In my someday dreamed of camping van I want to write & surf the web & generally have electricity. Since I highly doubt I’ll have room for a generator (plus they’re noisy and smelly and huge) solar seems like the obvious way to go.
Right off, let me invite anyone who knows more about RV solar systems than I do to dive in and point out any mistakes. I’d appreciate it!
So, let me break it all down for you so if you’re a novice like me you can see exactly how I arrived at my data.
 Step One: estimate your electrical needs.
 Find yourself a decent energy use calculator for RVs and plug in how much you’ll use your gadgets. Don’t forget your cell phone charger, laptop, lights, or margarita blender! I searched “RV Solar Panel Size Calculator” and “RV energy use calculator.” This netted a bunch of useless junk, but caught me these two gems: http://www.macandchris.com/electricalsystemsizing.htm and http://www.rvsolarsystemsonline.com/wattagecalculator/
 Novice Tip: some of the components of your solar system actually use electricity, including the battery charge controller, so don’t forget them in your calculations! You’ll notice I left them out of mine because I’m awesome like that (and because I didn’t know what types of charge controllers and inverters I’d need until the end).
 Find yourself a decent energy use calculator for RVs and plug in how much you’ll use your gadgets. Don’t forget your cell phone charger, laptop, lights, or margarita blender! I searched “RV Solar Panel Size Calculator” and “RV energy use calculator.” This netted a bunch of useless junk, but caught me these two gems: http://www.macandchris.com/electricalsystemsizing.htm and http://www.rvsolarsystemsonline.com/wattagecalculator/
About the math: This is where I’m still pretty hazy on how things convert back and forth. I’ve read that to convert a 120V AC amp to a 12V DC amp you multiply it by 10, (so a 2 amp plugin phone charger would use 20 amps on your 12V DC system), but then I see that a phone charger you plug into your car’s 12V DC cigarette lighter only draws 15 watts. And (if “watts are watts” and you simply divide 15 by 12 to get the amps that car phone charger pulls) that means the car charger only pulls 1.25 amps on a 12V DC system! 20 amps and 1.25 amps on the same 12V DC system are WAAAY different! So, until I find somebody who can explain this all in a way that makes sense, I’m leaving the converting AC to DC up to the pros. Find yourself a decent calculator to turn AC amps into DC, like this one. For converting watts to amps, check out this.
Here is an example of my dream RV solar set up worksheet:
CamperVan Solar System: Margaritas FTW!  
ITEM DESCRIPTION  12V DC AMPS DRAWN  HOURS USED  AMP HOURS (Ah) 
Chromebook charger – 65 watt 120V AC peak  5.417  8  43.336 
Phone Charger 15 watt 12V DC  1.25  8  10 
12V DC LED lights (four) – 0.125amp each  0.5  6  3 
120V AC blender  12  0.15  1.8 
12V DC Koolatron P65 Kargo Portable Cooler  5  24  120 
FanTastic Fan – 12V DC  3  24  72 
120V AC Blowdryer  125  0.15  18.75 
120V AC 1000 watt Microwave  83.333  0.35  63 
TOTAL  331.886 
 Take your total amp hours (in this case 331.886) and double it. Why? Because the (gasp) EXPENSIVE rechargeable batteries you’ll store all that free sunny goodness in should never be discharged (used up) more than halfway. That means, in this case, I’ll need a 12V battery bank that can store:
331.889 X 2 = 663.772 Ah
Just for safety sake, and because batteries don’t come in odd Ah ratings, round it up to 675Ah or 700Ah. Or even safer (for the batteries, but crappy for your wallet) 800Ah.
 Novice Tip: when buying your 12V battery bank, keep in mind that you don’t have to use 12V batteries. Instead you can use 6V as long as you keep them in pairs (2, 4, 6, etc.) and wire them together so they function just like a 12V battery.
 Oh, and a “Battery Bank” is just that – more than one battery wired together to function as a whole. So yeah, I don’t have to shell out tens of thousands of dollars to find ONE huge battery to satisfy my needs, but (as you’ll see) 675Ah – 800Ah in decent deep cycle batteries will still set me back.
 Now you can go battery shopping! Be sure to look for 12V DC batteries that will add up to your desired Ah, or for (multiple) pairs of 6V DC batteries that do the same thing. You’re going to want deep cycle batteries in the very least. I’ll discuss what (little) I know about battery types and why you should consider AMG if your wallet allows in a later post.
