Choose a controller for your system that can grow along with your needs. It ends up being cheaper to get a bigger controller now than having to buy an up-grade later so you can add panels to your system.  A controller with meters is an excellent choice.

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Solar Boost Charge Controllers

Link to Solar Boost article that explains in detail how these controllers work.

This is a relatively new technology now being used by various manufactures in some controllers and in some cases, it may be worth considering the extra expense of installing this type of controller into your system.

How MPPT Works

The maximum amount of power a PV array can produce varies with solar intensity, solar cell temperature, and module design. With these factors constant, the actual power that the module delivers varies with the voltage at which it is allowed to operate. A PV module is a constant current type device. As shown on a typical curve of PV module voltage vs. current, current remains relatively constant over a wide range of voltage. A typical 75 watt module is specified to deliver 4.45 amps at 17 volts with a cell temperature of 25°C.

A traditional PV controller connects the PV array directly to the battery when the battery is not fully charged. (The only time it actually processes the power is when it must be reduced to prevent overcharge.) When this 75 watt module is connected directly to a battery charging at 12 volts, it still provides about the same current. But, because PV voltage is now reduced to 12 volts by the battery rather than 17 volts, it can only deliver 53 watts to the battery. This wastes 22 watts of available power.

The MPPT technology charge controllers operates in a very different fashion. The maximum power voltage (VMP) at which the PV module delivers maximum power, in this case 17 volts. It then operates the PV module at 17 volts, which extracts the maximum available power. The MPPT controller continually recalculates the maximum power voltage as operating conditions change. Input power, in this case 75 watts, feeds a power converter which reduces the 17 volt input down to the battery voltage at the output, and correspondingly boosts the current. The full 75 watts which is being delivered at 12 volts would produce a charge current of 6.25 amps. A charge current increase of 1.8 amps or 40% is achieved by converting the 22 watts that would have been wasted into usable current. In reality, current increase will be somewhat less as a bit of power is lost in the conversion process.

How Much Charge Increase Will You See?

For a particular installation, the actual charge current increase will vary with PV temperature and battery voltage. Lower PV temperature increases VMP and thus the potential current boost. Lower battery voltage also increases the boost. In cool but comfortable temperatures with typical 75W modules, current increase normally varies between 10 to 25%, with up to 30% or more achieved in cold temperatures with a discharged battery. These controllers work quite well with higher voltage modules since a large portion of their power increase is due to higher VMP which traditional controllers can't make use of. In hot weather, when PV voltage is lower and current boosting may not be possible, Solar Boost will pass current through with a very low voltage drop.

Other Features

Solar Boost also features 3-stage charge control (bulk, absorption and finish). This type of charging assures the most rapid charging but it reduces water loss and battery damage during long periods of excess energy. It has a manual equalize setting for periodic battery maintenance. It is adjustable to the amp-hour size of the battery bank, to optimize both energy transfer and battery life. You can also connect it to an external shunt so that its decision to drop to finish charge is based on the net current flow measured right at the battery. A temperature compensation probe is additional.

The Solar Boost 50 is so well designed that it would be a winner even without the MPPT function. It can even accept a 24V PV array to charge a 12V battery bank. This reduces the minimum array wiring to 1/4 the size. Instructions are excellent.

Pricing and Purchasing

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