I am a huge solar advocate. We have a tent trailer that has 250 watts of panels. Prior to our current trailer we have owned for 5 years, we had another with 120 watts of power. I also have a sophisticated battery monitor that I have used over the years to measure amps in/out, etc. We have not connected our trailer batteries to the electric grid in 7 years. And we have camped well over 700 days and never had a dead battery. So some of the things about our tent trailer are appropriate for this discussion.
Number one: You need to determine how much power your devices will consume each day. That is how many amp hours (AH) you need to replace. Since we use lead acid batteries in our camper we cannot go below 50% of our battery bank's rated capacity, as the batteries can get damaged. Our panels are sized appropriately. In normal weather, even in the winter, our battery bank is usually fully charged by 10:00 AM. We are usually not in the camper during the day, so we need enough capacity to get through the nights. Typically our usage includes lights, water pump for dishes and showers, and operating a forced air heater during the night. We have replaced our 12 volt incandescent light bulbs with 12 volt fluorescent lights, which use less juice. We find LEDs do not work well for general lighting. Our refrigerator runs on LPG. We also have a LPG catalytic heater that uses no electricity and 75% less gas than the furnace. We use that when night temps get into the 30s F or below. The catalytic heater puts out about 50% of the furnace's BTUs and provides radiant heat. So sizing our system included a reduction in electrical consumers.
Number two: solar panels are rated at the max they can put out. With high quality panels like we use (Kyocera) we can get up to 95% of the rated power out of them under ideal conditions. For fixed installations like ours, there are other methods to get the full 100%, but not applicable to this conversation.
Number three: You need to determine how much sunlight your solar panel can collect. In So Cal in summer it is easy to get full power during the middle of the day. The season (e.g. winter) of the year and latitude (e.g. Seattle) can reduce this significantly.
Number four: The panels must be angled directly at the sun for maximum solar collection. This is not too difficult for fixed installations, but impossible while moving. If you cannot tilt panels directly at the sun, then you need a larger solar array.
Number five: Shade is a killer. Because of the construction of most panels, shade hitting 10% or your panel can kill 50% of the output. So a panel on the top of your pack can be cut significantly from the shade of a wide brimmed hat.
Number six: Clouds. Clouds will reduce the power output, but you can still get some output in cloudy weather.
Number seven: You need to size your solar system to anticipate all the items above. That is, you need much more capacity than your can theoretically produce each day.
Number eight: Worrying about charging your system. We do not worry about our panels and don't check the system much anymore. They just sit up on the roof and keep things in balance. We can go for many rainy days, and have enough reserve capacity to get by for a week of constant rain. Although we do not do it, we can power a microwave or an electric toaster. Also will your panel get damaged in use? Our panels are warrantied for 20 years and are designed to withstand hail.
Bottom line: You need to do a lot of research and calculations to size a system, as Bob G has posted earlier. Bad weather and shade can leave you with no power to run your electrical devices. For this reason I do not carry much in the way of devices when backpacking. Normally a camera and a headlight. If the camera goes dead, I will live. A spare battery for the headlamp and it will last for a very long time. A map and compass instead of a GPS 99% of the time. I cannot get cell phone coverage in most places I hike, so it stays in the car. Also if I had to constantly mess around with positioning my solar panel while walking, I would find the trip less enjoyable and frustrating.
Here is a picture of our panels.