SOLAR TUTORIAL

Preliminary

More and more consideration is given to solar solutions to power requirements every year. Rising energy costs, environmental concerns, parasitic drains, remote locations or unavailability of AC power are some of the driving factors. Solar technology has evolved and is more efficient and cost effective than in prior decades, but the cost per watt or physical size of the panels required for the necessary power demand must be evaluated for each situation. Whole house installations, or high amp priority communication installations require site specific evaluations, involving regional climate history, and appropriate engineering. We generally do not engage in this type of evaluation.

We specialize in small applications, like standby starting batteries (generators, pumps), equipment trailers, heavy equipment, gate openers, radio repeaters, data loggers, security, etc., and maintenance of boat or motorhome house and starting batteries (not bulk charging).

Along with the rising interest in solar power, are questions on applications that don't lend themselves to a solar solution. Two of the most frequent are trolling motor applications, and golf carts. The amps and voltages for most of these require a physically large, complicated and expensive array, usually much more than acceptable to the customer.

Much more could be written in a solar tutorial, including solar insolation, mean peak solar radiation, latitude maps, etc., but at some point it becomes overwhelming. Our intention in this tutorial is a brief but encompassing section that sheds light on how solar panels/systems function, the components involved, and what they are suited for, particularly in smaller applications.

Sizing for Solar

The main factors for utilizing solar are available sunlight in the location, and daily power consumption in the application. These will dictate solar wattage required, and amp hours of battery storage, for nighttime and reserve. The solar panel (or array) should be oriented to true south and tilted. The tilt angle should approximate the latitude of the location, 25 to 46 degrees in the continental United States. Latitude plus 15 degrees biases the panel for winter months, and latitude minus 15 degrees biases for summer months. For trailers or other mobile applications that may be parked in any direction, we recommend a flat mount, though this is not the solar ideal.

Solar panels are rated at peak output, which is based on beneficial solar incident angle (close to perpendicular), and bright sunlight. This can boil down to 4 to 6 hours a day (or less) output for calculation purposes. There may be more output before and after this window, but power is reduced.

Most solar panels are 12 or 24 volt DC output. They can be series/parallel connected for desired array voltage (12, 24, 36, 48), and amperage. Most small applications are 12 volts. A 12 volt panel may have an open circuit voltage (not connected to anything) of 16 to 22 volts, which drops to battery voltage when connected. The further the load/control system is from the panel/array, the higher the system voltage should be, as wire length increases resistance, which increases losses. Increasing wire diameter helps, but only to a degree. A chart follows showing approximate panel wattage/amperage correlation.

Total load for an application is calculated, in watts or amps, and this number multiplied by the hours required for operation (i.e. 24 hours, for a 24 hour application) gives the amp hours needed in battery storage, and the number of amps (or watts) the solar array must replace in available peak sunlight hours.

Controllers

Controllers are recommended for 5 watt and larger panels. Controllers do a couple of things. First, they regulate the output from a solar panel as it charges the battery. They have a cut-in voltage, where they start passing current to charge the battery, and as the battery reaches a full charge, they regulate current to maintain the charged battery voltage without overcharging. Second, they prevent electrical feedback from the battery through the solar panel at night, which will occur if not blocked. Some smaller panels have a blocking diode built in to prevent this, if no controller is used. Some controllers will not pass voltage/current unless they see a viable battery voltage, so just checking the controller at unconnected terminals may be misleading. The load may be connected to the controller if it has provisions, or straight to the battery.

Battery Storage

The amps produced in a solar system are usually stored in deep cycle lead acid batteries (flooded, AGM, or Gel). Batteries also serve as a buffer in electronic applications, such as trying to use solar to charge phones, laptops, cameras, etc. in a camping or similar environment. Some devices can be run directly off a solar panel, but they must be designed for voltage variation. Battery capacity (amp hours) for an application would be a minimum of twice the daily amp hour consumption (for battery life and safety margin), and usually several times daily load, to allow for overcast conditions, etc. For more, see the battery tutorial.

An example:
You have a wildlife monitoring camera that consumes 250 milliamps (ma) at 12 volts DC. It operates 24 hours a day. 250 milliamps is .250 amps, so the daily load is .250 amps x 24 hours = 6 amp hours. You would need at least a 12 amp hour battery, preferably a 30 to 35 amp hour battery. For a solar panel, a 20 to 40 watt unit should work fine, depending on the geographical location (and maybe season of use, i.e. summer only, or all year). A controller would be used in this system.

A more complicated example:
You have a remote data logger that spends most of the time in a standby mode, at 150ma (milliamps), and transmits once an hour for 5 minutes at 2 amps, both at 12 volts. The standby calculation would use 55 out of 60 minutes per hour, and is:
(55/60) x 24 hours/day x .150 amps = 3.3 amp hours/day.
The transmit calculation uses 5 minutes per hour and is:
(5/60) x 24 hours/day x 2 amps = 4 amp hours/day.
So, the total draw is 7.3 amp hours from the battery in a day. You would use an 18 amp hour battery minimum, preferably a 40 to 55 amp hour battery or larger. A 30 to 50 watt panel would be advisable, depending on geographical location. A controller would be used.

Inverters

Inverters are frequently used in DC applications, including small solar, converting the stored DC to 120 volts AC. They are inefficient devices on the order of 10 to 20%, so using DC powered devices is recommended when possible. For more information, see the inverter tutorial.