How to Select the Right Solar Panel

When considering a solar panel it is important to understand your power needs in the wilderness. By understanding your power needs you will be able to address the best battery bank and solar panel solution.

Chart of average current in mA Versus Solar Panel Mounting Position

Total Up All Your Power Needs

There are now so many electronic devices that hikers commonly use when hiking. I made a short list of the electronic gear I took on my John Muir Trail hike along with the battery size in mAh. I also included the approximate daily power I consumed. Your power consumption will vary.

  • iPhone 12 Mini Smartphone (2,227mAh battery, use about 1,200 mAh a day, depleted on day 1.9)
  • inReach Mini Messenger (1,250 mAh battery, use about 200 mAh a day, depleted on day 6.25)
  • Petzyl CORE Headlamp Rechargeable Battery (1,250 mAh battery, use about 100 mAh a day, depleted on day 12.5)
  • Expect to use 1,200 to 1,500 mAh per day.
  • External Battery (10,000 mAh battery)

Determine Your Charging Strategy

Assuming all electronics are fully charged on day 1 we can then estimate how long the external battery pack will extend the use of electronics. The smartphone will run out of power at the end of the second day and require to be charged. The inReach Mini messenger will not need to be charged until day 7 of the hike. The Petzyl CORE rechargeable battery will not need to be recharged until day 13. This means the external battery will primarily used to keep the smartphone charged. Assuming the delay of charging the inReach and Petzyl CORE rechargeable battery the external battery will be depleted on day 6 or 6,500*/1,200 = 5.4. This is exactly what was observed on my John Muir Trail hike. Most long-distance hikers resupply every 4 to 5 days and a 10,000 external battery bank is sufficient for their power needs. For the John Muir hike we were hiking 10 days between resupplying or charging.  So we decided to carry a lightweight solar panel instead of a heavy 20,000 mAh external battery.

*We can assume the external battery will provide only 65% of the listed power due to loss of energy during the transfer process.

Chart of Solar panel charging current versus time of day

Optimal Time of Day to Charge

We have found that the best time for charging using a solar panel is in the middle of the day when the sun is more direct. In the morning and evening hours the sun intensity is the low and the charging rate is minimal. As the day progresses sun intensity increases, between 11 AM and 4 PM, the charging rate will be the highest. As evening approaches the light intensity will diminish reducing the charge rate. The exact optimal timing will depend on the season and where you are located.

I like to take my longest break in the middle of the day so I can get out of the sun. This is great time to position the solar panel for optimal performance. I keep my panel attached to my pack during  the process to save time and also insure I don’t leave the panel behind.

Chart of the impact of shadow on charging current

Keep Panel Free of Shadows

We looked at how the solar panel charge rate was impacted when partially covered by a shadow. A shadow covering just 25% of the solar panel dropped the charge rate by about 50%. It is very important to keep the solar panel out of the shade for optimal performance.

Battery banks charging on a bunch of power strips

Smaller Discharged Batteries Charge Faster

We discharged 5,000 mAh of power from a small (5,000 mAh) and a large (10,000 mAh) battery. We then looked at the charge rate of 5,000 mAh into these two batteries. We found that the a fully discharged small external battery (5,000 mAh) charged much faster than a half charged large (10,000 mAh) external battery. Because of this we recommend carrying two smaller external batteries instead of one larger external battery. This will increase your charge rate and provide a backup battery.

Phone charging on a solar panel charger

Keep Battery Away From Direct Sunlight

It is very important to keep the battery away from direct sunlight. If the external battery pack heats up it will not charge as fast. Extreme temperatures can actually permanently damage the battery.

Keep Your Solar Panel Clean

It is important to keep your solar panel clean of dirt and debris. Accumulated dirt can drastically reduce the efficiency of your solar panel.

Summary

  • Orient the solar panel towards the sun to maximize charging performance.
  • Mount the solar panel to the top of the pack instead of the side.
  • Charge the more discharged battery first to increase the charge rate.
  • Carry two smaller batteries instead of one large battery, in general smaller batteries charge faster.
  • Keep the solar panel clean.
  • Keep solar panel free of shadows.
  • Keep your external battery away from direct sunlight. Excessive heat can destroy a battery.