Renewable energy systems have proven to be reliable methods of powering telecommunication systems in places where conventional electricity is unavailable or impractical. RTU, microwave repeaters, television and radio repeater sites are commonly located on mountaintops or otherwise remote sites not easily accessed during winter and inclement weather. Solar, wind and hydrogen fuel cells provide excellent sources of clean, reliable power to keep batteries charged in these locations. Even if grid power is available at these sites, a renewable energy system can provide security as a back-up in the event of grid failure.

SINGLE VOLTAGE SYSTEM SIZING
To size a simple (single voltage) repeater or RTU site we first need to evaluate the power consumption of the radio in both stand-by and transmit mode. We also need to determine the duty cycle (amount of time the radio is actually transmitting data during a 24 hour period) of the radio. Once we have this information, we need to know the amount of solar or wind resources available at the actual radio site. We also need to establish a period of autonomy for our system, which means that in the event of extreme overcast or calm winds, we want to know how long the system needs to be operable. This information will determine the size of the battery bank.
EXAMPLE: Let’s take the case of a simple RTU to monitor the flow of fluid in a pipeline. The equipment is 12 volt and draws .48 amps in stand-by mode and 3 amps in transmit mode. Transmit occurs for 1 second every 20 seconds around the clock. The system is located on a mountain top near Asheville, North Carolina and needs to have 10 days (minimum) of autonomy.
First, let’s calculate the standby power consumption; .48 amps X 24 hours = 11.52 amp hours per day consumed.
Next, let’s calculate the transmit power consumption; 1 second per 20 seconds is the equivalent of 3 seconds per minute or 180 seconds per hour or 4,320 seconds per day. This is equal to 1.2 hours of transmit per day at 3 amps. Multiply 1.2 hours X 3 amps and we get 3.6 amp hours of power consumed during the 4,320 transmit cycles. Next, let’s add the power consumption of the stand-by and transmit cycles and factor in 15% inefficiency for wire losses, battery inefficiency and losses in the electrical path. 11.52 amp hours + 3.6 amp hours = 15.12 (total power consumption) amp hours per day. 15.12 amp hours X 1.15 (inefficiency factor) = 17.38 amp hours per day that have to be produced to keep up with the demand of the radio as well as cover losses in the wire and batteries.
Next, we can calculate the battery size for 10 days of autonomy by multiplying the amp hour per day consumption by 10. This tells us we need a 174 amp hour battery to carry our loads for 10 days in the event of overcast or calm winds. Another factor that warrants consideration is battery temperature. Batteries loose storage capacity as they get colder so if we encounter a temperature of less than 80 degrees Fahrenheit (26.5 degrees C.) we need to increase the battery bank accordingly. For instance, a battery at 32 degrees Fahrenheit (0 degrees C.) will only have 65% of the storage capacity as the same battery at 80 degrees F. (26.5 degrees C.). If we assume the overcast or calm wind period will occur during 32 degree weather, we should increase the battery size by 35% to compensate for the cold battery. Therefore we need to multiply 174 amp hours X 1.35 which will equal 235 amp hours. We can then look for a 12 volt battery (or a combination of 2 volt or 6 volt batteries) with a storage capacity of 235 amp hours.