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Sizing your PV Panels
Photovoltaic panels are a rapidly maturing technology and as volume increases, prices have been dropping. If you haven’t priced panels for a few years, your instinct may be buy the smallest panel you think you can get away with, but this would be a mistake. Higher wattage panels in many cases are only slightly more expensive than lower wattage panels. In terms of price per watt, the best value is currently in the 20 volt panels commonly used in grid-tied residential applications which can often be found for the same price as a 12 volt panel with half the power rating. These panels can be used with 12 volt batteries if you have the right charge controller.
Once your PV panel has been selected, you should consider a mounting rack. Having an adjustable rack will make it straightforward to optimize the angle of the panel for maximum energy collection as well as easing reuse of the system at a different location. An automatically tracking panel rack will produce more power by continuously adjusting the angle of the panel to be perpendicular to the sun’s rays, but the added cost and complexity is generally not advisable for this type of application.
Choosing a Charge Controller
Since solar panels put out varying voltage levels depending on the intensity of the sunlight and batteries require different voltage levels depending on the current charge level, a device called a Charge Controller (sometimes referred to as a regulator) is required to regulate the voltage and prevent over-charging the batteries as well as preventing reverse voltage (so power doesn’t flow out of the batteries and into the solar panels at night). Charge controllers may also provide additional fuse or circuit breaker protection. There are several types of charge controllers available. Ensure that the controller you purchase is designed to work with the type of batteries you have chosen, as different batteries types have difference charging profiles. While they are slightly more expensive, a Maximum Power Point Tracking (MPPT) controller is generally recommended for this type of application as it is the most efficient and wastes less power in converting voltages than Pulse Width Modulated (PWM) controllers. Many MPPT controllers will also allow you to use commonly available (and thus cheaper) 20 volt panels designed for grid-tied applications without wasting the extra voltage they provide when connected to a 12 volt battery. Another feature that is highly recommended is a Load Control or Low Voltage Disconnect (LVD). This disconnects the load from the battery when the battery is nearly depleted, preventing an over-discharge that can permanently damage the battery.
Charge controllers are rated by current and voltage. For all systems, the nominal battery output voltage should match the voltage of your batteries (typically 12 V for small installations). For PWM controllers, the nominal PV input voltage should also match the battery and panel voltage (again, typically 12 V). For MPPT controllers, the nominal PV input voltage should greater than or equal to the panel voltage, and the maximum PV open circuit voltage should be greater than or equal to the panel open circuit voltage (VOC). The continuous-use input current rating of the controller should be greater than or equal to the short circuit current of the panel (ISC) multiplied by a 1.25 safety factor. If the current rating of the controller is not specified as for continuous use, then use a safety factor of 1.56 instead.
The battery and charge controller need to be enclosed in a weather-proof box, however it is important that the box is not air-tight. In the event that a lead-acid battery is overcharged, some hydrogen gas may be discharged. Normally this gas would dissipate harmlessly into the atmosphere, but in a confined, air-tight box enough gas may build up to become dangerous. Thus it is a good practice to use an enclosure with a vent to prevent the build-up of gas.
Wiring and Fuses
If your charge controller does not include integrated fuses or circuit breakers, it is good practice to add in-line fuses on the positive wires from the solar panel(s) to charge controller, charge controller to battery, and charge controller to load. Fuses and wires should be sized based on the maximum current for that wire multiplied by a 1.25 safety factor. Wires should be kept as short as possible to minimize losses (especially from the controller to the battery which carries the highest current in a MPPT system). Wire size should be selected to handle the maximum current (plus 25% safety factor), though using a wire size larger than the minimum is fine and will reduce losses, particular in runs which are necessarily long, like from the panels to the controller.
The Eco Online website has a useful wire sizing calculator, although it uses cross-sectional area instead of American Wire Gauge (AWG) to define the wire size. The table below can be used to cross-reference AWG to area.
The guidelines presented here are somewhat conservative. Additional constraints like size, weight and cost may force you to compromise on certain aspects of the design.
Good luck with your solar-powered projects, and feel free to post questions about this article in the comments section below.
Crompton, T. (2000), Battery Reference Book 3rd ed, Newnes, Oxford
Messenger, R., and Ventre J. (2005), Photovoltaic Systems Engineering 2nd ed, CRC Press, Boca Raton
Smith, G., et al (1991), Maintenance and Operation of Stand-Alone Photovoltaic Systems, Sandia National Laboratories, Albuquerque