Every now and then I get a request about running the station off the grid. Here are some numbers and calculations to help with figuring out what battery and solar panel is required to get the setup through the winter months in a worst case scenario.
The station including all sensors consumes 0.65W, that is 130mA at 5V.
Lets assume the station is located in Seattle WA, where the shortest day is a bit more than 8Hrs and consider that we might only get about 50% of the max sun influx during the winter.
During the 16Hrs of night the station will consume 16H * 0.13A = 2AH , Using a safety factor of 3 shows us that we should use a battery with a capacitance of at least 6AH.
During the 8Hr day the solar panel needs to provide the current to run the station ( 0.13A) as well as excess current to charge back the battery. Since there is only 8Hrs of light, the 2AH from the day needs to be replenished in 8Hrs, so 2AH/8 = 0.25A. Together with the running current that totals to 0.38A. Considering the 50% cloudiness factor from above we double that number to come up with a requirement of about 0.75A from the solar cell.
When looking through Ebay for small solar panels that can stand up to the elements we find the smallest panels come in 10W nominal, for between $20 and $40 a piece. These panels have a max current (shortcut) of around 0.7A, ideal for this setup.
For a minimal setup and to eliminate losses in voltage conversion etc I’m using a 6V 12AH sealed led lead acid battery. To prevent the battery from overcharging in the summer I have a 7.2V shunt regulator across the battery that can handle 10W. A low-drop-out (LDO) 5V linear regulator provides the regulated power for the station.
An other solution is by using a 12V 15-17AH SLA battery with a 10-20W solar panel. Connect this together with an off the shelf solar charge regulator ( check EBAY, many to choose from for ~$20 ) . Make sure that the charge regulator has one or two 5V USB outputs. This provides enough power to also run the WiFi hot-spot as well. Run it at it’s lowest power setting for best performance.
One thing to consider when working in cold climate is that the Battery doesn’t like the cold. If there is a building put the battery in the warmest place and insulate it by sticking it into layers of Styrofoam etc..
An other thing to think about is to build a solar tracker for the panel. There are a few excellent YouTube videos that detail how one can make a tracker with a RC servo and two light sensitive resistors plus a servo tester. Care must be taken however to not spend more power in tracking than the solution provides. An Arduino based approach that only adjust once every 15 minutes and sleeps the rest might be in order.