 Figure out how many solar watts you’ll need to produce and how many solar panels you can fit. Go back to that original total amp hour number (331.886) and round it up (to 345 or 350 in this case). That is how many amp hours your solar panels (and generator and wind turbine, if you want to add them) will need to replace every day you use your van/RV. You’d think you just round it up to 350Ah and go find solar panels that make that much power, but NO, nature likes to complicate these things. You see, solar panels are rated by how many watts they are capable of producing under perfect conditions. A tiny cloud or over hanging leaf or rainy day will muck things up and leave you without available power (since once you see the price of batteries you’d rather die than chance overtaxing your battery bank)! Since I won’t have the luxury of backup power in the form of a generator I will need to be EXTRA cautious about my batteries and should underestimate how many watts I’ll be able to produce. I’ll even need to add a battery charge monitor so I can keep and eye on my battery levels so I don’t over tax them.
 First, check out the available space on your van or RV roof and guesstimate how many solar panels you can cram up there. You might want to consider adding a ladder rack to the top and attaching the solar panels to the rack so your vents and whatnot can stick out underneath. This maximizes your available space while still allowing for ventilation and all that breathey goodness. If you’re lucky you might be able to fit two panels with the current space, or up to six with the ladder rack trick.
 Next, divide your rounded total amp hour number by the projected number of panels you can fit. Let’s assume I’ll be super lucky and will fit four on my dreamwagon.
 First, check out the available space on your van or RV roof and guesstimate how many solar panels you can cram up there. You might want to consider adding a ladder rack to the top and attaching the solar panels to the rack so your vents and whatnot can stick out underneath. This maximizes your available space while still allowing for ventilation and all that breathey goodness. If you’re lucky you might be able to fit two panels with the current space, or up to six with the ladder rack trick.
350Ah / 4 panels = 87.5Ah each
The result (87.5) is the number of amp hours each of your solar panels (plus generator, etc) will actually need to crank out on any given day.
 After some research and math I found that if you divide the solar panel’s watt rating by about 2.4 to 3.05 you get a decent estimate of how many actual watts the professionals think any given panel will put out. This means all you need to do is multiply your Ah per panel number by 2.4 and then 3.05 and you’ll get an estimated range of the panel wattage you’ll need. So:
87.5 X 2.4 = 209.28
And
87.5 X 3.05 = 266.875
So, I’ll need four panels that are each rated to produce between 209.28 watts and 266.875 watts. I found a big (but not too big) Kyocera panel that is rated at 260 watts. It sits at the top of the range nicely. It is a Kyocera KD260GXLFB2. Because it can crank out so many watts, a MPPT style charge controller is needed to convert the power down to 12V DC. That adds expense, but will allow me to get the amp hours I need for that blow dryer and margarita blender!
 Now it’s time to calculate for your charge controller!
 First, find your TOTAL solar panel watts:
4 panels X 260 watts = 1040 total watts

 Second, convert the total watts to amps at 12V DC. I used this handy calculator. I ended up with 86.67 amps at 12V DC. I’ll round this up to 90 amps for convenience.
 Next, look at the type of charge controller your solar panel(s) need. I know I’ll be looking for an MPPT.
 Now, hit the internet or your local solar supply store to find one with the amps and type you need! I’ll be looking for a MPPT type with at least 90 amps at 12V DC.
 If you’re going to run regular appliances that take household current (like my beloved blender) you’re going to need a 12V DC to 120V AC inverter. To find out the watt size you’ll need, add up all the amp hours used by the 120V AC appliances you’ll be packing. Here’s my example:
 Next, multiply this number by 12 and round up:
126.886 X 12 = 1522.632
1522.632 rounded up is 1600 watts, so I’ll need to look for a 1600 watt 12V DC to 120V AC inverter.
 Now that you know how much power you’ll need, the size of your battery bank, the number and wattage rating of your solar panels, the type and amps of your charge controller and inverter info you can shop like a fiend! I know this will keep me up all night and once I do fall asleep I’ll be dreaming solar panels!
I’d like to thank Jerry & Cynthia for sharing a basic rundown of their RV’s solar system and install and, I’m assuming, Mac & Chris for their more indepth electrical needs worksheet & I couldn’t have gotten as decent (as it is) an estimate without the great info at http://www.rvsolarsystemsonline.com/wattagecalculator/. I also found RapidTables.com invaluable for helping me convert watts to amps.