regulators

(latest technology and most expensive type) Up to 40% improvement on top of the PWM, which is about 70% on top of the basic unit. These, in effect, disconnect the solar cell from the battery bank to allow it to ‘run free of the battery’ and obtain its best performance. This means, in effect, you have 2 separate units, the solar cell side, which is allowed to operate free from the battery voltage restrictions, so the software can continually track and determine the most effective maximum power point that the cell can achieve at that moment in time, depending on weather and other environmental factors, such as temperature. That side of the product is 100% geared to maximize the harvesting of the suns energy. This also allows for different cell ratings to be used with different voltages and high temperature swings with no adverse effects and no setup required.

The big advantage of the MPPT is that the battery and the solar cell are not directly connected. This allows the battery charging side of the regulator free to concentrate on the battery / power requirements and effectively use the power available from the solar cell. The power is delivered to it via a highly efficient power conversion process, which not only maintains the max power point (MPP) in the higher voltage levels (when the sun is out) but continues to generate power even under low light conditions when the MPP is below the battery voltage (see fig 4) as the MPPT has a voltage boost function. This boost functions allows the device to effectively absorb that last little bit of power that can be obtained from the solar cell. Not only is this an effective function on a day to day basis, but, over time, if the cells become damaged, dirty or less efficient the MPPT continues to pull the power when a normal reg will have long since stopped working. Unlike a PWM regulator, an MPPT can be used with ‘on’ or ‘off’ grid systems. This system is also easy to install as there is no setup depending on different cell types or environmental temperature, making it totally automatic.

Advantage: Maximum performance from cell, low light / dirty cell operation when all other units have become ineffective. Absolutely necessary for: Large solar cells 100 watt + where every bit of possible power is required to be harvested, from 100 watt - 1000 watt Disadvantages, higher cost. Please note on the Power curve graph the this technology will harvest between the Red, Yellow and Green sections.

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