What are you charging and how much power do you need?
If you are just charging AA batteries you don't need a very powerful panel but you do need the right adapters. A kit like the Guide 10 Battery Pack will charge four AA batteries in a few hours when combined with a 7 watt panel.
If you are just charging a small cell phone or mp3, a 4 or 5 watt panel will work. However, many smart phones like the iPhone 4 are picky about what will charge them. In general, you need at least a 7 watt panel to be sure you can charge most smart phones. Keep in mind, you will usually need a DC charging device unless your panel has a USB output and your device can be charged by USB cable. The most common DC charging device is a 12 volt DC car charger (AKA cigarette charger). Most cell phones or small portable electronic devices have a car charging accessory you can buy for less that $20. You then have to make sure your solar charger has the female car charger adapter that attaches to the panel. Most panels come with this but some require you to buy one separately.
If you want to start charging more energy-hungry items like an iPad or multiple devices at once, things start to get more complicated and expensive. In general, you will need a panel with at least least 15 watts of power if not more. Even that may not work, you may need a battery and or and inverter. If you want to start charging laptops, you almost certainly need a battery and inverter. And if you want to charge AC devices, you are now in a new category. You will now need a pretty heavy duty panel (25+ watts, a battery, and a DC-to-AC inverter). It used to be hard to make those three components work well together. But now Goal Zero has a number of kits like the Goal Zero Escape 150 Adventure Kit that make it pretty easy.
Be careful of buying less wattage panels just to save money. Often you can get by with a smaller panel, but you will end up frustrated as charging anything takes more time than it's worth. We experienced a lot of frustration in our tests with some 4-6 watt chargers with powering our cell phones. It would sometime take 3-6 hours to get a full charge when an outlet or higher wattage panel would take 1-2 hours.
Solar Accessories
Some panels come with just a cigarette adapter. Others come loaded with alligator clips, extension cables, USB connectors and charging indicators. To make things worse, often it is hard to tell what accessories are included with a panel and which you have to buy separately. Make sure you know what accessories you need and which ones actually come with your panel.
One accessory that we love that is rarely included with panels is a cigarette splitter (the previous link goes to our favorite model). It is a very useful device, as long as you panel has enough output (7-12+ watts).
Weight and Compactness
If you are buying a portable solar panel for travel or backpacking, weight and compactness is very important. Unfortunately, it is pretty hard to tell from photos and even the listed specs how much a panel weighs. When comparing weights, make sure you are comparing apples to apples. Sometimes manufacturers list the weight of just the panel. Other times they list the weight of the panel plus all the accessories.
Price Per Watt of a Solar Panel
The easiest way to determine value in a solar panel is the price per watt. Take the price of the panel and divide by the number of watts. For example, if your panel costs $70 and is 7 watts, that is $10 per watt. $10-15 per watt is a good value. Anything more than $20 a watt is too expensive unless the panel is also coming with valuable accessories, batteries or inverters.
Solar Powered Electric Bike: Trek Valencia+ with the Goal Zero Sherpa 120 Kit
Credit: Chris McNamara
Solar Chargers vs. External Battery
In many cases you may be better served by an external USB battery. They typically hold four to five smart phone charges, operate anytime of the day, and are usually half the cost and bulk of a solar panel. As a general rule, a panel is typically better if you're going to be away from a power source for more than four days. For most other cases, an external battery may be the way to go. The ultimate solution may be to pair a panel with an external battery. It all comes down to what you need to power and for how long. See our complete External Battery Review for a more detailed discussion on this topic.
Is Buying a Solar Panel Worth It?
Solar power from a portable panel is still generally more expensive than electricity from the grid. But if you don't have access to the grid and you need to power something, a portable solar panel is often the only option.
What if you want to power your home office? Or power your camping accessories without running your car engine? Portable solar panels are still expensive per watt. However, two factors are often missed when deciding if solar is worth it: 1) When you are generating your own power, you usually use way less 2) Generating your own power is pretty fun.
For example, during this test we found it would be fun to use the Goal Zero Sherpa 120 Kit to power our home office (laptop, 24" LCD monitor, USB hub and external hard drive). We found it was possible as long as we turned off any accessories we were not using (lamps, printers, extra hard drives, scanners). So while the price per watt may still have been high with our solar panels, overall we were more aware of our power usage and overall used much less electricity. We did the same thing with our Trek Valencia+ pedal-assist electric bike. We found we could extend the range of the bike by taking the Sherpa 120 kit with us. Suddenly we needed the car much less. In fact, since I got the the pedal-assist electric bike and driving mileage has gone from 1,000-2,000 miles a month to 250-750. My car gets 28mpg so right there I am saving $75-175 a month.
More importantly, if you are like me, it's just fun to create little systems like this. Building your own electric grid and transportation system is the grownup version of building a fort or a tree house. I encourage you to try it out. Manufacturers like Goal Zero have 30-day return policies if it isn't all we've talked it up to be.
Goal Zero Nomad 7 Solar Panel on a sailing trip in the San Juan Islands.
Credit: Chris McNamara
