First, before you do anything else, go up on your roof and measure to see how much room you have. You should only buy what will fit on your roof. Then see what different panels have for different sizes, maybe you can fit more small ones, or, maybe you can only fit one big one.
Of course you can always buy a portable suitcase system or flexible panels and not mount them on the roof, that's another discussion.
Many people suggest you add up all your electrical use and get enough to meet those needs. But solar has gotten so cheap I say start with your wallet and buy all you can afford to spend right now. After all, if you can't afford it, why bother finding out what your needs are?
Also remember that you are buying for the worst day of the year--a storm that lasts for a week in the winter when the days are already short and the sun never rises very high. If you buy JUST enough for the summer, you WILL run out in the winter. Plus, you are buying for latitude, northern cities get less sun that southern cities.
Here are my recommendations.
Minimal Power User = 100 watts. This person mainly recharge his phone, laptop, camera and a few other devices. Buy the 100 watt kit and adjust your use to it. If it's all you need, then you are done.
Average Vandweller = 200 watts. The majority of vandwellers need and can be satisfied with 200 watts. It meets all the basic needs plus, 12 volt compressor fridge, fans or even TV.I have 190 watts on my trailer and I've never been unhappy with it. If you have the money, buy a 200 watt kit. If it's not enough, buy another 100 watt kit. Yes, you have two systems, but I consider that a blessing since you have redundancy; if one system fails you still have power. I have three distinct systems and I like it! Or, get the 200 watt kit and get the upgraded controller so you can add another one to it later when you can afford it. But controllers are so cheap now I think just buying a new cheap one is better.
Power user = 400 watts. If you have the money, start out with a 300 or 400 watt. You'll never have to worry about power even in a storm and nearly all of us can find a way to use extra electricity. I use my extra power to run a microwave and watch Satellite TV. [img=21x21]file:///C:/Users/Bob/AppData/Local/Temp/msohtmlclip1/01/clip_image001.gif[/img]
Why increments of 100 watts? Because you can get 100 watt Renogy panels cheap with free shipping. If you jump up to the big panels like 200 or bigger, they are cheaper per watt, but the cost of shipping is so high it will end up being much more.
But, if you are near where you can get big panels and they fit your van better, get them instead.
Be aware that higher watt panels are usually higher voltage and require a MPPT controller.
Starting with your wallet is just my personal bias, lots of people will disagree, hopefully they will speak up and explain there thinking.
Bob
