What power generation output do I need from my solar panel to keep my external battery pack from fully discharging? The answer is that it depends upon three considerations: (1) Your geographic location; (2) The orientation and quality of sun vs. shade throughout each day of the year; (3) The highest number of consecutive overcast days anticipated during the poorest power-generation month of the year for your location.
Fortunately, there is an excellent article with a link to a free online photovoltaic calculator that can help you calculate your needs, thus avoiding the costly and time-consuming process of experimenting to determine them through trial and error: https://www.voltaicsystems.com/blog/estimate-solar-irradiance-iot-device/
When entering input to the photovoltaic calculator, use the “off-grid” tab of the calculator. Enter the solar panel’s peak output in watts into the field labeled “Installed peak PV power [Wp]”. Enter the external battery’s storage capacity in watt-hours into the field labeled “Battery capacity [Wh]”. Enter the discharge cutoff limit for the battery into the field labeled “Discharge cutoff limit [%]”. If you don’t know the discharge cutoff limit for your battery and you are using a Lithium ion polymer battery, a conservative assumption is to input 40 into that field. Enter the power consumption per day in units of watt-hours into the field labeled “Consumption per day [Wh]”.
Since the goTenna Mesh device’s internal battery has a storage capacity of 0.589Ah, and the battery voltage is 3.7V, the amount of power you would need for one day is 0.589Ah x 3.7V = 2.1793Wh. That’s the power you would need if the energy conversion between the output of the solar panel to the input of the external battery, and back again to the output of the external battery, to the input of the goTenna Mesh was 100% efficient. In real world conditions, the energy conversion is far less than 100% efficient. The linked article states “With lithium batteries, we typically assume 50% of that power is lost going into the battery and back out to the device. That power loss comes from heat as electrical energy is transferred to chemical form and back again, as well as from regulation.” If you want to assume 50% efficiency due to energy conversion and regulation, then you’ll need to adjust your input to the photovoltaic calculator. You would enter “4.3586” into the field labeled “Consumption per day [Wh]. (That is 2 x 2.1793Wh = 4.3586Wh)
Next, enter the angle that the solar panel is raised above horizontal into the input field labeled “Slope [degrees]”. As noted in the linked article, the optimal angle for power production varies with month and geographic location. Be sure to recalculate the power production at various slope angles to determine the optimal slope for the worst power-producing month of the year, which will be December in the northern hemisphere, and June in the Southern hemisphere. If you exercise the photovoltaic calculator for Brooklyn, New York, you’ll find the optimal slope in December is a tilt of about 70 degrees up from horizontal. Next, enter the Azimuth angle in the input field labeled “Azimuth [degrees]”. In the Northern hemisphere, if you are able to orient the solar panel so that it is aimed directly to the South, that will provide the best power-production. In this case, enter “0” into the Azimuth input field. Finally, click the button labeled “Visualize Results” to obtain a bar graph that illustrates results expected for each month of the year for the power consumed (labeled “energy output”) and for the excess power delivered by the panel (labeled “energy not captured”).
One final consideration: If the solar panel will not have an unobstructed view of the sun throughout the day each day of the year, you will need to scale down the power output value you enter for the solar panel in the photovoltaic calculator. (i.e., if the sun tracks behind trees, buildings, and/or clouds during the course of the day, you’ll need to make a conservative assumption about the reduction in power production that will be output from the solar panel to the external battery.) Let’s say that you know the panel will be placed in a location where it will only have direct line of sight to the sun for 50% of the daylight hours on a clear day. Let’s also say that passing clouds will cause intermittent exposure to the full intensity of the sun during the periods of the day when there should be an unobstructed line of sight. You might make the conservative assumption that the output power of the solar panel is only 25% of the rated peak value, and adjust the input field in the photovoltaic calculator accordingly to help you understand expected performance under these conditions. The idea is to take these things into consideration to ensure that the panel you purchase has a high enough energy output to prevent full depletion of the external battery pack and the goTenna’s internal battery. As previously mentioned, you need to keep your relay node up and running continuously, or a manual intervention will be necessary to depress the power button and to place the device in relay mode once the solar panel recharges the batteries in the system. For this reason, it’s best to err on the side of selecting a solar panel that produces too much, rather than too little power. (It will save you the aggravation of periodically visiting your stationary relay nodes to reset them. Depending on the difficulty of accessing them, this may be “priceless”.)
In the end, you’ll still need to try out the system you design to verify adequate performance in the varying conditions at your location, but the photovoltaic calculator and the tips above will help you select the right solar panel for your needs the first time.